Università degli studi di Modena e Reggio Emilia

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.

In this work the numerical results of the flow around a 5:1 rectangular cylinder at Reynolds numbers 3 000 and 40 000, zero angle of attack and smooth incoming flow condition are presented. Implicit Large Eddy Simulations (ILES) have been performed with a high-order accurate spatial scheme and an implicit high-order accurate time integration method. The spatial approximation is based on a discontinuous Galerkin (dG) method, while the time integration exploits a linearly-implicit Rosenbrock-type Runge-Kutta scheme. The aim of this work is to show the feasibility of high-fidelity flow simulations with a moderate number of DOFs and large time step sizes. Moreover, the effect of different parameters, i.e., dimension of the computational domain, mesh type, grid resolution, boundary conditions, time step size and polynomial approximation, on the results accuracy is investigated. Our best dG result at Re=3 000 perfectly agrees with a reference DNS obtained using Nek5000 and about 40 times more degrees of freedom. The Re=40 000 computations, which are strongly under-resolved, show a reasonable correspondence with the experimental data of Mannini et al. (2017) and the LES of Zhang and Xu (2020).

Turbulence is investigated in the lee of an open-cell metal foam layer. In contrast to canonical grids metal foams are locally irregular but statistically isotropic. The solid matrix is characterised by two lengths the ligament thickness and the pore diameter. A direct numerical simulation is conducted on a realistic metal foam geometry for which and the porous layer thickness is five times the pore diameter. The Reynolds number based on the pore size is corresponding to a Taylor-scale Reynolds number. Closer to the foam than two pore diameters the pressure and turbulent transports of turbulent kinetic energy are non-negligible. In the same region undergoes a steep decrease whereas the dissipation coefficient increases like. At larger distances from the porous layer the classical grid turbulence situation is recovered where the mean advection of turbulent kinetic energy equals dissipation. This entails a power-law decay of turbulent quantities and characteristic lengths. The decaying exponents of integral Taylor and Kolmogorov scales are close to one-half indicating that the turbulence simulated here differs from Saffman turbulence. Analysis of the scaling exponents of structure functions and the decorrelation length of dissipation reveals that small-scale fluctuations are weakly intermittent.

A Direct Numerical Simulation is carried out to study a turbulent wake. The flow configuration is typical of grid turbulence investigations, but in place of a regular grid or fractal grid, the initially uniform flow passes through a three-dimensional, irregular yet statistically isotropic porous matrix. A synthetic, periodic, open cell metal foam of porosity ε = 0.92 is the geometry selected. The flow is at a Reynolds number based on the mean pore diameter dp and the freestream velocity U∞ of Redp = 4000. An approximation to homogeneous and isotropic decaying turbulence is achieved in the lee of the porous layer. Statistics reported include isotropy indicators, skewness, flatness, velocity autocorrelations, the integral scale of turbulence and compensated spectra. Dissipation of turbulent kinetic energy is calculated from its definition and from some known approximations based on different hypotheses, results extracted provide practical advice for experimentalists and give an insight in the isotropic features of the flow.