Parametric, response surface, and adjoint optimizations of the underbody diffuser of a generic ground vehicle

Abstract

It is a known fact in the literature that diffusers improve vehicle aerodynamics in terms of lift and drag forces. However, their effectiveness and efficiencies are quite sensitive to the design parameters. In this article, the underbody diffuser of a generic ground vehicle, Ahmed body, was optimized via CFD software of Ansys Fluent, step by step with parametric, response surface, and discrete adjoint techniques, respectively. Three numerical models (parametric, response surface, and adjoint models) have been created considering optimum force output for each optimization method, and qualitative and quantitative assessments were made on these models to compare flow characteristics and force coefficients with the base model. As a result, force coefficients are lower by 48.6, 49.3, and 52.8 counts for C-D, 499.6, 547.4, and 528.2 counts for C-L at parametric, response surface, and adjoint optimization phases compared with the base model, respectively. It is observed that diffusers help to improve flow transition and relief underbody and reduce the airflow over the upperbody zone by directing it to the underbody. However, detailed quantitative assessments show that it comes out that the contribution of the underbody to total C-D may increase due to lower pressure distribution diffuser upsweep caused by higher mass flow beneath the model. These results demonstrate the importance of effective utilization and further controlling of underbody flow when using an underbody diffuser.