Sensitivity analysis of hybrid methods for the flow around the ahmed body with application to passive control with rounded edges

Abstract

A comparative study of SST RANS and three hybrid approaches (SAS, DDES and SBES methods) is performed to analyse the flow around the 25∘ Ahmed body. The DDES approach is then selected as the reference model, based on comparison of numerical results. The main study is based on grid and time step influence over the simulation results. All results are compared to in-house experiments. The three grids 15l, 20l and 30l (related to the number of prism layers in the boundary layer) show a fair prediction for both drag Cd and lift Cl coefficients. The best prediction is obtained with the 30l grid with an accurate prediction of Cd and Cl values as well as the main flow characteristics, compared to experiments. Another part of this paper focuses on the time step effect on the computational accuracy. Then, the flow around various rounded edge Ahmed body configurations is compared to experiments in order to validate the robustness of the numerical methodology. The rounded edge flow topologies are therefore deeply investigated. It is shown that they act as passive flow control and significantly influence the flow topology and aerodynamical coefficients. In fact, the rounded strategy modifies the flow detachment and weakens the onset of C-pillar vortices, which directly reduce drag and lift coefficients.