Abstract
This article presents the results of an experimental investigation into the impact of rake, or inclination of the underfloor, on the aerodynamics of a bluff body equipped with an underbody diffuser. An extensive wind tunnel campaign, utilising a remotely-actuated model for faster data acquisition, showed that introducing rake results in a downforce increase at all ride heights and diffuser angles, with the strongest effect occurring at low ride heights. Surface pressure measurements on the underbody revealed this to be caused by three main effects. Firstly, a large increase in loading at the front of the floor, due to the inclination of the floor with rake angle and subsequently an increase in the pressure pumping effect. Secondly, a reduction in the suction peak at the throat of the diffuser, which leads to reduced pressure recovery in the diffuser, and less likely separation at high diffuser angles or low ride heights. Thirdly, stronger streamwise vortices along the edges of the underfloor and diffuser, which generate downforce directly due to their low-pressure cores, but also introduce upwash under the model, further inhibiting separation in the diffuser. As the related drag penalty is minimal, aerodynamic efficiency is also improved with increasing rake angle.
