Numerical Study of the Impact of a Curved Ground-effect Diffuser on the Aerodynamic Performance of a Bluff Body

This study numerically investigates the aerodynamic performance of several curved ground-effect diffuser geometries integrated into an inverted Ahmed body. The proposed diffuser configurations were designed to reduce abrupt transverse flow-direction changes inside the diffuser and improve aerodynamic efficiency through enhanced pressure recovery and controlled vortex development. Steady Reynolds-averaged Navier–Stokes simulations were performed using the SST turbulence model. The numerical methodology was validated against experimental pressure-coefficient data reported in the literature, showing a discrepancy of approximately 3.79%. Several diffuser geometries, including triangular, elliptical, curved-elliptical, and double-elliptical configurations, were analyzed for two diffuser-length ratios. The flow behavior was evaluated through pressure distributions, wake development, and the formation of longitudinal counter-rotating vortices generated along the diffuser edges. The results indicate that diffuser performance depends strongly on the interaction between pressure recovery and vortex development. The double-elliptical and elliptical diffusers exhibited the best aerodynamic performance by promoting compact and coherent vortex structures, stronger underbody suction, and reduced wake growth. In contrast, the triangular diffuser generated complex vortex interactions and larger wake disturbances, resulting in poorer aerodynamic efficiency. For the largest diffuser-length ratio, the proposed diffuser geometries improved the downforce-to-drag ratio by up to 18% relative to the benchmark configuration. These results suggest that curved diffuser geometries can significantly improve the downforce-to-drag ratio without appreciably increasing drag, providing practical guidance for the design of underbody diffusers in racing and street cars.