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ADRIANO DAVIDE SERAFINO LEONFORTE

CULTORE DELLA MATERIA
Dipartimento di Scienze e Metodi dell'Ingegneria

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Pubblicazioni

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

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

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 - 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.

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 - 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 - 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 - 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 - 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.

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 - 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 = U∞ · D/ν = 3 × 103 where U∞ 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.

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.