Numerical Study of the Flow Around 25° Ahmed Bodies with Hybrid Turbulence Models

摘要

The flow topology around sharped edges and rounded edges Ahmed bodies has been investigated. For the sharp case, DDES model globally shows the best prediction for drag and lift coefficients with respectively errors around -2.5 and 3.8%. The size of the recirculation bubble, equal to 75% compared to 78% experimentally, is also very well predicted with DDES model. SAS seems to suffer in the vicinity of separation zone and acts like RANS models in this area. The higher level of turbulent kinetic energy (not shown here) explains the shorter recirculation bubble obtained with SBES method. For the rounded case, the DDES model shows again the best prediction for both drag and lift coefficient. The Cp values along two planes over the back of the body state that the three turbulence model give very good agreement with experiments. The Cp plots explain the flow topology modification between the two cases. Due to rounded edges at the transition between the roof and the rear slant surface, the flow separation disappears leading to pressure recovery all along the rear surface. The rounded edges on the sides of the body delay the onset of the longitudinal vortices and reduce their intensity. These two major modifications in the flow topology lead to drag and lift reduction. Globally, the main flow structures for both cases have been recovered using Hybrid RANS/LES methods.