On the Relationship between the Vortices from an Underbody Diffuser in Ground-Effect and the Resulting Downforce

This study numerically investigates the vortex flow that is generated within the underbody diffuser of a bluff body. Previous experimental studies have identified the existence of three main types of trailing vortex structures that correlate with the aerodynamic behavior of an underbody diffuser in ground-effect. The “force enhancement” behavior of the underbody diffuser results in a pair of longitudinal counter-rotating vortices that are generated off of the sidewalls within the upswept section of the underbody diffuser. By comparing the aerodynamic forces with the resulting flow field induced by the trailing vortex pair, this investigation aims to identify a relationship between the circulation of the vortices and the resulting downforce. It is hypothesized that the circulation generation of the vortex pair is directly linked to the generation of the downforce, just as with a wing. The bluff body features an elliptical nose, a straight midsection, and a thin-walled underbody diffuser with an upswept angle of 17 degrees. The flow is simulated using the opensource computational fluid dynamics solver OpenFOAM, which solves the Reynolds Averaged Navier-Stokes equations with a turbulence model and wall functions using the SIMPLE algorithm. The ride height of the bluff body is varied while the flow Reynolds number based on the length of the body is kept constant at 1.68 × 10⁶. It is shown that during the “force enhancement” behavior of the lift curve, the lift coefficient is linearly related to the circulation of the counter-rotating vortex pair. This trend is shown at multiple axial locations within the diffuser flow. Moreover, the circulation of the vortex pair increases with axial position within the diffuser. Downstream of the diffuser exit plane the circulation of the trailing vortex pair begins to decrease.