Experimental and LES analysis of the flow around a 5:1 rectangular cylinder

The high Reynolds number flow around a 5:1 rectangular cylinder is experimentally and numerically analyzed. The flow is characterized by shear layer separation from the upstream edges. Vortical structures of different sizes form from the roll-up of these shear layers, move downstream and interact with the classical vortex shedding in the wake. The corresponding mean flow is characterized by a recirculation region along the lateral surface of the cylinder, ending close to the trailing edge. The mean flow features on the cylinder side have been shown to be highly sensitive to the shear layer dynamics, which is influenced by set up parameters both in numerical simulations and in experiments. The results of 5 experimental set-ups and 35 Large Eddy Simulations are analyzed herein to highlight the impact of the shear layer dynamics on the lateral mean recirculation characteristics and, in turn, on the near wake flow features and on some bulk quantities of practical interest. Experiments and numerical simulations are carried out at zero angle of attack and Reynolds number Re=DU/ν=40000, being D the crossflow dimension, U the freestream velocity and ν the kinematic viscosity of air. Almost-sharp upstream corners (r/D=0.0005) and different values of the corner roundings are considered. LES simulations are carried out with sharp edges and different values of the rounding of the upstream edges matching the experimental ones and for different numerical and modeling settings. The shear-layer dynamics and the dimensions of the mean recirculation zone vary considerably in these experiments and simulations, allowing us to single out meaningful trends. The growth of the velocity fluctuations along the shear layers detaching from the upstream corners is highly correlated with the location of the onset of Kelvin-Helmholtz instability and, in turn, with the length of the mean recirculation on the cylinder side. This, in turn, influences the near wake features.