Importance of sub-grid scale modelling for accurate aerodynamic simulations

The Ahmed body is one of the most well-investigated vehicle bodies for aerodynamic purposes. Despite its simple geometry, the flow around the body, especially at the rear, is very complex as it is dominated by a large wake with strong interaction between vortical structures. In the present study, the flow around the 25° Ahmed body has been investigated using Large Eddy Simulations (LES) and compared to high-resolution particle image velocimetry (PIV) measurements. Special emphasis was put on studying three commonly used Sub-Grid Scale (SGS) models and their ability to capture vortical structures around the Ahmed body. The ability of the SGS models to capture the near-wall behavior and small-scale dissipation is crucial for capturing the correct flow field. Excellent agreement between simulations and PIV measurements were seen when using the dynamic Smagorinsky-Lilly and the Wall-Adopting Local Eddy-Viscosity SGS models, respectively. However, the standard Smagorinsky-Lilly model was not able to capture the flow patterns when compared to the PIV measurements due to shortcomings in the near-wall modelling in the standard Smagorinsky-Lilly model, resulting in overpredicted separation.