Aerodynamics and Experimental Optimisation of Automotive Underbody Diffusers in the Presence of Rake

Abstract

This research project was focused on two related topics—hardware-in-the-loop aero- dynamic optimisation, and aerodynamics of automotive underbody diffusers in the presence of rake, defined as an inclination of the underfloor with respect to the ground. Two experimental systems were used for automatic, closed-loop optimisation trials, and for mapping of aerodynamic performance. Each consisted of an Ahmed-type body with a diffuser, with three controlled degrees of freedom, i.e. the model’s height above the ground, and inclinations of the underfloor and diffuser plates. The systems were equipped with force acquisition for optimisation and performance quantification purposes, and with surface pressure measurements to inspect the underlying flow patterns.The high-speed system was used for real-time optimisation runs using a range of algorithms in order to determine their suitability to problems of this type. Population-based algorithms, and genetic algorithms in particular, were found to provide the most reliable convergence in spite of the noise and hysteresis in the measurements. Reductions in pre-sampling delay and sampling time decreased the average function evaluation time without negatively impacting convergence performance, whereas combinatorial optimisation was used to minimise actuation overheads. Subsequently, both methods were shown to improve overall optimisation efficiency during experimental trials.Finally, the impact of rake on diffuser aerodynamics was investigated through quasi- static variations of the three degrees of freedom. Introducing rake was found to induce significant pressure recovery beneath the underfloor, causing strong suction under the front of the body and increased downforce. Furthermore, two counter-rotating vortices were observed along the edges of the underfloor, whose formation and strength depended on the configuration of the model, and which significantly affected the stall characteristics of the diffuser