Resolved dynamics and subgrid stresses in separating and reattaching flows

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.