Experimental and numerical study on the shear-layer instability in a 5:1 benchmark rectangular cylinder

The high-Reynolds-number flow around simple bluff bodies is often complex, as it clearly emerges from the results of the benchmark BARC on the aerodynamics of a rectangular 5:1 cylinder, which has been investigated for a decade so far. In addition to the apparent dispersion of both experimental and numerical data, the benchmark emphasized the fact that several large-eddy and detached-eddy simulations for a perfectly smooth incoming flow gave solutions in disagreement with the experiments when the numerical dissipation was reduced by either lowering the numerical or turbulent viscosity or refining the computational mesh in the spanwise direction. In those cases, a flow field with short mean recirculation bubbles on the lateral sides of the rectangular cylinder was obtained, similar to that observed in the wind tunnel in highly turbulent flow. In the present work, this problem is addressed focusing on the shear-layer instability downstream of the separation at the front sharp corners. Large-eddy simulations were carried out using the spectral code Nek5000 varying the amount of subgrid-scale dissipation and the spanwise mesh resolution. In addition, experimental measurements with hot-wire anemometers were performed to further characterize the shear-layer dynamics and the main features of the Kelvin- Helmholtz instability.