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Diego ANGELI
Professore Associato
Dipartimento di Scienze e Metodi dell'Ingegneria
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Pubblicazioni
2024
- Comprehensive modeling of ventilation systems for Nearly Zero Energy Buildings
[Relazione in Atti di Convegno]
Sedoni, Roberto; Cannistraci, Gioele; Santangelo, Paolo Emilio; Angeli, Diego; Romani, Marco; Fioravanti, Luca
abstract
In the present work, a lumped-parameter model of a multifunctional ventilation unit for residential applications was developed in the Simulink environment, also relying on the Simscape toolbox with Moist Air and Two-phase fluid libraries. A simple, yet effective method to analyze and optimize the efficiency of the combined HVAC – air distribution system is proposed. To investigate the impact of boundary conditions on system performance, a parametric study of different installation conditions was also carried out, including outdoor air and indoor air temperature, humidity, static pressure, filter fouling, pressure drop in the intake and distribution ducts. The model highlights a strong decrease in the useful cooling/heating heat flow rate produced by the system as the installation and maintenance conditions become more challenging.
2023
- A lumped-parameter model of a smart ventilation unit for Nearly Zero Energy Buildings
[Relazione in Atti di Convegno]
Sedoni, Roberto; Cannistraci, Gioele; Santangelo, Paolo Emilio; Angeli, Diego; Romani, Marco; Fioravanti, Luca
abstract
In the present work, a simple model of a ventilation unit used for residential purposes is proposed, which was developed by means of MATLAB Simulink and the Simscape toolbox, also including the Moist Air and Two-Phase fluid libraries. This study falls in the realm of air conditioning in nearly Zero Energy Buildings. The model presented here is focused mostly on the aeraulic system. A parametric analysis of various installation conditions was conducted to assess and enhance the combined heat pump and the air distribution system performance, under various operating conditions. Therefore, the overall approach included several parameters, such as outdoor and indoor air temperature, humidity, static pressure, pressure drop in the intake and the distribution ducts, and filter fouling. The model serves as a predictive tool to evaluate the effectiveness of the whole system, in both design and off-design conditions; notably, critical conditions are emphasized, which are associated to severe fouling conditions, making the use of an additional fan ineffective.
2023
- ACCURATE REDUCED-ORDER MODELLING OF PLATE-FIN HEAT EXCHANGERS
[Relazione in Atti di Convegno]
Grespan, Mattia; Leonforte, Adriano; Cavazzuti, Marco; Calò, Luigi; Angeli, Diego
abstract
2023
- Experimental measurements and CFD modelling of hydroxyapatite scaffolds in perfusion bioreactors for bone regeneration
[Articolo su rivista]
D’Adamo, Alessandro; Salerno, Elisabetta; Corda, Giuseppe; Ongaro, Claudio; Zardin, Barbara; Ruffini, Andrea; Orlandi, Giulia; Bertacchini, Jessika; Angeli, Diego
abstract
In the field of bone tissue engineering, particular interest is devoted to the development of 3D cultures to study bone cell proliferation under conditions similar to in vivo ones, e.g. by artificially producing mechanical stresses promoting a biological response (mechanotransduction). Of particular relevance in this context are the effects generated by the flow shear stress, which governs the nutrients delivery rate to the growing cells and which can be controlled in perfusion reactors. However, the introduction of 3D scaffolds complicates the direct measurement of the generated shear stress on the adhered cells inside the matrix, thus jeopardizing the potential of using multi-dimensional matrices. In this study, an anisotropic hydroxyapatite-based set of scaffolds is considered as a 3D biomimetic support for bone cells deposition and growth. Measurements of sample-specific flow resistance are carried out using a perfusion system, accompanied by a visual characterization of the material structure. From the obtained results, a subset of three samples is reproduced using 3D-Computational Fluid Dynamics (CFD) techniques and the models are validated by virtually replicating the flow resistance measurement. Once a good agreement is found, the analysis of flow-induced shear stress on the inner B-HA structure is carried out based on simulation results. Finally, a statistical analysis leads to a simplified expression to correlate the flow resistance with the entity and extensions of wall shear stress inside the scaffold. The study applies CFD to overcome the limitations of experiments, allowing for an advancement in multi-dimensional cell cultures by elucidating the flow conditions in 3D reactors.
2023
- Measurements and scaling of buoyancy-induced flows in ventilated tunnels
[Articolo su rivista]
Salizzoni, P.; Peruzzi, C.; Marro, M.; Cingi, P.; Angeli, D.; Kubwimana, T.; Mos, A.
abstract
We investigate the ventilation conditions required to control the propagation of smoke, produced by a tunnel fire, in the presence of two inertial forcings: a transverse extraction system and a longitudinal flow. For that purpose, we performed a series of experiments in a reduced-scale tunnel, using a mixture of air and helium to simulate the release of hot smoke during a fire. Experiments were designed to focus on the ventilation flows that allow the buoyant release to be confined between two adjacent extraction vents. Different source conditions, in terms of density and velocity of the buoyant release, were analysed along with different vent configurations. Experiments allowed us to quantify the increase of the extraction velocity needed to confine the buoyant smoke, overcoming the effect of an imposed longitudinal velocity. Vents with a rectangular shape, and spanning over the whole tunnel width, provide the best performance. Finally, we studied the stratification conditions of the flow, individuating four regimes. Interestingly, when the stratification conditions fade out, as both the longitudinal flow and vertical extraction flows increase, the flow dynamics becomes almost independent of the forcing induced by the presence of buoyant smoke, which eventually acts as a passive scalar transported by the flow.
2022
- Direct Numerical Simulation of natural, mixed and forced convection in liquid metals: selected results
[Articolo su rivista]
Fregni, A.; Angeli, D.; Cimarelli, A.; Stalio, E.
abstract
Selected results of three Direct Numerical Simulations are presented, on relevant test cases for the thermal hydraulics of liquid–metal-cooled nuclear reactors, encompassing a wide spectrum of turbulent convection regimes. The first test case is a Rayleigh-Bénard cell at a Grashof number Gr=5×107, representative of the conditions in the unstably stratified layer of coolant in a reactor pool in both standard operating conditions and emergency situations, e.g. shutdown of the cooling system. The second case is the mixed convection in a cold-hot–cold triple jet configuration, representative of liquid streams exiting from the core into the pool, and relevant for the modeling of thermal striping and thermal fatigue phenomena on the vessel containment walls. The third case is the fully-developed flow in a vertical bare rod bundle with triangular arrangement and a pitch-to-diameter ratio P/D=1.4, in both forced and mixed convection conditions. These regimes respectively represent normal operation or decay heat removal conditions in reactor cores. The availability of these numerical databases will allows for an in-depth analysis of the turbulent flow and heat transfer in liquid metals under different convection regimes, and is also relevant for the development, calibration and validation of turbulent heat transfer models.
2022
- Flow solutions around rectangular cylinders: The question of spatial discretization
[Articolo su rivista]
Corsini, R.; Angeli, D.; Stalio, E.; Chibbaro, S.; Cimarelli, A.
abstract
The aerodynamics of blunt bodies with separation at the sharp corner of the leading edge and reattachment on the body side are particularly important in civil engineering applications. In recent years, a number of experimental and numerical studies have become available on the aerodynamics of a rectangular cylinder with chord-to-thickness ratio equal to 5 (BARC). Despite the interest in the topic, a widely accepted set of guidelines for grid generation about these blunt bodies is still missing. In this work a new, well resolved Direct Numerical Simulation (DNS) around the BARC body at Re=3000 is presented and its results compared to previous DNSs of the same case but with different numerical approaches and mesh. Despite the simulations use different numerical approaches, mesh and domain dimensions, the main discrepancies are ascribed to the different grid spacings employed. While a more rigorous analysis is envisaged, where the order of accuracy of the schemes are kept the same while grid spacings are varied alternately along each spatial direction, this represents a first attempt in the study of the influence of spatial resolution in the Direct Numerical Simulation of flows around elongated rectangular cylinders with sharp corners.
2022
- Liquid flow in scaffold derived from natural source: experimental observations and biological outcome
[Articolo su rivista]
Salerno, Elisabetta; Orlandi, Giulia; Ongaro, Claudio; D'Adamo, Alessandro; Ruffini, Andrea; Carnevale, Gianluca; Zardin, Barbara; Bertacchini, Jessika; Angeli, Diego
abstract
This study investigates the biological effects on a 3D scaffold based on hydroxyapatite cultured with MC3T3 osteoblasts in response to flow-induced shear stress (FSS). The scaffold adopted here (B-HA) derives from the biomorphic transformation of natural wood and its peculiar channel geometry mimics the porous structure of the bone. From the point of view of fluid dynamics, B-HA can be considered a network of micro-channels, intrinsically offering the advantages of a microfluidic system. This work, for the first time, offers a description of the fluid dynamic properties of the B-HA scaffold, which are strongly connected to its morphology. These features are necessary to determine the FSS ranges to be applied during in vitro studies to get physiologically relevant conditions. The selected ranges of FSS promoted the elongation of the attached cells along the flow direction and early osteogenic cell differentiation. These data confirmed the ability of B-HA to promote the differentiation process along osteogenic lineage. Hence, such a bioactive and naturally derived scaffold can be considered as a promising tool for bone regeneration applications.
2022
- Numerical evaluation of head losses in plate and bar heat exchangers
[Relazione in Atti di Convegno]
Grespan, Mattia; Leonforte, Adriano; Cavazzuti, Marco; Calò, Luigi; Angeli, Diego
abstract
2022
- Two-phase flow characterization through recurrence quantification analysis of the dominant features of experimental dynamics
[Articolo su rivista]
Pagano, A.; Angeli, D.
abstract
This paper describes a novel approach to the analysis of the experimental void fraction time series detected in air–water upward two-phase flows within a vertical channel. Singular Value Decomposition (SVD) is applied to the experimental void fraction time series in order to assess a novel state space spanned by the principal components of the flow dynamics; in fact, this was demonstrated in a previous study to be effective in separating the dominant features from noise-like dynamics and, above all, in evidencing the existence of recurrent behaviours. As recurrence is a typical and fundamental signature of low-order deterministic nonlinear dynamics, the present study aims at reaching a detailed insight on the recurrent structures that characterize the experimental void fraction time series, which were measured during an experimental campaign encompassing the entire spectrum of patterns in vertical air–water upward flows. The main novelty of the present study is the adoption of the tools collectively known as Recurrence Quantification Analysis not directly to the experimental time series, as it is increasingly proposed in the analysis of two-phase flows, but to their most important principal components. The goal is to obtain a more reliable characterization of two-phase flow patterns, which is based only on the relevant features and, hence, is unbiased by noisy high-order dynamics. Reported results show that the present approach indeed provides a powerful tool for the characterisation of the variety of complex dynamics exhibited by two-phase flows, as well as for flow pattern classification.
2021
- An integrated approach for the analysis and modeling of road tunnel ventilation. Part I: Continuous measurement of the longitudinal airflow profile
[Articolo su rivista]
Levoni, P.; Angeli, D.; Cingi, P.; Barozzi, G. S.; Cipollone, M.
abstract
The knowledge of the flow field inside road tunnels under normal operation, let alone fire conditions, is only approximate and partial. The reason is that while the full three-dimensional, unsteady problem is out of reach of numerical methods, on the other hand accurate measurement of the airflow in road and railway tunnels constitutes an extremely demanding task. The present work, structured as a twofold study, takes up the challenge and proposes an original integrated experimental and numerical approach for the analysis and modeling of flow inside a road tunnel and its ventilation systems, aiming at defining a methodology for the creation of “digital twins” of the system itself, on which advanced ventilation and smoke control strategies can be tested and fine-tuned. In this first part, an innovative experimental facility for the continuous acquisition of the longitudinal velocity profile along the whole length of a road tunnel has been designed and built. The facility consists of a survey rake with five bidirectional vane anemometers, which is mounted on a small electric vehicle that can travel through the tunnel at constant speed. This paper reports the design procedure of the measurement facility, with particular focus on the conception and realization of the vehicle carrying the survey rake. Results of the first experimental campaign carried out under the 11611 meters long Mont Blanc road tunnel are presented to corroborate the validity of the approach adopted and the accuracy of the measurement chain.
2021
- An integrated approach for the analysis and modeling of road tunnel ventilation. Part II: Numerical model and its calibration
[Articolo su rivista]
Cingi, P.; Angeli, D.; Cavazzuti, M.; Levoni, P.; Stalio, E.; Cipollone, M.
abstract
The present work represents the second and final part of a twofold study aiming at the definition and validation of an integrated methodology for the analysis and modeling of road tunnel ventilation systems. A numerical approach is presented, based on the Finite Volume integration of the 1D mechanical and thermal energy conservation equations on a network of ducts, representing the ventilation system of the 11.6 km long Mont Blanc Tunnel. The set of distributed and concentrated loss coefficients, representing dissipation of mechanical energy by friction in each part of the ventilation system, is calibrated against a rich experimental dataset, collected throughout a dedicated set of in situ tests and presented in the first part of the work. The calibration of the model is carried out by means of genetic optimization algorithms. Predictions of the flow field using the calibrated parameters are in remarkable agreement with the experimental data, with an overall RMS error of ± 0.27 m/s, i.e. of the same order of the accuracy of the measurement probes. Further validation against a selection of field data recorded by the tunnel monitoring and control system is brought forward, highlighting the robustness and potential general applicability of the proposed approach.
2021
- Direct numerical simulation of transition in a differentially heated vertical channel
[Articolo su rivista]
Cingi, P.; Cimarelli, A.; Angeli, D.
abstract
The transition mechanisms of natural convection flows ensuing in a fluid layer between two differentially heated vertical plates at Prandtl number Pr = 0.71 are investigated by means of Direct Numerical Simulations. In accordance with several previous studies, results show a first bifurcation from the so-called laminar conduction regime to steady convection at Rayleigh number Ra = 5708. On the other hand, the subsequent transition to turbulence appears to be accompanied by a great sensitivity to some fundamental numerical choices such as domain size, accuracy, resolution and amplitude of the imposed perturbations. Results reveal the occurrence of a bifurcation branch which leads the system to chaos via a second bifurcation to a steady-state, a Hopf bifurcation and, seemingly, a period-doubling cascade. Although the described scenario compares well with previous findings, some doubts persist upon the possible pitfalls in the use of numerical simulation for the study of transition in this kind of systems. Indeed, several numerical aspects are found to become of crucial importance for the prediction of the dynamical behaviour and heat transfer rate of the system.
2021
- Influence of the Thermophysical Model on the CFD Analysis of Oil-Cooled Transformer Windings
[Relazione in Atti di Convegno]
Salerno, E.; Leonforte, A.; Angeli, D.
abstract
A disc-type winding of an oil-immersed power transformer is modeled with Computational Fluid Dynamics. Different approaches are implemented to evaluate the feasibility of the Boussinesq approximation: (i) constant fluid properties, (ii) variable viscosity and thermal diffusivity and (iii) temperature-dependent fluid properties. Temperature and flow distributions are reconstructed and put into relation with physical phenomena and model assumptions. Their comparison suggests that numerical results are fairly sensitive to the thermophysical model as long as the buoyancy force is a relevant component of the flow. Nonetheless, all the cases converge to very close predictions of the hot-spot value and location, with possibly positive implications for the use of reference parameters when deriving flow and heat transfer correlations for this topic.
2020
- A collaborative effort towards the accurate prediction of turbulent flow and heat transfer in low-Prandtl number fluids
[Articolo su rivista]
Shams, A.; Roelofs, F.; Tiselj, I.; Oder, J.; Bartosiewicz, Y.; Duponcheel, M.; Niceno, B.; Guo, W.; Stalio, E.; Angeli, D.; Fregni, A.; Buckingham, S.; Koloszar, L. K.; Villa Ortiz, A.; Planquart, P.; Narayanan, C.; Lakehal, D.; van Tichelen, K.; Jager, W.; Schaub, T.
abstract
This article reports the experimental and DNS database that has been generated, within the framework of the EU SESAME and MYRTE projects, for various low-Prandtl flow configurations in different flow regimes. This includes three experiments: confined and unconfined backward facing steps with low-Prandtl fluids, and a forced convection planar jet case with two different Prandtl fluids. In terms of numerical data, seven different flow configurations are considered: a wall-bounded mixed convection flow at low-Prandtl number with varying Richardson number (Ri) values; a wall-bounded mixed and forced convection flow in a bare rod bundle configuration; a forced convection confined backward facing step (BFS) with conjugate heat transfer; a forced convection impinging jet for three different Prandtl fluids corresponding to two different Reynolds numbers of the fully developed planar turbulent jet; a mixed-convection cold-hot–cold triple jet configuration corresponding to Ri = 0.25; an unconfined free shear layer for three different Prandtl fluids; and a forced convection infinite wire-wrapped fuel assembly. This wide range of reference data is used to evaluate, validate and/or further develop different turbulent heat flux modelling approaches, namely simple gradient diffusion hypothesis (SGDH) based on constant and variable turbulent Prandtl number; explicit and implicit algebraic heat flux models; and a second order turbulent heat flux model. Lastly, this article will highlight the current challenges and perspectives of the available turbulence models, in different codes, for the accurate prediction of flow and heat transfer in low-Prandtl fluids.
2020
- DIRECT NUMERICAL SIMULATION OF NATURAL, MIXED AND FORCED CONVECTION IN LIQUID METALS: SELECTED RESULTS
[Relazione in Atti di Convegno]
Fregni, A.; Angeli, D.; Cimarelli, A.; Stalio, E.
abstract
Selected results of three Direct Numerical Simulations are presented, on relevant test cases for the thermal hydraulics of liquid metal-cooled nuclear reactors, encompassing a wide spectrum of turbulent convection regimes. The first test case is a Rayleigh-Benard cell at a moderately high Grashof number, representative of the conditions in the unstably stratified layer of fluid in a reactor pool. The second case is the mixed convection in a cold-hot-cold triple jet configuration, representative of the mixing liquid streams exiting from the core into the pool, and relevant for the modeling of thermal striping and thermal fatigue phenomena on the vessel containment walls. The third case is the fully-developed flow in a vertical bare rod bundle with triangular arrangement and a large pitch-to-diameter ratio, in both forced and mixed convection conditions, representative of normal operation or decay heat removal flow conditions in the reactor core, respectively. The availability of these numerical databases will allow for an in-depth analysis of the turbulent flow and heat transfer in liquid metals under different convection regimes, and is also relevant for the development, calibration and validation of turbulent heat transfer models.
2020
- Development and calibration of a 1D thermo-fluid dynamic model of ventilation in tunnels
[Relazione in Atti di Convegno]
Cingi, P.; Angeli, D.; Cavazzuti, M.; Levoni, P.; Cipollone, M.
abstract
In complex, large civil infrastructures where ventilation has a crucial role for the safety of users in both normal operation and hazardous scenarios, the correct prediction of flow and heat transfer parameters is of fundamental importance. While full 3D simulation is applicable only to a limited extent, and the resort to 1D modeling is a common practice in both design and evaluation phases, the limitation of such models lies in the choice of transfer parameters, such as friction loss coefficients and heat transfer coefficients. In this work, an original approach based on the Finite Volume integration of the 1D flow and energy equations is presented. Such equations are to be solved on a network of ducts, representing the ventilation system in the 11.6 km long Mont Blanc Tunnel with a spatial resolution of 10 m. A preliminary calibration of a set of friction loss coefficients against a rich experimental dataset collected throughout a dedicated set of in situ tests is of particular concern here, as it is carried out by means of genetic optimization algorithms. Predictions of the flow field are in remarkable agreement with the experimental data, with an overall RMS error of - 0.42 m/s. Further refinements and possible parameter choices are also discussed.
2020
- Experimental and numerical analysis of a liquid aluminium injector for an Al-H2O based hydrogen production system
[Articolo su rivista]
Milani, M.; Montorsi, L.; Storchi, G.; Venturelli, M.; Angeli, D.; Leonforte, A.; Castagnetti, D.; Sorrentino, A.
abstract
This paper investigates pressurised injection system for liquid aluminium for a cogeneration system based on the Al–H2O reaction. The reaction produces hydrogen and heat which is used for super-heating vapour for a steam cycle. The aluminium combustion with water generates also alumina as a byproduct; the aluminium oxide can be recycled and transformed back to aluminium. Thus, aluminium can be exploited as energy carrier in order to transport energy from the alumina recycling plant to the place where the cogeneration system is located. The water is also used in a closed loop; indeed, the amount of water produced employing the hydrogen obtained by the proposed system corresponds to the oxidizing water for the Al/H2O reaction. The development of a specific test rig designed for investigating the liquid aluminium injection is presented in this research study. The injector nozzle is investigated by means of numerical thermal and structural analysis. The calculations are compared and validated against the experimental measurements carried out on ad-hoc developed test rig. A good agreement between the numerical results and the experimental values is found and the new design of the nozzle is devised.
2020
- Fully developed turbulent convection of Lead Bismuth Eutectic in the elementary cell of the NACIE-UP Fuel Pin Bundle
[Articolo su rivista]
Angeli, D.; Di Piazza, I.; Marinari, R.; Stalio, E.
abstract
One of the requirements for achieving high levels of safety in fourth generation nuclear power plants, is that core thermal-hydraulics can be simulated numerically at a good level of accuracy. To this aim, detailed validation of turbulent heat transfer models needs to be carried out and discussed, in relevant flow and heat transfer configurations, and for low Prandtl-number fluids. The focus of this research is the turbulent, fully developed convection in a heated rod bundle with a triangular arrangement, and a pitch-to-diameter ratio P/D=1.4. Statistics derived from a set of Direct Numerical Simulations (DNS) at the moderate Reynolds number of Re=8290 are compared to Reynolds Averaged Navier-Stokes (RANS) solutions where the closure is provided by the two-equation model SST k-ω and by the Reynolds-Stress Model. Comparisons are drawn for forced flow and in mixed convection conditions (Ri≈0.25). Global quantities extracted from experiments performed in very similar conditions are also compared against the numerical results. Profiles of the turbulent Prandtl number about the unit flow cell are also displayed and discussed. This work clarifies through detailed comparison that if, on the one hand, only the Reynolds-Stress model is able to reasonably capture important flow features like secondary flow components, on the other hand also the SST k-ω two-equation model considered is acceptably accurate in predicting the integral quantities of interest.
2019
- A collaborative effort towards the accurate prediction of turbulent flow and heat transfer in low-Prandtl number fluids
[Relazione in Atti di Convegno]
Shams, A.; Roelofs, F.; Tiselj, I.; Oder, J.; Bartosiewicz, Y.; Duponcheel, M.; Niceno, B.; Guo, W.; Stalio, E.; Angeli, D.; Fregni, A.; Buckingham, S.; Koloszar, L. K.; Villa Ortiz, A.; Planquart, P.; Narayanan, C.; Lakehal, D.; van Tichelen, K.; Jager, W.; Schaub, T.
abstract
This article reports the experimental and DNS database that has been generated, within the framework of the EU SESAME and MYRTE projects, for various low-Prandtl flow configurations in different flow regimes. This includes three experiments: confined and unconfined backward facing steps with low-Prandtl fluids, and a forced convection planar jet case with two different Prandtl fluids. In terms of numerical data, seven different flow configurations are considered: a wall-bounded mixed convection flow at low-Prandtl number with varying Richardson number (Ri) values; a wall-bounded mixed and forced convection flow in a bare rod bundle configuration for two different Reynolds numbers; a forced convection confined backward facing step (BFS) with conjugate heat transfer; a forced convection impinging jet for three different Prandtl fluids corresponding to two different Reynolds numbers of the fully developed planar turbulent jet; a mixed-convection cold-hot-cold triple jet configuration corresponding to Ri=0.25; an unconfined free shear layer for three different Prandtl fluids; and a forced convection infinite wire-wrapped fuel assembly. This wide range of reference data is used to evaluate, validate and/or further develop different turbulent heat flux modelling approaches, namely simple gradient diffusion hypothesis based on constant and variable turbulent Prandtl number; explicit and implicit algebraic heat flux models; and a second order turbulent heat flux model. Lastly, this article will highlight the current challenges and perspectives of the available turbulence models, in different codes, for the accurate prediction of flow and heat transfer in low-Prandtl fluids.
2019
- A fast algorithm for Direct Numerical Simulation of turbulent convection with immersed boundaries
[Articolo su rivista]
Angeli, D.; Stalio, E.
abstract
A parallel algorithm is presented for the Direct Numerical Simulation of convection flows in open or partially confined periodic domains, containing immersed cylindrical bodies of arbitrary cross-section. The governing equations are discretized by means of the Finite Volume method on Cartesian grids. The method presented includes a triperiodic Poisson solver employed irrespective of the actual boundary shape and a second order accuracy for the computational domain, including the near wall regions, when walls are defined as immersed boundaries. The numerical solution of the set of linear equations resulting from discretization is carried out by means of efficient and highly parallel direct solvers. Verification and validation of the numerical procedure is reported in the paper, for laminar and turbulent pipe flow, and for the case of flow around an array of heated cylindrical rods arranged in a triangular lattice. The formal accuracy of the method is demonstrated in laminar flow conditions, and DNS results in turbulent conditions are compared to available literature data, thus confirming the favorable qualities of the method.
2019
- A priori analysis and benchmarking of the flow around a rectangular cylinder
[Capitolo/Saggio]
Cimarelli, A.; Leonforte, A.; Angeli, D.
abstract
The flow around bluff bodies is recognized to be a rich topic due to its huge number of applications in natural and engineering sciences. Of particular interest is the case of blunt bodies where a reattachment of the separated boundary layer before the definitive separation in the wake occurs. One of the main feature of this type of flows is the combined presence of small scales due to the occurrence of self-sustained turbulent motions and large scales due to classical vortex shedding. The complete understanding of these multiple interacting phenomena would help for a correct prediction and control of relevant features for engineering applications such as wind loads on buildings and vehicles, vibrations and acoustic insulation, heat transfer efficiency and entrainment. Archetypal of these kind of flows is the flow around a rectangular cylinder. Many studies have been carried out in the past. The general aim is the understanding of the main mechanisms behind the two unstediness of the flow, the shedding of vortices at the leading-edge shear layer and the low-frequency flapping mode of the separation bubble, see e.g Cherry et al (J Fluid Mech, 144:13–46, 1984, [1]), Kiya and Sasaki (J Fluid Mech, 154:463–491, 1985[2]), Nakamura et al (J Fluid Mech, 222:437–447, 1991, citeNakamura).
2019
- Direct Numerical Simulation of a buoyant triple jet at low-Prandtl number
[Articolo su rivista]
Fregni, A.; Angeli, D.; Cimarelli, A.; Stalio, E.
abstract
Mixing of buoyant streams is a phenomenon of relevance in many practical cases like pollutant emission in the atmosphere, discharges from marine outfalls and cooling of fuel rods in nuclear reactors to name a few. A canonical configuration for this class of flows consists in three buoyant jets at different temperatures vertically entering a pool from the bottom. This work reports a Direct Numerical Simulation study performed on the triple jet configuration. The Reynolds number based on the average jet centerline velocity and jet width is set to Re=5000 and mixed convection regime is established at a Richardson number, Ri=0.25. In order to represent flows occurring inside liquid metal fast reactors, the selected Prandtl number is Pr=0.031. Statistics computed show that in the first stages of development, the three jets undergo a strong interaction. In that same region the shedding of large-scale vortices is originated accompanied by low-frequency undulations. Further from the inlet, the three jets are observed to coalesce in a single, isothermal stream. The analysis of momentum fluxes clarifies the mutual entrainment mechanism behind coalescence, which is commonly known as Coandă effect. At distances larger than ten times the jet width the self-similar characteristics of single and isothermal planar jets are recovered. The flow configuration presented includes several peculiar features, namely buoyancy effects at low Prandtl number, interaction between jets and the presence of multiple shear layers. This leads to an irregular behaviour of the turbulent diffusivity of momentum and heat as well as the misalignment between the temperature gradient and turbulent heat flux. Therefore the flow can be considered very complex and might constitute a demanding test bench for the development and validation of turbulence models.
2019
- Direct Numerical Simulation of turbulent mixed convection around a bundle of heated rods at low-Prandtl number
[Relazione in Atti di Convegno]
Angeli, Diego; Fregni, Andrea; Stalio, Enrico
abstract
The present work reports an overview of the results of Direct Numerical Simulations performed on the case of fully-developed, mixed convection flow of a liquid metal around a uniformly heated bundle of vertical rods. Finite-Volume computations are performed by an original discretization technique based on the representation of arbitrarily-shaped cylindrical boundaries on a non-uniform Cartesian grid. A domain consisting of four subchannels of a triangular lattice of rods with a pitch-to-diameter ratio P/D = 1.4 is considered as the reference geometry. A single friction Reynolds number value is simulated, namely Reτ = 550. Both forced and mixed convection regimes are investigated, buoyancy effects being introduced by imposing a Richardson number value of Ri = 0.25. A Prandtl number Pr = 0.031 is chosen to represent LBE as the working fluid. Instantaneous snapshots and relevant statistics of the velocity and thermal fields are reported here for the considered case, and integral results are compared against available literature data.
2019
- Direct numerical simulation of turbulent forced and mixed convection of LBE in a bundle of heated rods with P/D=1.4
[Articolo su rivista]
Angeli, D.; Fregni, A.; Stalio, E.
abstract
In this work, reference data obtained by means of Direct Numerical Simulations of fully-developed flow and heat transfer around a vertical rod bundle are presented. Finite-Volume computations are performed by an original discretization technique based on the representation of arbitrarily-shaped cylindrical boundaries on a non-uniform Cartesian grid. A periodic domain consisting of four subchannels of a triangular lattice of rods with a pitch-to-diameter ratio P/D=1.4 is considered as the reference geometry. A Prandtl number Pr = 0.031 is chosen to represent Liquid Lead-Bismuth Eutectic (LBE) as the working fluid. A single friction Reynolds number value is simulated, namely Reτ=550. Both forced and mixed convection regimes are investigated, buoyancy effects being introduced by imposing a Rayleigh number Ra = 5×105, corresponding to a Richardson number Ri = 0.22. Instantaneous snapshots and relevant statistics of the velocity and thermal fields are reported and discussed for the considered cases, focusing on the effect of aiding buoyancy on turbulent flow and heat transfer. Integral results are also compared against available literature data.
2019
- On negative turbulence production phenomena in the shear layer of separating and reattaching flows
[Articolo su rivista]
Cimarelli, A.; Leonforte, A.; De Angelis, E.; Crivellini, A.; Angeli, D.
abstract
The analysis of Direct Numerical Simulation data of the separating and reattaching flow over a blunt bluff body with sharp edges, reveals the presence of negative turbulence production mechanisms in the leading-edge shear layer. Contrary to what is commonly observed in fully developed turbulent flows, this phenomenon represents flow reversal of energy from the fluctuating field to the mean flow. The detailed study of the data reveals that at the origin of such mechanisms is a statistically positive relation between Reynolds shear stresses and vertical shear. We argue that such a positive relation is a result of large-scale interactions of the fluctuating field with the streamwise inhomogeneity. The analysis of time cospectra confirms this picture by highlighting the presence of a net separation of scales consisting in a range of small scales positively contributing to turbulence production in opposition to a range of large scales giving to a reversal of flow energy from the fluctuating to the mean field. By means of a reduced description of the interactions of the fluctuating field with the mean field given by a generalized mixing length hypothesis, we finally also provide conceptual arguments for the modelling of turbulence production in the transitional shear layer. A model for the mixing length is also proposed which is found to work nicely in shear flows. The simplicity of the formulation supports its use especially in experiments of wall-bounded turbulence.
2019
- Reference numerical database for turbulent flow and heat transfer in liquid metals
[Articolo su rivista]
Shams, A.; Roelofs, F.; Niceno, B.; Guo, W.; Angeli, D.; Stalio, E.; Fregni, A.; Duponcheel, M.; Bartosiewicz, Y.; Tiselj, I.; Oder, J.
abstract
Turbulent heat transfer is a complex phenomenon that has challenged turbulence modellers over various decades. In this regard, in the recent past, several attempts have been made for the assessment and further development/calibration of the available turbulent heat flux modelling approaches. One of the main hampering factors with respect to the further assessment of these modelling approaches is the lack of reference data. In the framework of the EU SESAME and MYRTE projects, an extensive effort has been put forward to generate a wide range of reference data, both experimental and numerical, to fill this gap. In that context, this article reports the numerical database that has been generated within these projects for various liquid metal flow configurations in different flow regimes. These high fidelity numerical data include seven different flow configurations: a wall-bounded mixed convection flow at low Prandtl number with varying Richardson number (Ri) values; a wall-bounded mixed and forced convection flow in a bare rod bundle configuration; a forced convection confined backward facing step (BFS) with conjugate heat transfer; a forced convection impinging jet for three different Prandtl fluids corresponding to two different Reynolds numbers of the fully developed planar turbulent jets; a mixed-convection cold-hot-cold triple jet configuration corresponding to Ri=0.25; an unconfined free shear layer for three different Prandtl fluids; and a forced convection infinite wire-wrapped fuel assembly. These high-fidelity numerical databases will serve the further development of turbulent heat transfer models by providing unique, new and detailed data for the thermal-hydraulic behaviour of liquid metals in various flow configurations.
2019
- Resolved dynamics and subgrid stresses in separating and reattaching flows
[Articolo su rivista]
Cimarelli, A.; Leonforte, A.; De Angelis, E.; Crivellini, A.; Angeli, D.
abstract
Direct numerical simulation data of the separating and reattaching flow around a blunt bluff body are used for the assessment of the combined role played by the numerical resolution and subgrid turbulence closure in large eddy simulation. The ability of the large-scale resolved field to capture the main flow features is first analyzed. The behavior of the intensity of the resolved fluctuations as a function of the filter lengths reveals a higher sensitivity of the resolved flow on a reduction of resolution in the streamwise direction rather than in the spanwise one. On the other hand, the analysis of the subgrid stresses shows the presence of two challenging phenomena, a reversal of flow of energy from the fluctuating to the mean field in the leading-edge shear layer and a backward energy transfer from small to large scale within the main recirculating bubble. These two phenomena challenge for subgrid closures that should be able to reproduce a flow of energy from the space of small unknown subgrid scales to drive the resolved mean and fluctuating motion. In particular, it is found that the formalism of subgrid viscosity models allows us to capture neither the negative turbulence production of the leading-edge shear layer nor the backward energy transfer within the main flow recirculation. On the other hand, the subgrid similarity models are able to capture both these two phenomena but, from a quantitative point of view, the intensity of the reproduced stresses is very weak. In conclusion, the need of subgrid closures based on a mixed modeling approach for the solution of the flow is envisaged.
2019
- Simulation and analysis of the jet flow patterns from supersonic nozzles of laser cutting using OpenFOAM
[Articolo su rivista]
Darwish, Mohamed; Orazi, Leonardo; Angeli, Diego
abstract
The operating pressure of gas-assisted laser cutting and the resulting exit jet pattern is one of the most important process parameters in high-pressure laser cutting. Many studies have been done to illustrate the effect of this parameter on both laser cutting quality and laser cutting capability. However, most of these studies have been done using conical nozzles. In this paper, the exit jet from supersonic nozzle has been studied, analyzed, and simulated under three different operating conditions, namely desired design, under-expansion, and over-expansion to illustrate the effect of these operating conditions on the dynamic characteristics of the exit jet. Quasi 1-D gas dynamics theory has been used to calculate the desired design operating condition, and then an axisymmetric 2-D model has been created using the OpenFOAM Ⓡ Computational Fluid Dynamics (CFD) toolbox to simulate the gas-assisted laser cutting flow through the modeled supersonic nozzle. Finally, the proposed simulations have been validated by comparing the results with experimental observations reported in previous literature. The effect of the turbulent viscosity has been considered through the proposed model to better simulate real conditions. Moreover, the model has been optimized to be effectively used for engineering purposes. The simulation results are qualitatively consistent with the reported experimental measurements and they demonstrate that in the case of supersonic nozzles, the exit jet pattern is characterized by high uniformity, absence of Mach disks, and bounded shape for a long distance especially under the desired design operating conditions.
2019
- Thermo-mechanical behaviour of an injection nozzle for a cogeneration system based on the aluminum/water reaction
[Relazione in Atti di Convegno]
Angeli, Diego; Castagnetti, Davide; Cingi, Pietro; Leonforte, ADRIANO DAVIDE SERAFINO; Melchionda, Filippo; Milani, Massimo; Montorsi, Luca; Sorrentino, Andrea; Zanni, Davide
abstract
The thermal behaviour of an injection nozzle for a prototype combustion chamber of a cogeneration system based on the reaction of liquid aluminum and water steam, is analyzed. The heat released by the oxidation of aluminum with water is exploited for super-heating the vapor of a steam cycle and simultaneously producing hydrogen. The only by-product is alumina, which can be transformed again into aluminum. From a thermo-mechanical point of view, the most critical part of the system is the injection nozzle, located at the end of a graphite pneumatic needle valve. The head of the injector is made by titanium and includes a Titalox ceramic nozzle characterized by a 0.5 mm calibrated hole. After a warm up cycle, the injector reaches temperatures of the order of 1000°C. During the subsequent cool-down phase, the different strain rates of the two materials could lead to mechanical failure. In this work, the heating and cooling transients of the injection zone are simulated by a Finite Volume approach. Temperature distributions are then transferred to a Finite Element structural solver in order to verify the resulting stresses. Temperature measurements taken during preliminary experimental tests provide a qualitative assessment of the reliability of the numerical predictions.
2018
- Direct numerical simulation of the flow around a rectangular cylinder at a moderately high Reynolds number
[Articolo su rivista]
Cimarelli, Andrea; Leonforte, Adriano; Angeli, Diego
abstract
We report a Direct Numerical Simulation (DNS) of the flow around a rectangular cylinder with a chord-to-thickness ratio B/D=5 and Reynolds number Re=3000. Global and single-point statistics are analysed with particular attention to those relevant for industrial applications such as the behaviour of the mean pressure coefficient and of its variance. The mean and turbulent flow is also assessed. Three main recirculating regions are found and their dimensions and turbulence levels are characterized. The analysis extends also to the asymptotic recovery of the equilibrium conditions for self-similarity in the fully developed wake. Finally, by means of two-point statistics, the main unsteadinesses and the strong anisotropy of the flow are highlighted. The overall aim is to shed light on the main physical mechanisms driving the complex behaviour of separating and reattaching flows. Furthermore, we provide well-converged statistics not affected by turbulence modelling and mesh resolution issues. Hence, the present results can also be used to quantify the influence of numerical and modelling inaccuracies on relevant statistics for the applications.
2018
- Direct numerical simulations for liquid metal applications
[Capitolo/Saggio]
Iztok, Tiselj; Stalio, Enrico; Angeli, Diego; Jure, Oder
abstract
Thermal Hydraulics Aspects of Liquid Metal cooled Nuclear Reactors is a comprehensive collection of liquid metal thermal hydraulics research and development for nuclear liquid metal reactor applications. A deliverable of the SESAME H2020 project, this book is written by top European experts who discuss topics of note that are supplemented by an international contribution from U.S. partners within the framework of the NEAMS program under the U.S. DOE. This book is a convenient source for students, professionals and academics interested in liquid metal thermal hydraulics in nuclear applications. In addition, it will also help newcomers become familiar with current techniques and knowledge.
2018
- Direct simulation of transition in a differentially heated vertical channel
[Relazione in Atti di Convegno]
Cingi, Pietro; Cimarelli, Andrea; Angeli, Diego
abstract
Transition to turbulence of natural convection flows ensuing in a fluid layer between two differentially heated vertical plates is a topic of substantial interest for many applications. Among these, notable examples are the air gaps in double-glazing panes or in ventilated façades, and passive heat exchangers. The correct prediction and control of flow regimes, air flow rates and heat transfer coefficients has a significant impact in the correct design of such elements and, in turn, on their efficiency.
In recent studies the early stages of transition have been explored by means of Direct Numerical Simulation (DNS) with high-accuracy pseudospectral codes. While all these studies correctly capture the first bifurcation from the so-called laminar conduction regime to steady convection, the detection of the subsequent transition to turbulence appears to be accompanied by a great sensitivity to some fundamental numerical choices, such as domain size, spectral resolution and amplitude of the imposed perturbations. In turn, these aspects become of crucial importance for the prediction of the heat transfer performance of the system.
In this work, the problem is tackled by means of a second-order, Finite-Volume based Direct Numerical Simulation technique, specifically devised for convection problems, and which already proved successful in the simulation of transitional scenarios. Results reveal the occurrence of a bifurcation branch which leads the system to chaos via a second bifurcation to a steady-state, a Hopf bifurcation and, seemingly, a period-doubling cascade. Such a scenario compares well with previous findings, except for minor discrepancies. All in all, though, some doubts persist upon the possible pitfalls in the use of DNS for the study of transition in this kind of systems.
2018
- Exploratory modeling and experimental investigation of a vibrating-stripe wind energy converter
[Articolo su rivista]
Allesina, Giulio; Cingi, Pietro; Gessani, Gabriele; Angeli, Diego
abstract
The future of portable devices is ultimately a challenge on energy conversion and storage, fought on two fronts: battery life and distributed availability of power sources. Nano-scale energy conversion devices are highly attractive for the market, but the non-PV systems still lack in efficiency and portability. Few years ago the Windbelt® system was presented with a new ingenious rotor-free approach for nanoscale wind energy harvesting through aeroelastic fluttering. Aim of this work is to provide a mathematical model with experimental validation to the scientific framework related to the Windbelt technology. An experimental device with variable length was built and tested under different belt tension conditions. The basis of the windbelt technology consists in the movement of a magnet linked to the belt that oscillates between two coils. In the tested device the movement of the magnet was recorded with a high speed camera and the images processed in order to evaluate instantaneously its rotation and vertical deviation from the resting position. Results of the experimental campaign were compared with the output of an aero-elastic model of the belt. The model is based on the assumption of a belt composed of two interconnected sections: the free belt section, whose aeroelastic behaviour is represented by a nonlinear lift coefficient, and the magnet section, where the motion is governed by the inertial and elastic forces. The results of the testing campaign and the model outputs showed a remarkable agreement for what concerns the frequency response of the system.
2018
- Numerical and Experimental Study of an Air Extraction System
[Relazione in Atti di Convegno]
Cingi, Pietro; Leonforte, ADRIANO DAVIDE SERAFINO; Angeli, Diego; Bellei, Umberto
abstract
A numerical and experimental analysis is performed on the case of an air extraction system for a relatively large kitchen, equipped with a reintroduction system blowing untreated air directly under the hood.
Such an approach, while leading to a notable energetic saving, concedes a decrease of the suction efficiency, since the extraction system is partially utilized to convey the clean air coming from the outside. At the same time, the presence of a jet, below the hood ceiling, directed towards the extraction filters, may or may not have a beneficial effect on the operators comfort.
A 2D numerical model of a central section of the hood is built, considering isothermal conditions. Numerical results are validated against the outcomes of an in vivo measurement campaign, carried out on an existing installation. The obtained model is then used to perform a preliminary parametric study, in order to determine the influence of the ratio between inlet and outlet flow rates on the local flow field, and, ultimately, on the system effectiveness.
2018
- On the structure of the self-sustaining cycle in separating and reattaching flows
[Articolo su rivista]
Cimarelli, A.; Leonforte, A.; Angeli, D.
abstract
The separating and reattaching flows and the wake of a finite rectangular plate are studied by means of direct numerical simulation data. The large amount of information provided by the numerical approach is exploited here to address the multi-scale features of the flow and to assess the self-sustaining mechanisms that form the basis of the main unsteadinesses of the flows. We first analyse the statistically dominant flow structures by means of three-dimensional spatial correlation functions. The developed flow is found to be statistically dominated by quasi-streamwise vortices and streamwise velocity streaks as a result of flow motions induced by hairpin-like structures. On the other hand, the reverse flow within the separated region is found to be characterized by spanwise vortices. We then study the spectral properties of the flow. Given the strongly inhomogeneous nature of the flow, the spectral analysis has been conducted along two selected streamtraces of the mean velocity field. This approach allows us to study the spectral evolution of the flow along its paths. Two well-separated characteristic scales are identified in the near-wall reverse flow and in the leading-edge shear layer. The first is recognized to represent trains of small-scale structures triggering the leading-edge shear layer, whereas the second is found to be related to a very large-scale phenomenon that embraces the entire flow field. A picture of the self-sustaining mechanisms of the flow is then derived. It is shown that very-large-scale fluctuations of the pressure field alternate between promoting and suppressing the reverse flow within the separation region. Driven by these large-scale dynamics, packages of small-scale motions trigger the leading-edge shear layers, which in turn created them, alternating in the top and bottom sides of the rectangular plate with a relatively long period of inversion, thus closing the self-sustaining cycle.
2018
- Pattern recognition by Recurrence Analysis in the flow around a bluff body
[Relazione in Atti di Convegno]
Angeli, Diego; Cimarelli, Andrea; Leonforte, ADRIANO DAVIDE SERAFINO; Pagano, Arturo
abstract
The identification of scales, patterns and structures in a turbulent flow, starting from a high-fidelity experimental or numerical database, is an aspect of primary importance for the understanding of the dynamics of transition to turbulence and the energy cascade. In this frame, the present study represents a preliminary effort to evaluate a time series analysis tool, the so-called Recurrence Analysis (RA), for the identification of the dominant features of a relatively complex flow. The test case is represented by the turbulent flow around a rectangular cylinder with a chord-to-thickness ratio $C/D = 5$, for a Reynolds number value $Re=3000$. The problem at hand has already been tackled by means of a well-resolved Direct Numerical Simulation, whose results highlighted the presence of a multiplicity of scales and structures. Due to this interesting combination of features, the case appears as a promising benchmark for the development of a novel tool for pattern recognition. To this aim, the system dynamics are condensed to pointwise observations at various abscissas along the flow. The analysis is aimed at verifying whether or not it is possible to isolate regular structures that: i) represent characteristic features of the flow and ii) can be used to distinguish the various phase of the shear-wake development along the flow. Results, cast in the form of Recurrence Plots (RP), reveal that the main scales of the flow, reflected in the sampled time series, are associated with well-defined recurrent patterns. This encouraging outcome leaves room for further utilization of the technique for the description of transitional and turbulent flows in thermo-fluid dynamics.
2018
- Solution-Processing of Copper Nanowires for Transparent Heaters and Thermo-Acoustic Loudspeakers
[Articolo su rivista]
Bobinger, Marco; La Torraca, Paolo; Mock, Josef; Becherer, Markus; Cattani, Luca; Angeli, Diego; Larcher, Luca; Lugli, Paolo
abstract
In this study, we present a copper nanowires (CuNWs) based spray deposition process for the fabrication of transparent heaters and thermo-acoustic loudspeakers. We developed a scalable and solution-based synthesis process for CuNWs, which allows to fabricate spray deposited transparent electrodes that show performances comparable to indium tin oxide based TEs, at much lower material and deposition costs. Without any post-processing, the CuNWs films exhibit a sheet resistance as low as 12.6 Ω/□ at a high transparency of 77%. CuNW-based transparent heaters and thermo-acoustic loudspeakers are accurately characterized and modeled in both the thermal and the acoustic domain, showing performances aligned with the state-of-the art.
2017
- A priori and a posteriori analysis of the flow around a rectangular cylinder
[Capitolo/Saggio]
Cimarelli, A.; Leonforte, A.; Franciolini, M.; Angelis, E. De; Angeli, D.; Crivellini, A.
abstract
The definition of a correct mesh resolution and modelling approach for the Large Eddy Simulation (LES) of the flow around a rectangular cylinder is recognized to be a rather elusive problem as shown by the large scatter of LES results present in the literature. In the present work, we aim at assessing this issue by performing an a priori analysis of Direct Numerical Simulation (DNS) data of the flow. This approach allows us to measure the ability of the LES field on reproducing the main flow features as a function of the resolution employed. Based on these results, we define a mesh resolution which maximize the opposite needs of reducing the computational costs and of adequately resolving the flow dynamics. The effectiveness of the resolution method proposed is then verified by means of an a posteriori analysis of actual LES data obtained by means of the implicit LES approach given by the numerical properties of the Discontinuous Galerkin spatial discretization technique. The present work represents a first step towards a best practice for LES of separating and reattaching flows.
2017
- Aerodynamic Study of Advanced Airship Shapes
[Articolo su rivista]
Cimarelli, Andrea; Madonia, Mauro; Angeli, Diego; Dumas, Antonio
abstract
The present paper reports a numerical study of the aerodynamic properties for a novel disc-shaped airship. Different configurations are considered, some of which present a circular opening connecting the bottom and top surface of the airship. The aim of the study is to understand the flow dynamics, in order to define the aerodynamic efficiency and the stability properties of the flying vehicle. Such information is crucial for the design of the propulsion system and of the mission profile of these innovative airships. Results show that, in general, disc-shaped airships are characterized by large values of drag and small levels of lift. Interestingly, it appears that lift keeps increasing up to very high angles of attack. This feature is found to be related with strong tip effects, which induce a significant flow of air from the high-pressure region at the bottom surface to the low-pressure region at the top surface. This air flow energizes the upper boundary layer, thus contrasting the flow separation on the top surface. This phenomenon is found to be useful for the stability properties of the airship: in fact, it shifts the center of pressure closer to the geometrical center of the airship, hence implying a reduction of the aerodynamic moment. The role of openings is also addressed and found to positively contribute to the stability properties of the airship, by further reducing the levels of aerodynamic moment.
2017
- Application of Recurrence Analysis to the period doubling cascade of a confined buoyant flow
[Relazione in Atti di Convegno]
Angeli, Diego; Corticelli, Mauro Alessandro; Fichera, A.; Pagano, A.
abstract
Recurrence Analysis (RA) is a promising and flexible tool to identify the behaviour of nonlinear dynamical systems. The potentialities of such a technique are explored in the present work, for the study of transitions to chaos of buoyant flow in enclosures. The case of a hot cylindrical source centred in a square enclosure, is considered here, for which an extensive database of results has been collected in recent years. For a specific value of the system aspect ratio, a sequence of period doublings has been identified, leading to the onset of chaos. RA is applied here to analyse the different flow regimes along the route to chaos. The qualitative visual identification of patterns and the statistics given by the quantitative analysis suggest that this kind of tool is well suited to the study of transitional flows in thermo-fluid dynamics.
2017
- Characterization and modelling of transparent heaters based on solution-processed copper nanowires
[Relazione in Atti di Convegno]
Bobinger, Marco; Mock, Josef; Becherer, Markus; Torraca, Paolo La; Angeli, Diego; Larcher, Luca; Lugli, Paolo
abstract
In this study, we present an environmentally friendly and solution-based synthesis for copper nanowires (CuNWs) at a moderate process temperature. Transparent electrodes (TEs) are fabricated by spray-deposition and evaluated in terms of their electro-optical performance. Using ImageJ, the CuNW diameters are determined in an automated and reproducible way. Without any post-processing, the films show a sheet resistance as low as 12.6 Ohm/sq at a high transparency of 77 %. Further, CuNW-based transparent heaters are characterized and accurately modelled using the Crank-Nicolson finite method that accounts for the heat losses and the resistance-temperature dependence of the films.
2017
- Features of a blue-sky transition in an autonomous convective flow
[Articolo su rivista]
Angeli, D.; Corticelli, M. A.; Fichera, A.; Pagano, A.
abstract
The nonlinear dynamics of a buoyant air flow in the gap formed by a horizontal cylindrical heat source and a concentric isothermally cooled square enclosure have been recently studied, in the limits of a two-dimensional approximation, for different values of the system aspect ratio, defined as the ratio between the cavity side length and the minimum gap width. Accurate analyses have revealed that, for low values of the aspect ratio, the dominant pattern is the swaying motion of a buoyant plume, which undergoes a period-doubling cascade for increasing values of the Rayleigh number. On the other hand, a substantially different behaviour has been preliminarily observed for higher values of the aspect ratio, since, in a thinner gap, the flow exhibits Rayleigh-Bénard-like cellular patterns above the cylinder. The aim of this communication is to describe the peculiar route to chaos which has been detected for this second type of geometry. The scenario consists in the progressive winding of a periodic limit cycle on an attractor whose topology resembles that of a French horn, which is a typical signature of the codimension-2 Shilnikov bifurcation from periodic dynamics to chaos. A similar feature, known as the blue-sky catastrophe, has never been reported so far for confined convection of pure fluids.
2017
- Infrared, transient thermal, and electrical properties of silver nanowire thin films for transparent heaters and energy-efficient coatings
[Articolo su rivista]
Bobinger, Marco; Angeli, Diego; Colasanti, Simone; La Torraca, Paolo; Larcher, Luca; Lugli, Paolo
abstract
n this study, we investigate the infrared and electrical propertiesas well as the thermal response of transparent silvernanowire (AgNW) based thin-film heaters, when subjected toJoule heating. Controlling the number of layers and hence thedeposition time, our spray-coating technique allows to modulatethe thermal and electrical properties of the thin films in a precisemanner. In addition, this technique enables the fabrication ofhomogeneous and large-area heaters, which, in terms of theirelectro-optical properties, nicely compare to the performances ofstate-of-the-art AgNW transparent electrodes. The thermal response and the electrical properties are accurately reproducedby a purposely developed physical model, which shows that thetemperature dependence of the AgNW film resistance is loweredby a factor of 2 compared to bulk silver, independently of thenumber of deposited layers. Compared to uncoated glass,the emissivity decreases by 58% at a coverage rate of 58%. At thesame time, the AgNW film can sustain a transparency as high as81.3%. Therefore, AgNW-based thin films can be used as a low-emissivity coating, for e.g., energy-efficient window glazingapplications. Finally, we accurately determine the fragmentationtemperature of AgNWs, which sets the ultimate limitation of usefor heating applications.
2017
- Naturally ventilated double-skin façade in modeling and experiments
[Articolo su rivista]
Dama, Alessandro; Angeli, Diego; Larsen, Olena Kalyanova
abstract
The modeling activity presented in this work aims at the assessment of a simplified model, named BS model, which was specifically developed for integration of DSF in Building Simulation. The BS model is based on a pressure loop and on an integral approach to the heat transfer along the vertical channel. It considers buoyancy as a function of the average temperature in the channel. The wind action is taken into account by means of wind pressure coefficients (Cp) on the façade openings. The focus of this study is the experimental validation of the modeling “core”: the natural ventilation through the DSF. The validation is based on the dataset of the experimental campaign conducted on a DSF test facility, the “Cube”, in Denmark, under IEA ECBCS ANNEX 43/SHC Task 34. Hourly simulations were performed with the BS model for the 15 days of the experimental campaign. A CFD modeling activity was also carried out on a selection of four cases, extracted from the experimental benchmark and representative of different temperature and pressure boundary conditions. The results show that the BS model presents a good level of agreement with the experimental data in predicting the mass flow rate and the heat removed by ventilation. Although the two experimental methods used to determine the airflow rate in the DSF cavity produce in many cases divergent results, it was possible to distinguish valid experimental results for comparison with the BS model. This was possible thanks to a thorough analysis of the experimental procedure together with the insight provided by the model into the determination of the driving wind and thermal differential pressures. In particular, by selecting only the measurements associated to sufficiently low wind fluctuations in the hourly averaged data, a good degree of correlation was found between the predicted total driving pressure and the flow measurements. Concerning the four cases investigated also by means of CFD, the agreement between the BS and CFD models is remarkable in terms of outlet temperatures and in the prediction of flow reversal.
2017
- Notes and experiments on the statics of capillary columns
[Relazione in Atti di Convegno]
Barozzi, Giovanni Sebastiano; Corticelli, Mauro Alessandro; Angeli, Diego; Salerno, Elisabetta
abstract
Experimental results are summarized, concerned with the statics of liquid columns in capillary tubes under non-standard conditions. Three configurations are considered: 1. inclined capillary tubes; 2. capillary effect in the horizontal branch of an L-shaped tube; 3. capillary columns in a vertical tube. The effect of inclination on capillary rise (1.) had already been explored in [1] using water in glass-tubes, and it was found that the vertical rise progressively reduces for increasing the tube inclination. This behavior is now confirmed for n-Hexadecane (C16H34). For L-shaped capillaries (2.), the length of the horizontal branch of the tube occupied by the liquid is detected, as a function of the elevation of the branch itself over the feeding pool. The statics of suspended liquid columns (3.) is investigated for two configurations, namely: i. freely suspended columns, and, ii. edge-ending columns. In the latter case, the evaporation transient is also tracked. Even if the experimental basis is limited, the results are sufficient to highlight some of the peculiar features of the statics of capillary columns under the above conditions. In particular, it is shown that the contact angle hysteresis plays a fundamental role in all the cases considered.
2017
- Numerical simulation of mixing buoyant jets: Preliminary studies
[Relazione in Atti di Convegno]
Angeli, Diego; Cimarelli, Andrea; Fregni, Andrea; Stalio, Enrico; Shams, Afaque; Roelofs, Ferry
abstract
Preliminary numerical analyses are reported for the case of three vertical planar mixing jets at different temperatures, in view of a forthcoming reference DNS to be performed in the frame of the SESAME European project. The reference case stems from the well-known PLAJEST triple jet experiment, but with a relative increase of the buoyancy effect, achieved by reducing the Reynolds number by a factor 5. Pre-production DNS runs for the reference case and a complementary RANS parametric analysis with varying Prandtl and Richardson numbers are carried out, highlighting that a suitable configuration for a benchmark can be obtained with a hotter central jet and colder lateral jets.
2017
- Parametric Analysis of a Large-Scale Cycloidal Rotor in Hovering Conditions
[Articolo su rivista]
Xisto, Carlos M.; Leger, J. A.; Páscoa, J. C.; Gagnon, L.; Masarati, P.; Angeli, Diego; Dumas, Antonio
abstract
In this work, four key design parameters of cycloidal rotors, namely the airfoil section, number of blades, chord-to-radius ratio,
and pitching axis location, are addressed. The four parameters, which have a strong effect on rotor aerodynamic efficiency, are analyzed with
an analytical model and a numerical approach. The numerical method, which is based on a finite-volume discretization of two-dimensional
unsteady Reynolds averaged Navier-Stokes equations on a multiple sliding mesh, is proposed and validated against experimental data.
A parametric analysis is then carried out considering a large-scale cyclogyro, suitable for payloads above 100 kg, in hovering conditions.
Results demonstrate that the airfoil thickness significantly affects the rotor performance; such a result is partly in contrast with previous
findings for small-scale and microscale configurations. Moreover, it will be shown that increasing the number of blades could result in a
decrease of the rotor efficiency. The effect of chord-to-radius will demonstrate that values of around 0.5 result in higher efficiency. Finally it is
found out that for these large systems, in contrast with microscale cyclogyros, the generated thrust increases as the pitching axis is located
away from the leading edge, up to 35% of chord length. Furthermore, the shortcomings of using simplified analytical tools in the prediction of
thrust and power in nonideal flow conditions are discussed.
2017
- QUADRIGEN: uso eco-sostenibile dell’alluminio come vettore energetico per la produzione combinata di energia elettrica, termica e idrogeno
[Altro]
Montorsi, Luca; Castagnetti, Davide; Angeli, Diego
abstract
“QUADRIGEN: uso eco-sostenibile dell’alluminio come vettore energetico per la produzione combinata di energia elettrica, termica e idrogeno”
2017
- Routes to chaos of natural convection flows in vertical channels
[Articolo su rivista]
Cimarelli, Andrea; Angeli, Diego
abstract
The aim of the present study is the analysis of the transition to turbulence of natural convection flows between two infinite vertical plates. For the study of the problem, a number of Direct Numerical Simulations (DNSs) have been performed. The continuity, momentum and energy equations, cast under the Boussinesq assumption, are tackled numerically by means of a pseudospectral method, through which the three-dimensional domain is decomposed with Chebychev polynomials in the wall-normal direction and with Fourier modes in the wall-parallel directions. For low Rayleigh number values, the predictions of the flow regimes are consistent with the classical analytical results and linear stability analyses. In particular, the first bifurcation (Ra ≈ 5800) from the so-called laminar conduction regime to steady convection is correctly captured. By increasing the Rayleigh number beyond a second critical value (Ra ≈ 10200), the flow regime becomes chaotic. This transition to chaos is found to be related with the amplification of spanwise instabilities occurring at scales larger than the channel gap, H. The study of the return of the system from the chaotic regime to the laminar base flow reveals a phenomenon of hysteresis, i.e. the chaotic regime persists even at Ra-values lower than the second critical value. From a numerical point of view, the predicted flow regimes appear to be extremely sensitive to the domain size, grid resolution and perturbation amplitude. These aspects are shown to be of crucial importance for the prediction of the heat transfer performance, and, hence, should be taken into consideration when numerical methods are used for the simulation of real-world problems.
2017
- Structure of turbulence in a flow around a rectangular cylinder
[Capitolo/Saggio]
Leonforte, Adriano; Cimarelli, Andrea; Angeli, Diego
abstract
The behaviour of the flow over a finite blunt plate with square leading and trailing edge corners at moderate Reynolds number is studied by means of a Direct Numerical Simulation. The chord-to-thickness ratio of the plate is 5 and the Reynolds number is Re = Uâ · D/ν = 3 à 103 where Uâ and D are the free-stream velocity and the thickness, respectively. The flow separates at the leading edge corner developing in a strong free-shear. The flow reattaches on the solid surface upstream the trailing edge and evolves in typical large-scale shedding beyond it. To the authors knowledge, this is the first time that high-fidelity three-dimensional data are produced to analyze in detail the flow features of such a system. Preliminary results on the flow topology will be presented in this work. In particular, the streamlines of the mean flow and the instantaneous three-dimensional turbulent structures via λ2 vortex criterion will be examinated.
2016
- A new aircraft architecture based on the ACHEON Coanda effect nozzle: flight model and energy evaluation
[Articolo su rivista]
Trancossi, Michele; Madonia, Mauro; Dumas, Antonio; Angeli, Diego; Bingham, Chris; Das, Shyam Sumanta; Grimaccia, Francesco; Marques, Jose Pascoa; Porreca, Eliana; Smith, Tim; Stewart, Paul; Subhash, Maharshi; Sunol, Anna; Vucinic, Dean
abstract
Purpose: Aeronautic transport has an effective necessity of reducing fuel consumption and emissions to deliver efficiency and competitiveness driven by today commercial and legislative requirements. Actual aircraft configurations scenario allows envisaging the signs of a diffused technological maturity and they seem very near their limits. This scenario clearly shows the necessity of radical innovations with particular reference to propulsion systems and to aircraftarchitecture consequently. Methods: This paper presents analyses and discusses a promising propulsive architecture based on an innovative nozzle, which allows realizing the selective adhesion of two impinging streams to two facing jets to two facing Coanda surfaces. This propulsion system is known with the acronym ACHEON (Aerial Coanda High Efficiency Orienting Nozzle). This paper investigates how the application of an all-electric ACHEONs propulsion system to a very traditional commuter aircraft can improve its relevant performances. This paper considers the constraints imposed by current state-of-the-art electric motors, drives, storage and conversion systems in terms of both power/energy density and performance and considers two different aircraft configurations: one using battery only and one adopting a more sophisticated hybrid cogeneration. The necessity of producing a very solid analysis has forced to limit the deflection of the jet in a very conservative range (±15°) with respect to the horizontal. This range can be surely produced also by not optimal configurations and allow minimizing the use of DBD. From the study of general flight dynamics equations of the aircraft in two-dimensional form it has been possible to determine with a high level of accuracy the advantages that ACHEON brings in terms of reduced stall speed and of reduced take-off and landing distances. Additionally, it includes an effective energy analysis focusing on the efficiency and environmental advantages of the electric ACHEON based propulsion by assuming the today industrial grade high capacity batteries with a power density of 207 Wh/kg. Results: It has been clearly demonstrated that a short flight could be possible adopting battery energy storage, and longer duration could be possible by adopting a more sophisticated cogeneration system, which is based on cogeneration from a well-known turboprop, which is mostly used in helicopter propulsion. This electric generation system can be empowered by recovering the heat and using it to increase the temperature of the jet. It is possible to transfer this considerable amount of heat to the jet by convection and direct fluid mixing. In this way, it is possible to increase the energy of the jets of an amount that allows more than recover the pressure losses in the straitening section. In this case, it is then possible to demonstrate an adequate autonomy of flight and operative range of the aircraft. The proposed architecture, which is within the limits of the most conservative results obtained, demonstrates significant additional benefits for aircraft manoeuvrability. In conclusion, this paper has presented the implantation of ACHEON on well-known traditional aircraft, verifying the suitability and effectiveness of the proposed system both in terms of endurance with a cogeneration architecture and in terms of manoeuvrability. It has demonstrated the potential of the system in terms of both takeoff and landing space requirements. Conclusions: This innovation opens interesting perspectives for the future implementation of this new vector and thrust propulsion system, especially in the area of greening the aeronautic sector. It has also demonstrated that ACHEON has the potential of renovating completely a classic old aircraft configuration such as the one of Cessna 402.
2016
- Characterization and modeling of the thermal and electrical properties of transparent silver nanowire thin-film heaters
[Relazione in Atti di Convegno]
Bobinger, Marco; Colasanti, Simone; Lugli, Paolo; Angeli, Diego; La Torraca, Paolo; Larcher, Luca
abstract
In this study, we present the characterization and the modeling of transparent silver nanowire thin-film heaters in terms of their transient thermal response when subjected to Joule heating and their electrical properties. The electrical properties, which showed a conductance-temperature dependence that is reduced down to a factor of 2 compared to the value for bulk silver, could be modeled accurately by simulation results. In addition, our transparent electrode deposition technique, i.e. spray-coating, allowed for an excellent reproducibility and provided homogeneous and large films that compare to state-of-the-art silver nanowire transparent electrode performance.
2016
- Mathematical Model of a Constructal Coanda Effect Nozzle
[Articolo su rivista]
Trancossi, Michele; Stewart, Jill; Subhash, Maharshi; Angeli, Diego
abstract
This paper analyses the ACHEON Coanda effect nozzle for aircraft propulsion, based on the dynamic equilibrium of two jet streams. The ACHEON concept, and, in particular, the HOMER nozzle, which is its main component, are presented, together with the literature milestones from which the idea originally stems. A subsystem analysis inspired by the principles of Constructal Theory is presented for the current
architecture. A mathematical model of a 2D case of the system is developed, focusing on the combined effect of the mixing of the two streams and the Coanda adhesion over a convex surface. A validation of
the model is also reported, based on 2D CFD analyses, under the hypothesis of incompressible flow. Results highlight that, in spite of its relative simplicity, the model produces accurate results.
2016
- On the bimodal nature of a confined buoyant plume. Part I: A topological and analytical insight
[Articolo su rivista]
Pagano, A; Angeli, Diego
abstract
The series of bifurcations leading a confined thermal plume from stationary behaviour to deterministic chaos has been recently reported for the case of a horizontal cylindrical heat source within an air-filled, isothermally cooled square cavity. A refined sampling of the bifurcation parameter range revealed that, in the limit of the 2D approximation, transition unfolds through a period doubling cascade; however, a peculiarity of the observed scenario resides in the occurrence of a window of quasiperiodic behaviour at the beginning of the period doubling cascade. In particular, the quasiperiodic range splits the regions of stability of the period-1 and period-2 limit cycles, hinting at the bimodal nature of the subsequent subharmonic cascade. Within this frame, new analyses are presented here, to achieve of a deeper insight on the birth, evolution and disappearance of quasiperiodic flow. Particular attention has been given to the study of the transient dynamics leading to the asymptotic flows, especially in the vicinity of the bifurcation points.
2016
- On the bimodal nature of a confined buoyant plume. Part II: Flow structure echoes in state space
[Articolo su rivista]
Angeli, Diego; Pagano, A.
abstract
This work represents the second part of a twofold study aiming at a detailed characterisation of the low order dynamics of a buoyant plume arising from an enclosed horizontal cylindrical heat source, for which chaos unfolds through a period doubling cascade preluded by a window of quasiperiodicity. In part I, this behaviour has been found to be related to the inherent bimodal nature of the flow, and an analytical characterisation of the asymptotic flows in the vicinity of the bifurcation points has been carried out.In this second part, the thermo-fluid dynamic origin of this bimodal flow is investigated. A novel approach is proposed based on the observation of the existing correspondences between the recurrent formation of cellular flow structures, and the global structure of the system dynamics, as described by the attractor in an appropriate state space. In particular, the two modes are shown to correspond to the existence of two alternate and distinct sequences of flow patterns, and the locking between them corresponds to the synchronisation of analogous events in the evolution of the flow structures. Moreover, reported results show that, within the same framework, the period doubling cascade generated by the extinction of the quasiperiodic flow can be seen as a consequence of the modulation of the cellular flow structures.In light of the remarkable similarities with experimental evidences of other buoyancy-driven flows, the proposed approach may represent a useful paradigm for the analysis of transitional, confined convection problems. Phenomenological analysis of the bimodal behaviour of a buoyant plume.Identification of the birth and disappearance of different flow structures in the two modes.State-space representation of the two modes and of the time instances relative to different phases in the system evolution.Persistence of the bimodal phenomenology throughout the whole transition to chaos.
2016
- Optimal pitching schedules for a cycloidal rotor in hovering
[Articolo su rivista]
Andrisani, Andrea; Angeli, Diego; Dumas, Antonio
abstract
Purpose: The purpose of this paper is to define an optimal pitching profile for the blades of a cycloidal rotor which minimizes the mean power consumption for a given mean thrust of the rotor. Design/methodology/approach: A simple analytical model of the kinematics and aerodynamics of a cycloidal rotor is defined first to obtain expressions for thrust and power depending on the pitching profile and geometrical parameters of the rotor. Then, Lagrange optimization is applied to obtain the optimal pitching schedule under hovering conditions. Finally, results of the theoretical analysis are compared with those of a two-dimensional computational fluid dynamics (CFD) model. Findings: Results of the optimization suggest that the optimal profile is a combination of sinusoidal functions. It is shown that the adoption of the optimal pitching schedule could improve the power efficiency of the rotor by approximately 25 per cent. Practical implications: The possibility to increase the efficiency of a cycloidal rotor by acting on its pitching schedule could be a significant factor of success for this alternative propulsion concept. Originality/value: The present work represents the first attempt at a definition of an optimal pitching profile for a cycloidal rotor. Moreover, although being carried out on the basis of simplified analytical considerations, the present investigation sets a methodological framework which could be successfully applied to the design of similar kinds of systems.
2016
- Wind and buoyancy driven natural ventilation in double skin façades
[Articolo su rivista]
Dama, Alessandro; Angeli, Diego
abstract
The computational fluid dynamics (CFD) modelling activity presented in this work aims at investigating the reliability of the assumptions employed in a double skin façades (DSFs) simplified model, developed for the integration of naturally ventilated DSFs in building simulation (BS) tools. The simplified fluid-dynamic model considers both the wind action, by means of pressure coefficients (Cp) on the openings, and the buoyancy inside the ventilated channel. Both the BS and the CFD models have been assessed using the database of an experimental campaign carried out in a full scale test facility, named ‘The Cube’, and made available by the Department of Civil Engineering of Aalborg University. The results show a good agreement between the CFD and BS models in terms of predicted temperature increase, with a maximum deviation of 15%. Both models exhibit a high sensitivity to the imposed differential wind pressure, which depends on the Cp source employed. The determination of proper Cp values for the specific case is, hence, a crucial aspect. Moreover, the CFD analysis offers a deeper insight on surface heat transfer and suggests the need to take into account the interplay between thermal and wind-driven convection, which ultimately gives rise to a mixed convection phenomenon.
2015
- A fast algorithm for Direct Numerical Simulation of natural convection flows in arbitrarily-shaped periodic domains
[Relazione in Atti di Convegno]
Angeli, Diego; Stalio, Enrico; Corticelli, Mauro Alessandro; Barozzi, Giovanni Sebastiano
abstract
A parallel algorithm is presented for the Direct Numerical Simulation of buoyancy-induced flows in open or partially confined periodic domains, containing immersed cylindrical bodies of arbitrary cross-section. The governing equations are discretized by means of the Finite Volume method on Cartesian grids. A semi-implicit scheme is employed for the diffusive terms, which are treated implicitly on the periodic plane and explicitly along the homogeneous direction, while all convective terms are explicit, via the second-order Adams-Bashfort scheme.
The contemporary solution of velocity and pressure fields is achieved by means of a projection method. The numerical resolution of the set of linear equations resulting from discretization is carried out by means of efficient and highly parallel direct solvers. Verification and validation of the numerical procedure is reported in the paper, for the case of flow around an array of heated cylindrical rods arranged in a square lattice. Grid independence is assessed in laminar flow conditions, and DNS results in turbulent conditions are presented for two different grids and compared to available literature data, thus confirming the favorable qualities of the method.
2015
- Buoyancy-driven turbulent convection in a bundle of vertical heated cylinders
[Abstract in Atti di Convegno]
Angeli, D.; Stalio, E.
abstract
Background Buoyant, turbulent convective heat transfer around cylindrical rods arranged in bundles is a technically relevant heat transfer configuration which finds application in steam generators, cooling of reactor core fuel assemblies and heat exchangers in general. Most of the research performed so far considered forced convection conditions on vertical rod bundles, corresponding for example to the configuration of a nuclear reactor primary loop. Fewer works have focused on the effect of buoyancy, with or without an external source of momentum. In their experimental investigation, Hallinan and Viskanta [4] employed a thermosyphon loop to determine the average heat transfer coefficients for water under natural circulation conditions in a tube bundle containing twenty-one tubes; their work is mainly focused on the favorable effect of grid spacers on heat transfer enhancement. El Genk et al. performed experiments of upflow- and downflow-forced turbulent and laminar convection, natural convection and buoyancy-assisted combined convection of water in a uniformly heated square lattice of seven [2] and nine [3] rod bundles with variable pitch-to-diameter ratio, Reynolds and Rayleigh number. They proposed heat transfer correlations and concluded that the rod arrangement only negligibly affects the overall Nusselt number in both forced and natural convection regimes. Concerning the numerical modeling of this class of flows, only very recent works resort to the Large Eddy Simulation [5], and even less frequently, to the Direct Numerical Simulation [7]. This is largely due to the geometric complexity of the flow domain and the difficulties related to the adoption of numerical techniques allowing for sufficiently accurate results.
2015
- Experimental and numerical investigation of liquid-metal free-surface flows in spallation targets
[Articolo su rivista]
Batta, A.; Class, A. G.; Litfin, K.; Wetzel, T. h.; Moreau, V.; Massidda, L.; Thomas, S.; Lakehal, D.; Angeli, Diego; Losi, G.; Mooney, K. G.; Van Tichelen, K.
abstract
Accelerator Driven Systems (ADS) are extensively investigated for the transmutation of high-level nuclear waste within many worldwide research programs. The first advanced design of an ADS system is currently developed in SCK•CEN, Mol, Belgium: the Multi-purpose hYbrid Research Reactor for High-tech Applications (MYRRHA). Many European research programs support the design of MYRRHA. In the framework of the Euratom project 'Thermal Hydraulics of Innovative nuclear Systems (THINS)' a liquid-metal free-surface experiment is performed at the Karlsruhe Liquid Metal Laboratory (KALLA) of Karlsruhe Institute of Technology (KIT). The experiment investigates a full-scale model of the concentric free-surface spallation target of MYRRHA using Lead Bismuth Eutectic (LBE) as coolant. In parallel, numerical free surface models are developed and tested which are reviewed in the article. A volume-of-fluid method, a moving mesh model, a free surface model combining the Level-Set method with Large-Eddy Simulation model and a smoothed-particle hydrodynamics approach are investigated. Verification of the tested models is based on the experimental results obtained within the THINS project and on previous water experiments performed at the University Catholic de Louvain (UCL) within the Euratom project 'EUROpean Research Programme for the TRANSmutation of High Level Nuclear Waste in Accelerator Driven System (EUROTRANS)'. The design of the target enables a high fluid velocity and a stable surface at the beam entry. The purpose of this paper is to present an overview of both experimental and numerical results obtained for free surface target characterization. Without entering in technical details, the status, the major achievements and lessons for the future with respect to model development are described as well as some applications, which were carried out within the work package 'multi-phase flow' of THINS.
2015
- Fluid-dynamic characterisation of the Mont Blanc tunnel by multi-point airflow measurements
[Articolo su rivista]
Levoni, Paolo; Angeli, Diego; Stalio, Enrico; Agnani, Elia; Barozzi, Giovanni Sebastiano; Cipollone, M.
abstract
Long road and railway tunnels necessitate of a fine-tuned control of the ventilation system to be activated, for both safety and air quality maintenance reasons. This, in turn, requires that the main fluid-dynamic parameters of the tunnel are known with sufficient accuracy, so that the system behaviour can be predicted with reasonable confidence under standard and emergency operative conditions. As a first step in the modelling of the complex system embodying the 11,611. m long Mont Blanc road tunnel, and its ventilation facilities, a movable 5-point survey rake was designed and built-up, for detecting the distribution of the axial velocity on the tunnel cross-section. Two extensive experimental campaigns were carried out, where the airflow-rates were either measured at different stations along the tunnel length (C1), or at a fixed location, under purely axial main-flow conditions and varying the number of the activated axial fan pairs (C2). A simplified dynamic model of the tunnel was developed, and the airflow data from the experiments were used for the model fine-tuning, additional CFD analyses having provided extra information on concentrated pressure losses connected with air inlet and discharge through the tunnel ends. The Mont Blanc tunnel was finally characterised in terms of friction factors and jet-fans installation efficiency.
2015
- Heat transfer along the route to chaos of a swaying thermal plume
[Relazione in Atti di Convegno]
Angeli, Diego; Corticelli, Mauro Alessandro; Fichera, A; Pagano, A.
abstract
Detailed analyses have been recently reported on the low order dynamics of a thermal plume arising from a horizontal cylindrical heat source concentric: to an air-filled isothermally cooled square enclosure, together with those of the related flow structures, in the limit of the 2D approximation. In particular. within the range of O < Ra < 3 RaL-T, With Ram corresponding to the loss of stability of the stationary buoyant plume, the entire evolution from a periodic limit cycle (P1) to the birth of chaos through a period»doubling cascade has been fullyexplored. With this respect, special attention has been given to the window of quasiperiodic dynamics onto a T;-torus that is observed to separate the monoperiodic dynamics from the biperiodic dynamics onto a P1 and a Pg-liniit cycle, respectively. The results of these analyses hint at the bimodal nature of the overall dynamics. in general, and of the subharmonic cascade, in particular, which are still under investigation. Although relevant on a dynamical perspective, a with a main reflection on the laminar-turbulent transition, the observed oscillations appear to be characterised by comparable amplitudes and to be determined by similar evolutions of the flow pattern evolutions, so that their role on the overall heat transfer rate is expected to be marginal. Vi/'ithin this frame, the present study aims at reporting the influence played by the observed dynamics of the thermal plume and of the [low structures on the global heat transferrate. In particular, the aim is the assessment of the correlation between the Rayleigh number and the average Nusselt number on the cylinder surface, as well as the effect on the latter of the observed series of bifurcations.
2015
- Modelling mechanically ventilated double skin facades with integrated shading device
[Relazione in Atti di Convegno]
Dama, Alessandro; Angeli, Diego
abstract
Double skin façades (DSF), are typically composed by two transparent envelope elements separated by a ventilated airspace. Such a technology can be applied both in new and existing buildings and may combine architectural value with energy efficiency. In the most common mechanically ventilated configurations the DSF air inlet came from the indoor environment and the outlet air returns to the HVAC system. Integrated shading devices are positioned between the skins. The assessment of the energy performance of buildings with DSFs requires proper dynamic simulation tools, based on models capable of predicting DSF heat transfer under variable boundary conditions, at the price of a reasonable computational effort. Many DSF simplified models have been developed and implemented in building simulation tools, but the validation of these tools is still an open issue, especially in presence of shading devices. The CFD modelling activity presented in this work aims at supporting the assessment of a DSF simplified model, specifically developed for the dynamic simulation of heat transfer in buildings. Such a model is based on an integral approach to the vertical channel, which is assumed to be separated into two channels when the shading device is used. Averaged surface heat transfer coefficients, depending on the geometry and flow regime, are adopted in order to represent convection inside the channels, according to the available correlations. The dataset of a measurement campaign, which was performed in a twin test facility on a mechanically ventilated DSF adopting both Venetian and roller blinds, was used to validate both the CFD model developed for this study, and the implementation of a former simplified model suitable for building simulation. The CFD approach allows for an assessment of the assumptions and hypotheses employed by the simplified model. Moreover, the CFD analyses provide a deeper insight on important aspects such as, the presence and impact of recirculation, the development of velocity and temperature profiles.
2015
- Modelling natural ventilation in double skin facade
[Relazione in Atti di Convegno]
Angeli, Diego; Dama, Alessandro
abstract
The assessment of the energy performance of buildings with Double Skin Facades (DSF) requires proper dynamic simulation tools, based on models capable of predicting heat and mass transfer in the DSF under variable boundary conditions, at the price of a reasonable computational effort. Many DSF simplified models have been developed and implemented in building simulation tools, but the validation of these tools is still an open issue, especially for the prediction of the mass flow rate in naturally ventilated DSF. The CFD modelling activity presented in this work aims at investigating the reliability of the assumptions and hypotheses employed in the simplified model, which was specifically developed for the dynamic simulation of heat transfer in buildings. Both the CFD and simplified models have been tested and evaluated on an experimental case study, using the database provided by a research program developed under IEA ECBCS Annex 43/SHC Task 34, reporting the results of a measurement campaign conducted on an a transparent naturally ventilated DSF tested in Denmark, in an experimental facility called "the Cube".
2015
- Transition to Chaos of Natural Convection Flows in Differentially Heated Vertical Slots
[Relazione in Atti di Convegno]
Angeli, Diego; Cimarelli, Andrea; Leonforte, ADRIANO DAVIDE SERAFINO; De Angelis, Elisabetta; Dumas, Antonio
abstract
Aim of the present study is the preliminary numerical analysis of the transition to turbulence of natural convection of air between two infinite vertical plates, with a focus on the effect of modeling and discretization choices on the predicted transitional patterns. In particular, the effect of the domain size, grid resolution and perturbation amplitude are explored. For the study of the problem, different Direct Numerical Simulations (DNS) have been performed. The
governing equations of the problem are the continuity, momentum and temperature equations under the Boussinesq assumption. Such equations are tackled numerically by means of a pseudospectral method which discretizes space with Chebychev polynomials in the direction normal to the walls and with Fourier modes in the wall-parallel directions.
For low Rayleigh number values, the predictions of the flow regimes are consistent with the results classical analytical results and linear stability analyses. In particular, the first bifurcation from the so-called conduction regime to steady convection is correctly captured. By increasing the Rayleigh number beyond a second critical value (Ra ≈ 10200), the predicted flow regime is observed to be extremely sensitive to all the above-mentioned numerical parameters, which lead to physically sound results only in some cases. In particular, the appearance of 3D structures is seen to be linked either to the adoption of a large enough domain or to the superimposition of finite-amplitude disturbances to the initial flow field.
2014
- A note on capillary rise in tubes
[Relazione in Atti di Convegno]
Barozzi, Giovanni Sebastiano; Angeli, Diego
abstract
Results are presented from an experiment on capillary rise of water in inclined small-bore (Φ = 1.92 mm) cylindrical
tubes. A series of thick-walled tubes with different lengths and taken from the same glass rod was used, and the
angle of inclination towards the vertical, α, was varied from 0° to 88°.
Results indicate that the capillary rise progressively reduces for increasing α, contrary to Jurin’s law predictions. It
is observed further that the meniscus seems not to change in shape while varying the tube orientation. Possible
consequences of the above observations on the statics of capillary tubes are commented. In the frame of the
discussion, an alternative and more general approach for the derivation of the Jurin’s law is proposed.
2014
- Appearance of quasiperiodicity within a period doubling route to chaos of a swaying thermal plume
[Relazione in Atti di Convegno]
Angeli, Diego; Corticelli, Mauro Alessandro; Fichera, A; Pagano, A.
abstract
The birth, evolution and disappearance of quasiperiodic dynamics in buoyancy-driven flow arising from an enclosed horizontal cylinder are analysed here, by numerical means, in the limit of the 2D approximation. The governing equations are solved on orthogonal Cartesian grids, giving special treatment to the internal, non-aligned boundaries. Thanks to the adoption of a high level of re finement of the Rayleigh number range, quasiperiodicity was observed to emerge from a periodic limit cycle (P1), and to turn into its omologous orbit with doubled period (P2), eventually evolving into a classical period-doubling route to chaos, for further increases of the Rayleigh number. The present study gives a deeper insight to what appears to be an imperfect period doubling bifurcation through a quasiperiodic T2-torus. The approach used is based on the classical tools for time series analysis. The distribution of the power spectral densities is used to search for and characterise the existence of relations between the frequencies of the P1, T2 and P2 dynamics. The topology of the orbits, as well as their evolution within the quasiperiodic window, are analysed with the aid of phase space representation and Poincar è maps.
2014
- High altitude airship cabin sizing, pressurization and air conditioning
[Relazione in Atti di Convegno]
Dumas, A.; Angeli, D.; Trancossi, M.
abstract
This paper aims at defining a design methodology for the global thermodynamic performance of a high altitude airship cabin. This design method applies to different systems, which could not use the traditional air conditioning plant layout based on bleed air intake from the compressor stage of jet engines. In the case of electrically propelled green vehicles and airships, other energy sources must be exploited. The MAAT EU FP7 project presents an innovative, energetically self sufficient, airship system based on cruiser-feeder architecture. Both the cruiser and feeder are fed by photovoltaic energy. The energy storage system by electrolysis and fuel cells with intermediate energy storage by hydrogen and oxygen is characterized by high temperature energy dispersions (about 800-1000°C for High temperature SOFC cells). This situation encourages the definition of a novel pressurization and air conditioning system. A preliminary cabin sizing with some structural considerations, an energetic evaluation of the thermal insulation of the cabin and a general balance of the energy production system are provided.
2014
- Natural convection in asymmetric triangular enclosures heated from below
[Relazione in Atti di Convegno]
O. M., Kamiyo; Angeli, Diego; Barozzi, Giovanni Sebastiano; M. W., Collins
abstract
Triangular enclosures are typical configurations of attic spaces found in residential as well as industrial pitched-roof buildings. Natural convection in triangular rooftops has received considerable attention over the years, mainly on right-angled and isosceles enclosures. In this paper, a finite volume CFD package is employed to study the laminar air flow and temperature distribution in asymmetric rooftop-shaped triangular enclosures when heated isothermally from the base wall, for aspect ratios (AR) 0.2 ≤ AR ≤ 1.0, and Rayleigh number (Ra) values 8 × 105 ≤ Ra ≤ 5 × 107. The effects of Rayleigh number and pitch angle on the flow structure and temperature distributions within the enclosure are analysed. Results indicate that, at low pitch angle, the heat transfer between the cold inclined and the hot base walls is very
high, resulting in a multi-cellular flow structure. As the pitch angle increases, however, the number of cells reduces, and the total heat transfer rate progressively reduces, even if the
Rayleigh number, being based on the enclosure height, rapidly increases. Physical reasons for the above effect are inspected.
2013
- A Revised Approach for One-Dimensional Time-Dependent Heat Conduction in a Slab
[Articolo su rivista]
Caffagni, A.; Angeli, Diego; Barozzi, Giovanni Sebastiano; Polidoro, Sergio
abstract
Classical Green’s and Duhamel’s integral formulas are enforced for the solution of one
dimensional heat conduction in a slab, under general boundary conditions of the first
kind. Two alternative numerical approximations are proposed, both characterized by fast convergent behavior. We first consider caloric functions with arbitrary piecewise continuous boundary conditions, and show that standard solutions based on Fourier series do not converge uniformly on the domain. Here, uniform convergence is achieved by integrations by parts. An alternative approach based on the Laplace transform is also presented, and this is shown to have an excellent convergence rate also when discontinuities are present at the boundaries. In both cases, numerical experiments illustrate the improvement of the convergence rate with respect to standard methods.
2013
- Buoyancy-induced transitional flows around an enclosed horizontal cylinder: an experiment
[Articolo su rivista]
Fiscaletti, D.; Angeli, Diego; Tarozzi, L.; Barozzi, Giovanni Sebastiano
abstract
An experiment is performed to detect the onset of time-dependent
flow regimes within a water-filled square-sectioned cavity containing a horizontal cylindrical heat source. Data are recorded on the cavity cross-section, where velocity fields and local temperature values are detected using a 2D PIV rig,
and thermocouples, respectively. Numerical predictions of the experimental runs are carried out, where the system is modelled as a 2D feature.
Results are provided for nine values of the leading parameter, the modified Rayleigh number Raq, ranging from 1.48x10^4 to 1.21 x10^5. The system is witnessed to undergo a transition from steady-state, laminar flow to unsteady oscillatory flow at Raq = 5.88x10^4.
Experimental data and numerical predictions are compared and cross-validated. The evolution of the flow throughout the bifurcation is described by velocity and temperature profile plots, 2D field visualizations and Fourier analysis of experimental time series. The influence of the flow regime on the overall heat transfer rate of the system is finally discussed.
2013
- Full scale CFD modeling of the Mont Blanc tunnel ventilation system
[Relazione in Atti di Convegno]
Agnani, Elia; Angeli, Diego; I., Spisso; Levoni, Paolo; Stalio, Enrico; Barozzi, Giovanni Sebastiano; M., Cipollone
abstract
A
exible and versatile full-scale CFD modeling strategy of the air
ow in a road
tunnel is described. As applied to the case of the Mont-Blanc tunnel, the model includes the
entire tunnel length and main characteristic elements of its hybrid ventilation system, including
lateral air intake, longitudinal jet fans and air extraction vents. Each of these elements has
been modeled, tested and calibrated independently with the help of experimental data collected
on-site. In order to simplify the generation of the computational model, seven elementary
modules (each 50 m long) have been singled out and meshed independently; such modules, if
properly combined, can represent the whole tunnel or just one of its segments. Automatic mesh
manipulation scripts were implemented to combine and merge such modules and generate the
whole model, which consist of more than two hundred million cells. The validity of the modeling
strategy, when appropriate, is veried by means of steady state simulations performed using data
from previous in vivo experimental campaigns. Results demonstrate that the deviation between
numerical and experimental data is, in most cases, lower than the measurement error of the
experimental procedure.
2013
- Mathematical modelling of a two streams coanda effect nozzle
[Relazione in Atti di Convegno]
Trancossi, Michele; Maharshi, Subhash; Angeli, Diego
abstract
This paper analyses the ACHEON Coanda effect based propulsion nozzle for aircraft propulsion based on the dynamic equilibrium of two jet streams. It presents a large bibliographic analysis and the ACHEON concept and, in particular, the HOMER Nozzle, that is its main component. The Constructal optimization process that allows defining this architecture has presented. A preliminary mathematical model of a 2D case of the system has presented, focusing on the combined effect of the mixing effect of the two streams and the Coanda Effect Adhesion over a convex surface. A CFD preliminary validation has presented in uncompressible regime. The results have been evaluated in 2D cases.
2013
- Modeling conjugate heat transfer to a protrusion exposed to bulk air flow by OpenFOAM®
[Relazione in Atti di Convegno]
P., D'Andria; Angeli, Diego; G., Ruocco; M. V., De Bonis
abstract
In the last few years open source codes have attracted growing interest because of their favorable impact in modeling costs. The heat transfer and flow solvers implemented in the open-source OpenFOAM code are exploited in this paper, to solve a model used to describe the heating of discrete substrate samples. The aim is to determine the effect of process configuration and geometry on the heating performance due to a bulk air flow, and to report on computational characteristics of the code at stake. The model allows to disregard one of the most limiting parameters in such modeling, i.e. the average heat transfer coefficient at the auxiliary air/heated substrate interface. Such assumption is limiting as it refers to average conditions and unspecified geometry variations. The presented model then relies upon a finite-volume solution of time–dependent differential equations, for simultaneous and conjugate heat transfer in a two–dimensional domain, without any inference in such empiricism. After proper validation with literature data and previous work, the solution is discussed by presenting velocity and temperature fields, emphasizing on the conjugate nature of the process. Due to its flexibility and generality, the model can be used in common industrial optimization, even in the assumption of a laminar flow field.
2013
- Nonlinear dynamics of a confined buoyant flow
[Articolo su rivista]
Angeli, Diego; A., Pagano
abstract
The sequence of bifurcations leading to the onset of chaotic flow is determined numerically, for the case of a buoyant plume arising from a horizontal cylinder, centred in a square-sectioned, air-filled enclosure. In the frame of the 2D assumption, a specifically-developed high resolution simulation procedure is adopted, with the aim of achieving a detailed description of the transitional dynamics occurring within the system. A large number of simulations are performed, allowing for an accurate estimate of the critical values of the main system parameter, the Rayleigh number Ra, at which bifurcations occur. A single value of the geometric aspect ratio A of the system is considered, for which transition is found to be characterized by an imperfect period-doubling cascade, an uncommon behaviour in thermofluid systems. Peculiarities of the route to chaos are highlighted, such as the existence of a window of quasiperiodic flow, and the detection of high-order period orbits.
2013
- PIV experiments on a valve for the handling of liquids
[Relazione in Atti di Convegno]
Salerno, Elisabetta; Levoni, Paolo; Angeli, Diego; Barozzi, Giovanni Sebastiano
abstract
Particle Image Velocimetry (PIV) is employed to investigate the flow field inside the dummy of a prototype valve to be used for the industrial dosage of liquids. Velocity fields are reconstructed in three regions of the valve passages and for two values of the flow rate. Distilled water is used as the working fluid. Optical limitations were faced during the setup of the apparatus and a description of the expedients adopted is reported. Time-averaged velocity fields reveal the presence of critical fluid behaviours such as throttling, asymmetric flows and vortex generation. Those effects strongly affect the valve performance. The quality of PIV measurements is verified by comparing the experimental data with the results of RANS-based numerical simulations. Contour maps and velocity profiles obtained with the two methods show a very good agreement, thus demonstrating the reliability of PIV in this specific context. A first attempt was also made to characterize the jet flow at the valve outlet using PIV. It is found that in this case the accuracy of the measurements deteriorates, due to the presence of fluid-dynamic instabilities at the jet surface.
2012
- Analysis of the bifurcating orbits on the route to chaos in confined thermal convection
[Relazione in Atti di Convegno]
Angeli, Diego; A., Pagano; Corticelli, Mauro Alessandro; A., Fichera; Barozzi, Giovanni Sebastiano
abstract
Bifurcating thermal convection flows arising from a horizontal cylinder centred in a square-sectioned enclosure are studied numerically, with the aim of achieving a more detailed description of the sequence of transitions leading to the onset of chaos, and obtaining a more precise estimate of the critical values of the main system parameter, the Rayleigh number Ra. Only a value of the geometric aspect ratio A of the system is considered, namely A = 2:5, for which a period-doubling cascade was previously observed. Results give evidence of new and interesting features in the route to chaos, such as a window of quasiperiodic flow and the detection of high-order period orbits.
2012
- Experimental Analysis of Unsteady Natural Convection Around a Horizontal Heater in a Water-Filled Enclosure
[Relazione in Atti di Convegno]
D., Fiscaletti; Angeli, Diego; Barozzi, Giovanni Sebastiano
abstract
Particle Image Velocimetry (PIV) measurements are carried out to analyse the buoyancy-induced flows originating from a
horizontal heated cylinder enclosed in a square-sectioned cavity, filled with distilled water.
A description of the experimental setup is provided, alongside with a discussion of critical issues in the measurement process
and thermal conditioning of the system. Repeatability of the tests is assessed in both steady-state and unsteady conditions.
Results are provided for four different values of the leading parameter, the modified Rayleigh number Raq, ranging from
1:48 104 to 8:62 104. The system is witnessed to undergo a transition from steady-state, laminar flow to unsteady oscillatory
flow. The evolution of the flow throughout the bifurcation is described by velocity profile plots and 2D field visualizations.
The suitability of the technique for the analysis of transitional natural convection regimes is confirmed by the quality of the
experimental data, and the agreement with comparative numerical computations.
2012
- Numerical analisys of Weakly turbulent mixed convection flows in a horizontal pipe
[Relazione in Atti di Convegno]
Errico, Orsola; Angeli, Diego; Barozzi, Giovanni Sebastiano; Stalio, Enrico; Corticelli, Mauro Alessandro
abstract
Weakly turbulent developing flows in a heated horizontal pipe are investigated numerically, focusing on the effects of mixedconvection, and for two different thermal boundary conditions (BCs), the uniform heat flux condition and the uniform walltemperature condition. The Reynolds number, based on the pipe diameter and bulk velocity, is set equal to Re = 5750 and the molecular Prandtl number Pr = 12. For the sake of comparison, the Grashof number values GrH = 3×108 and GrT = 2.3×1010 are chosen for isoflux and isothermal boundary conditions, respectively. Turbulence is modeled by using alternative RANS approaches. Four RANS models are considered, namely the realizable k-e (RKE), the renormalization-group k-e (k-e RNG), the Gibson’s q-z model, and the Shear-Stress Transport k-w model (k-w SST). Results allow to discriminate the performance ofthe different turbulent models and give hint on the effects of the buoyancy induced flow over the leading forced component.The effect of the thermal boundary condition is finally discussed.
2012
- Numerical simulations of turbulent heat transfer in a channel with one wavy wall
[Relazione in Atti di Convegno]
Errico, Orsola; Cavazzuti, Marco; Angeli, Diego; Stalio, Enrico
abstract
Wavy surfaces are encountered in a large variety of applications, and are well-known for enhancing heat and mass transfer mechanisms. The present study numerically investigates the flow dynamics and heat transfer for turbulent flow in a channel with one flat and one wavy wall. Investigations have been conducted for a Prandtl number Pr = 0.71 and Reynolds numbers Re = 13840 and Re = 19 000, based on the bulk velocity and the hydraulic diameter. Direct Numerical Simulations (DNSs) have been performed for a deep understanding of the dynamic effects on the heat transfer mechanisms for the case of turbulent flow in the channel with one wavy wall.
The performance of two different Reynolds-Averaged Navier Stokes (RANS) turbulence models, namely the k-omega SST and the q-zeta, selected for their favorable characteristics, is assessed against the DNSs results. The applicability of the two selected RANS model is ascertained from a qualitative point of view.
2012
- T.A.L.P.A.: an innovative facility for cotinous longitudibnal air flow profile acquisition in tunnels
[Relazione in Atti di Convegno]
Levoni, Paolo; A., Scorcioni; Angeli, Diego; Stalio, Enrico; Barozzi, Giovanni Sebastiano; M., Cipollone
abstract
The knowledge of the flow field inside road tunnels under fire conditions and normal operation is only approximate andpartial. The reason is that while the full three-dimensional, unsteady problem is out of reach of numerical methods, on theother hand accurate measurement of the airflow in road and railway tunnels constitutes an extremely challenging task.Aiming at performing easy and accurate in situ measurements of the flow field in road tunnel under different conditions, aninnovative experimental facility for the continuous acquisition of the longitudinal velocity profile has been designed and built.The facility is made up with a survey rake with five bidirectional vane anemometers, which is mounted on a small electricvehicle that can travel through the tunnel at constant speed. This paper reports the design procedure of the measurementfacility, with particular focus on the conception and realization of the vehicle carrying the survey rake. Results of the firstexperimental campaign carried out under the 11611 meters long Mont Blanc road tunnel are also presented to corroborate thevalidity of the approach adopted and the accuracy of the measurement chain.
2011
- Bifurcations of Natural Convection Flows from an Enclosed Cylindrical Heat Source
[Articolo su rivista]
Angeli, Diego; A., Pagano; Corticelli, Mauro Alessandro; A., Fichera; Barozzi, Giovanni Sebastiano
abstract
A numerical analysis of transitional natural convection from a confined thermal source is presented. The system considered is an air-filled, square-sectioned 2D enclosure containing a horizontal heated cylinder. The resulting flow is investigated with respect to the variation of the Rayleigh number, for three values of the aspect ratio A. The first bifurcation of the low-Ra fixed-point solution is tracked for each A-value. Chaotic flow features are detailed for the case A = 2.5. The supercritical behaviour of the system is investigated using nonlinear analysis tools and phase-space representations, and the effect of the flow on heat transfer is discussed.
2011
- Concept, design, construction and testing of an experimental facility for multi-point longitudinal air flow measurements in tunnels
[Relazione in Atti di Convegno]
Levoni, Paolo; Angeli, Diego; Stalio, Enrico; Barozzi, Giovanni Sebastiano; M., Cipollone
abstract
Performing accurate measurements during a set of fire tests in a road tunnel is a notoriously difficult task to carry out. As in addition the full three-dimensional, unsteady problem is out of reach of numerical methods, the knowledge of the flow field inside road tunnels under fire conditions and also during normal operation is in many cases only approximate and partial. As a consequence, also the influence of road tunnel ventilation on fire development and spread are still not clarified, even for the simple longitudinal ventilation. Aiming at performing easy and accurate in situ measurements of the flow field in road tunnel under different conditions, a portable experimental facility including a survey rake with five bidirectional vane anemometers has been designed and built. This paper reports the design procedure of the measurement facility, all the details of the measurement chain and also results of the first experimental campaign carried out under the 11611 meters long Mont Blanc road tunnel. Unlike data provided by the S.C.A.D.A., a monotonic axial velocity profile resulted, which is theoretically justified in the case of semi-transverse ventilation.
2011
- Experimental characterization of a road tunnel jet fan by direct measurement of the velocity components at the inflow and out flow section
[Relazione in Atti di Convegno]
Levoni, Paolo; Angeli, Diego; E., Canepa; Barozzi, Giovanni Sebastiano; M., Cipollone
abstract
Axial jet fans installed at the ceiling are worldwide diffuse devices for longitudinal ventilation control in road tunnels underordinary operating conditions and also for smoke control under fire conditions.An experimental characterization of one of the jet fans actually operating inside the tunnel Mont Blanc has been carried out bymeans of a multi-axial hot wire technique. Inflow and outflow instantaneous axial, radial and tangential velocity profiles havebeen measured in still air at the nominal speed of rotation of the fan. Time averaged velocity profiles and turbulent kineticenergy profile have been derived from instantaneous data. In the paper, details of the testing procedure, the experimental setup,the collected data and the post-processed profiles are reported.
2011
- Flow transitions in a Joule-heated cavity of a low-Prandtl number fluid
[Articolo su rivista]
Zhang, X; Angeli, Diego
abstract
The competition of buoyancy with electromagnetic body forces is examined numerically in terms of flow structure transitions by means of a two-dimensional unsteady, finite volume model. In the present numerical study, we consider a low-Prandtl liquid metal heated by Joule effect in a rectangular cavity with an aspect ratio of 2. The direct current provides heat to the process medium by a pair of plate electrodes, located at the cavity sidewalls. The simulations have been carried out for fixed values of the Prandtl number, Pr = 0.01, and of the Rayleigh number, Ra = 1.5 × 104, while the Hartmann number, Ha, varies from 0 to 104. The variation of Ha is found to have considerable effects on flow patterns and heat transfer inside the cavity. Several hitherto unknown flow structures are revealed, increasing in complexity with increasing Ha. Amongst the oscillatory flows predicted, intermittency and chaos are detected. The effect of Ha on the overall heat transfer performance of the system is also assessed.
2011
- Numerical simulation of forced convection over a periodic series of rectangular cavities at low Prandtl number
[Articolo su rivista]
Stalio, Enrico; Angeli, Diego; Barozzi, Giovanni Sebastiano
abstract
Convective heat transfer in laminar conditions is studied numerically for a Prandtl number Pr = 0.025,representative of liquid lead–bismuth eutectic (LBE). The geometry investigated is a channel with a periodicseries of shallow cavities. Finite-volume simulations are carried out on structured orthogonal curvilineargrids, for ten values of the Reynolds number based on the hydraulic diameter between Rem = 24.9and Rem = 2260. Flow separation and reattachment are observed also at very low Reynolds numbers andwall friction is found to be remarkably unequal at the two walls. In almost all cases investigated, heattransfer rates are smaller than the corresponding flat channel values. Low-Prandtl number heat transferrates, investigated by comparison with Pr = 0.71 results, are large only for uniform wall temperature andvery low Re. Influence of flow separation on local heat transfer rates is discussed, together with the effectof different thermal boundary conditions. Dependency of heat transfer performance on the cavity geometryis also considered.
2011
- Routes to chaos in confined thermal convectionarising from a cylindrical heat source
[Relazione in Atti di Convegno]
Angeli, Diego; A., Pagano; Corticelli, Mauro Alessandro; Barozzi, Giovanni Sebastiano
abstract
Natural convection ows arising from a horizontal cylinder centred ina square-sectioned enclosure are studied numerically. The sequence of bifurcations marking the transition of base fixed-point solutions to unsteady, chaotic ows is followed for increasing values of the Rayleigh number, and for two values of the enclosure aspect ratio, A. It is observed that, for the lower A-value, the route to chaosis triggered by a supercritical Hopf bifurcation, followed by a sequence of period- doublings, while, for the higher A-value, the symmetry of the system is broken by a pitchfork bifurcation, with periodic orbits originating from both branches, and eventually approaching chaos, exhibiting features typical of blue-sky catastrophes.
2011
- THREE-DIMENSIONAL TRANSITIONAL BUOYANCY-INDUCED FLOWSFROM A HORIZONTAL ISOTHERMAL CYLINDER IN A PERIODIC ENCLOSURE
[Relazione in Atti di Convegno]
Angeli, Diego; Corticelli, Mauro Alessandro; B., Niceno
abstract
The study presents a preliminary study of three-dimensional transitional natural convection flows, generated by a confined thermal source. The system considered is a periodic enclosure of square cross-section, containing a horizontal cylindrical source, placed in central position. The heat carrier fluid is air with Prandtl number Pr = 0:7. For such system, transition from steady-state tounsteady, chaotic flow has already been widely investigated under the 2D assumption, with respect to the variation of the Rayleighnumber and aspect ratio A. The numerical scheme is based on a second order finite-volume discretization of the Navier-Stokesequations, solved on orthogonal Cartesian grids. Internal boundaries non-aligned with the grid are subjected to special treatment by means of an original technique belonging to the class of Immersed Boundary Methods. Five computations for selected (Ra;A) couples are carried out for comparison with previously obtained 2D results in the transition range. A first hint is provided at the degree of validity of the 2D assumption for the detection of bifurcative dynamics, and the overall quality of the computational technique is evaluated successfully.
2011
- XT-ADS Windowless spallation target thermohydraulic design & experimental setup
[Articolo su rivista]
Class, Ag; Angeli, Diego; Batta, A; Dierckx, M; Fellmoser, F; Moreau, V; Roelofs, F; Schuurmans, P; Van Tichelen, K; Wetzel, T.
abstract
The objective of the European 6th framework Integrated Project (IP) EUROTRANS (EUROpean Research Programme for the TRANSmutation of High Level Nuclear Waste in an Accelerator Driven System) is to demonstrate the feasibility of transmutation of high level nuclear waste using subcritical Accelerator Driven Systems (ADS). The spallation target represents the most challenging new component in an ADS since it is the component coupling the accelerator and the nuclear core and is subjected to very high thermal load in a high radiation field. In this document the thermal hydraulic activities which led to reliable design rules for a windowless target are presented and the status of the heavy liquid metal target mock-up experiment at the KArlsruhe Liquid metal LAboratory (KALLA) are reported.
2010
- A Comprehensive Review of Natural Convection in Triangular Enclosures
[Articolo su rivista]
O. M., Kamiyo; Angeli, Diego; M. W., Collins; Barozzi, Giovanni Sebastiano; V. O. S., Olunloyo; S. O., Talabi
abstract
Natural convection in triangular enclosures is an important problem. It displays well the generic attributes of this class of convection, with its dependence on enclosure geometry, orientation and thermal boundary conditions. It is particularly rich in its variety of flow regimes and thermal fields as well as having significant practical application. In this paper, a comprehensive view of the research area is sought by critically examining the experimental and numerical approaches adopted in studies of this problem in the literature. Different thermal boundary conditions for the evolution of the flow regimes and thermal fields are considered. Effects of changes in pitch angle and the Rayleigh number on the flow and thermal fields are examined in detail. Although most of the past studies are in the laminar regime, the review extends up to the recent studies of the low turbulent regime. Finally, areas of further research are highlighted.
2010
- A Critical Review of Buoyancy-induced Flow Transitions in Horizontal Annuli
[Articolo su rivista]
Angeli, Diego; Barozzi, Giovanni Sebastiano; M. W., Collins; O. M., Kamiyo
abstract
The main mechanisms of transition of buoyancy-induced flows in the horizontal annulus betweencircular cylinders are reviewed, based on the available literature. Both experimental and theoreticalstudies are considered. The different scenarios for the evolution of the flow regimes and temperaturepatterns are tracked, for increasing values of the Rayleigh number, Ra. The occurrence of variousinstability and bifurcative phenomena is pointed out, and linked to other relevant parameters, such asthe radius ratio R and the Prandtl number, Pr. Although most of the relevant literature is on 2D cases, theeffect of the third dimension is considered as far as possible. Studies on the influence of the eccentricityof the inner cylinder on the laminar flow and the thermal asset are also reviewed. Finally, open questionsand topics for future research are hinted at.
2010
- Direct Numerical Simulation of Forced Convection over Steps at Low Prandtl Number
[Relazione in Atti di Convegno]
Angeli, Diego; Barozzi, Giovanni Sebastiano; Errico, Orsola; Stalio, Enrico; Tartarini, Paolo
abstract
Convective heat transfer in transitional and weakly turbulent conditions is investigated numerically for a Prandtl number Pr =0.025, representative of liquid lead-bismuth eutectic (LBE). The geometry selected is a periodic channel with cavities. Finitevolumesimulations are carried out on structured orthogonal curvilinear grids, for two different values of the Reynolds numberin the weakly turbulent range. The main features of the mean and instantaneous flow fields are described. The influence of flowseparation mechanisms and turbulent mixing on local and global heat transfer rates is also discussed, considering the effect ofdifferent thermal boundary conditions imposed at the channel walls.
2010
- Flow Transitions and Bifurcations of Buoyancy-induced Flows from an Eclosed Cylindrical Heat Source
[Relazione in Atti di Convegno]
Angeli, Diego; A., Pagano; Corticelli, Mauro Alessandro; A., Fichera; Barozzi, Giovanni Sebastiano
abstract
The study presents a numerical analysis of the dynamics of transitional natural convection flow regimes, generated by a confined thermal source. The system considered is a 2D enclosure of square cross-section, containing a horizontal cylindrical source,placed in central position. The heat carrier fluid is air with Prandtl number Pr = 0:7. The resulting flow is investigated withrespect to the variation of the Rayleigh number, based on the minimum gap width H between the cylinder and the enclosurewalls. Three values of the aspect ratio A are considered (A = 2:5; 3:¯3; 5;). The numerical scheme is based on a second orderfinite-volume discretization of the Navier-Stokes equations, solved on orthogonal Cartesian grids. Internal boundaries nonalignedwith the grid are subjected to special treatment. The first bifurcation of the low-Ra fixed-point solution is tracked foreach A-value. The transition to unsteady, chaotic flow is detailed for A = 2:5. The supercritical behaviour of the system isinvestigated using nonlinear analysis tools and phase space representations of the computed time series, and the effect of the flow features on heat transfer is discussed.
2010
- Flow Transitions and Bifurcations of Buoyancy-induced Flows from an Enclosed Cyindrical Heat Source
[Relazione in Atti di Convegno]
Angeli, Diego; A., Pagano; Corticelli, Mauro Alessandro; Barozzi, Giovanni Sebastiano
abstract
This study presents a numerical analysis of the dynamics of transitional natural convection flow regimes, generated by a confined thermal source. The system considered is a 2D enclosure of square cross-section, containing a horizontal cylindrical source, placed in a central position. The heat carrier fluid is air with Prandtl number Pr = 0.7. The resulting flow is investigated with respect to the variation of the Rayleigh number, based on the minimum gap width H between the cylinder and the enclosure walls. Three values of the aspect ratio A are considered (A = 2.5; 3.3; 5). The first bifurcation of the low-Ra fixed-point solution is tracked for each A-value. The transition to unsteady, chaotic flow is detailed for A = 2.5. The supercritical behaviour of the system is investigated using nonlinear analysis tools and phase space representations of the computed time series.
2009
- A comprehensive review on buoyancy-induced flow transitions in horizontal annuli
[Relazione in Atti di Convegno]
Angeli, Diego; Barozzi, Giovanni Sebastiano; Corticelli, Mauro Alessandro
abstract
The main mechanisms of transition of buoyancy-inducedflows in horizontal annuli are reviewed, based on the availableliterature. Both experimental and theoretical studies areconsidered. The different scenarios for the evolution of theflow regimes and temperature patterns are tracked, for increasingvalues of the Rayleigh number, Ra. The occurrenceof various instability and bifurcative phenomena is pointedout, and linked to other relevant parameters, such as the radiusratio, R and the Prandtl number, Pr. The effect of theaxial dimension is also considered. Finally, possible futureresearch issues on the subject are briefly hinted at.
2009
- Numerical simulation of forced convection over steps at low Prandtl number
[Relazione in Atti di Convegno]
Angeli, Diego; Cavazzuti, Marco; Stalio, Enrico
abstract
Convective heat transfer in laminar conditions is investigated numerically for a Prandtl number Pr = 0.025, representative of liquid lead-bismuth eutectic (LBE). The geometry selected is a periodic channel with a set of backward and forward steps. Finite-volume simulations are carried out on structured orthogonal curvilinear grids, for ten values of the Reynolds number up to the transitional range. It is shown how flow can undergo separation and reattachment also at very low-Re. The influence of flow separation mechanisms on local and global heat transfer rates is discussed, also considering the effect of different thermal boundary conditions imposed at the channel walls.
2009
- Optimization of heat exchanger enhanced surfaces under turbulent conditions
[Relazione in Atti di Convegno]
Cavazzuti, Marco; Angeli, Diego; Corticelli, Mauro Alessandro; Barozzi, Giovanni Sebastiano
abstract
The optimization of heat transfer enhancing surfaces for compact heat exchangers is discussed.The heat transfer passages are parameterized through Bézier curves, and the optimum choice of the parameters is sought using both multi-objective and single objective optimization techniques. The objectives of the optimization are the maximization of the heat transfer rate and the minimization of the pressure drop encountered by the fluid crossing the passage. The objectives are evaluated in nondimensional form in terms of the Nusselt number and the friction factor. The investigation is carried out by means of CFD analyses, the fluid flowing in the passages is air, and turbulent flow is assumed. Two different turbulence models (namely, k- and k-!) are adopted in the optimization processes and the effect of the choice of the turbulence model on the optimization outcome is discussed.
2009
- Towards a new Liquid Argon Imaging Chamber for the MODULAr project
[Articolo su rivista]
Angeli, Diego; Baibussinov, B; Ceolin, Mb; Battistoni, G; Benetti, P; Borio, A; Calligarich, E; Cambiaghi, M; Cavanna, F; Centro, S; Cieslik, K; Cocco, Ag; Dolfini, R; Berzolari, Ag; Farnese, C; Fava, A; Ferrari, A; Fiorillo, G; Gibin, D; Guglielmi, A; Mannocchi, G; Menegolli, A; Meng, G; Montanari, C; Muraro, S; Palamara, O; Periale, L; Picchi, P; Pietropaolo, F; Rappoldi, A; Raselli, Gl; Rossella, M; Rubbia, C; Sala, P; Satta, G; Varanini, F; Ventura, S; Vignoli, C.
abstract
The MODULAr project foresees the exploitation of a new liquid Argon imaging detector, of at least 20 kt fiducial mass, to be operated in a shallow depth location under the Gran Sasso Mountain. It will be devoted to study neutrino oscillations with an optimized off-axis CNGS neutrino beam. Cosmic neutrinos as well as proton decay will also be addressed. The MODULAr detector will vastly inherit from the technology developed for ICARUS-T600. However, such an increase in the volume over the current ICARUS-T600 needs to be carefully considered. It is concluded that a single, huge volume is an inoperable and uneconomical solution for many reasons. A very large mass is best realized with a modular set of many identical, independent units, each of about 5 kt, “cloning” the basic technology of the ICARUS-T600. Several of such modular units will be assembled to reach at least 20 kt as initial sensitive volume. The increase of the active volume of about one order of magnitude with respect to the ICARUS-T600 detector requires some specific R&D activity, which will be implemented in a ∼ 360 ton prototype unit (SLICE) of reduced length.
2008
- Influence of the Prandtl Number on Buoyancy-Induced Flow and Heat Transfer from an Enclosed Cylindrical Heat Source
[Relazione in Atti di Convegno]
Angeli, Diego; Corticelli, Mauro Alessandro; Barozzi, Giovanni Sebastiano
abstract
A numerical study of the natural convection flow arising from a confined thermal source is presented. The physical systemconsidered is a 2D enclosure of square cross-section, containing an isothermal horizontal cylindrical source. The heating body iscentred in the enclosure, and the resulting flow is investigated with respect to the effect of the Prandtl number, for different valuesof the Rayleigh number and of the aspect ratio. The numerical scheme is based on a second order finite-volume discretization ofthe Navier-Stokes equations, solved on orthogonal Cartesian grids. Internal boundaries non-aligned with the grid are subjectedto special treatment. The work focuses on the onset of steady and transitional flows. The asymptotic flow and thermal patternsare investigated, as well as the heat transfer performance of the system
2008
- Numerical Predictions for Stable Buoyant Regimes within a Square Cavity Containing a Heated Horizontal Cylinder
[Articolo su rivista]
Angeli, Diego; P., Levoni; Barozzi, Giovanni Sebastiano
abstract
Buoyancy-induced flow regimes are investigated numerically for the basic case of a horizontal cylinder centred into a long co-axial square-sectioned cavity. In the frame of the 2D assumption, the treshold for the occurrence of time-dependent behaviour is explored. Stable symmetric and non-symmetric steady-state solutions, as well as unsteady regimes are observed, depending on the Rayleigh number, Ra, and the aspect ratio of the cavity, d. Four d-values are considered (d = .2, .4, .6, and .8). Heat transfer results are correlated by a single equation covering the full subcritical region.
2007
- Preliminary Analysis of the Dynamics of Buoyancy-driven Flows from an Enclosed Cylindrical Heat Source
[Relazione in Atti di Convegno]
Angeli, Diego; Barozzi, Giovanni Sebastiano; A., Fichera; A., Pagano
abstract
The study presents a numerical analysis of the dynamics of time-dependent natural convection flow regimes, generated by a con- fined thermal source. The physical system considered is a 2D cavity of square cross-section, containing a horizontal cylindrical source, placed in central position. The heat carrier fluid is air at Prandtl number Pr = .7. The resulting flow is investigated with respect to the variation of the Grashof number, based on the gap between the upper cavity wall and the cylinder top. The relative dimensions of both cavity and source are fixed. The numerical scheme is based on a second order finite-volume discretization of the Navier-Stokes equations, solved on orthogonal Cartesian grids. Internal boundaries non-aligned with the grid are subjected to special treatment. Computations are carried out for four supercritical values of the Grashof number. Some preliminary con- siderations are drawn on the dynamical behaviour of the system, while representing the attractors of the computed time series in phase space, and observing their morphological structure.
2006
- Numerical Analysis of Natural Convection Flow and Heat Transfer from an Enclosed Cylindrical Heat Source
[Relazione in Atti di Convegno]
Angeli, Diego; P., Levoni; E., Caffagni; Barozzi, Giovanni Sebastiano
abstract
A numerical study of the natural convection flow arising from a confined thermal source is presented. The physical system considered is a 2D cavity of square cross-section, containing a horizontal cylindrical source, the heat carrier fluid being air at Prandtl number Pr = .7. The heating body is placed in central position; the resulting flow is investigated with respect to the variation of the Rayleigh number and the ratio between the source diameter and the cavity side length. Either Dirichlet and Neumann thermal boundary conditions are imposed on the cylinder surface. The analyses make use of two Navier-Stokes, finite- volume codes: a general-purpose CFD package, and a Direct Numerical Simulation software. Main issues of the work are the rise of steady and transitional flows and the heat transfer performance of the system, as influenced by the different boundary conditions at the heat source wall. A correlating equation for the average Nusselt number on the cylinder, previously obtained for the isothermal case, is validated for the uniform heat flux condition.
2006
- Studio Numerico e sperimentale della convezione naturale da cilindri orizzontali a basso numero di Rayleigh
[Relazione in Atti di Convegno]
Corticelli, Mauro Alessandro; Angeli, Diego; D., Ambrosini; D., Paoletti; A., Ponticiello
abstract
Nel presente lavoro si indaga il fenomeno della convezione naturale originata in aria da cilindri orizzontali di piccolodiametro, in regimi a basso numero di Rayleigh. Le analisi sono condotte sperimentalmente e numericamente. Particolareattenzione è riservata al transitorio di innesco della convezione stessa. La parte sperimentale è caratterizzata dall'utilizzo dimetodi ottici, strumenti versatili e potenti per ottenere misure termiche in convezione senza alterare il campotermofluidodinamico. Le simulazioni numeriche si avvalgono di una metodologia di Direct Numerical Simulation, sviluppataspecificatamente per applicazioni di convezione naturale e implementata su griglie cartesiane. I risultati ottenutinumericamente e sperimentalmente sono quindi confrontati fra loro e con i dati esistenti in letteratura.
2005
- Analisi numerica del flusso convettivo instazionario originato da sorgente termica in cavità chiusa
[Relazione in Atti di Convegno]
Angeli, Diego; Levoni, Paolo; Corticelli, Mauro Alessandro
abstract
Il lavoro intende esaminare il comportamento di due diversi codici di simulazione CFD nella soluzione del flusso confinato, instazionario, generato da un cilindro riscaldato, in regime di convezione naturale. Nello specifico, l’obiettivo della ricerca consiste nell’analisi delle analogie e delle differenze operative tra le tecniche di soluzione numerica e nella conseguente individuazione dei limiti di applicabilità dei due codici, nell’ambito della specifica tipologia di flusso, al variare dei parametri caratteristici del sistema; particolare attenzione è stata posta sulla previsione e interpretazione della tipologia di moto verso il quale il campo cinetico e termico evolvono, al variare del numero di Rayleigh.Vengono presentati i risultati di simulazioni bidimensionali effettuate mediante un codice industriale ed un codice DNS (Direct Numerical Simulation), sviluppato specificatamente per la soluzione del campo di moto in regime di convezione naturale.Le analisi prevedono valori crescenti del numero di Rayleigh, parametro caratteristico del sistema, e denotano flusso completamente laminare, per bassi valori di Ra, e transizione del campo di moto, per valori di Ra maggiori.Le soluzioni generate dai due codici dimostrano accordo soddisfacente, pur essendo riscontrabili sensibili differenze, sia in termini di precisione e accuratezza, che di tempi di calcolo.