Optimization of bluff body for minimum drag in ground proximity

Abstract

Three-dimensional Navier-Stokes analysis was applied to optimize the rear-end shape of a vehicle-like body in ground proximity. The flow analysis was coupled with an optimization program to find values of the shape parameters (backlight angle, boat-tail angle, and ramp angle) that produce a minimum aerodynamic drag coefficient. The approach of this method is to create a localized quadratic approximation to the objective function, in this case, drag coefficient, in terms of the three shape parameters. Values of the objective function are calculated using Navier-Stokes analysis of proposed optimum geometries; the localized objective function approximation is updated and used to select a new geometry, and the iteration process is repeated until the objective function converges to a minimum value. The optimum design geometry (17.8-deg backlight, 18.9-deg boat tail, 9.2-deg ramp) was obtained after 15 Navier-Stokes analyses. The predicted drag coefficient reduction was 0.10 referenced to the constantcross-section afterbody. It was observed that the optimum afterbody shape minimized the trailing vortices in the wake and also produced near zero afterbody lift force. The experimentally determined optimum, for a similar body, was very flat but fell in the range of 15- -18-deg backlight, 15- -22-deg boat tail, and 9- -14-deg ramp. The measured drag coefficient reduction was 0.13.