Aerodynamic analysis of a modified Ahmed Body: geometry effects on flow separation

This Master’s thesis focuses on the aerodynamic study of a modified version of the Ahmed Body benchmark. The primary objective is to investigate the relationship between geometry, adverse pressure gradient (APG), and flow separation. To achieve this goal, several geometries are analyzed, starting from the standard Ahmed Body configuration and modifying the rear slant with curved profiles represented by polynomial functions. All numerical simulations are conducted using the open-source CFD software OpenFOAM. The first part of the thesis discusses the theoretical aspects of aerodynamics and the computational methodology underlying the project. Subsequently, the study addresses the standard Ahmed Body with slant angles of 25° and 35°; this phase is dedicated to the validation of the numerical model. This is accomplished by comparing simulation results with experimental data available in the literature to ensure the reliability of the computational setup. The core of the research involves the analysis of the modified Ahmed body with different curved rear ends. Specifically, two distinct polynomials (related to a "more concave" profile and a "less concave" one) are selected for each investigated angle, ranging from 35° to 65° with a 5° step. To validate the separation points obtained from these simulations, the results are compared with a theoretical method for the prediction of turbulent flow separation, the Stratford’s criterion. Furthermore, the overall robustness of the study is substantiated by a grid independence study. Comparisons between the baseline mesh and a refined grid utilizing a wall-resolving approach demonstrate that the numerical results are independent of spatial discretization. Additionally, an investigation assesses the influence of moving ground versus fixed ground conditions on flow separation. In conclusion, the results from the various simulations are synthesized to provide a comprehensive understanding of the link between geometry, APG development, and the resulting flow separation.