Study of the Effect of Vertical Airfoil Endplates on Diffusers in Vehicle Aerodynamics

Abstract

Diffusers and the floor ahead of them create the majority of the downforce a vehicle creates. Outside motorsports, the diffuser is relatively unused, although its interaction with the ground is a consistent field of study owing to the aerodynamic benefits. The diffuser flow behavior is governed by three fluid-mechanical mechanisms: ground interaction, underbody upsweep, and diffuser upsweep. In addition, four different flow regimes appear when varying ride height, the vortices of which have great importance on downforce generation. The present study focuses on the diffuser’s fluid-dynamic characteristics undertaken within an academic framework with the objective of finding and understanding a high level of performance in these elements. Once the functioning of diffusers has been analyzed and understood, a new configuration is proposed: rear vertical airfoil endplates. The aim of the paper is to study the effect in performance of vertical airfoil endplates on diffusers in vehicle aerodynamics in a simplified geometry. The candidate to this geometry is the inversed Ahmed body, a geometry that is used as a model that simulates the flow behavior of car diffusers. Three different diffuser configurations are performed, namely 0° diffuser, 25° diffuser, and in the third case vertically installed rear vertical airfoil endplates are added to the 25° diffuser Ahmed body to change the flow field. These analyses are carried out by using open-source CFD simulation software OpenFOAM. An inlet velocity of 20 m/s is considered, as this is a typical velocity when cornering in motorsport. It is concluded that the 25° diffuser configuration generated more downforce than the 0° diffuser, which makes sense as the aim of adding a diffuser is to increase the amount of downforce produced. In addition, and as a result of the newly proposed configuration, the 25° diffuser Ahmed body with the vertical airfoil endplates emerges in a substantial increase of downforce thanks to the low-pressure zone generated at the back of the body.

Parametric, response surface, and adjoint optimizations of the underbody diffuser of a generic ground vehicle

“…As x does not explicitly appear in Equation (7), Equation (8) should only be computed once for each function of interest hence, the required cost is almost independent of the number of design parameters, computationally. This is advantageous for a 3D optimization process since the number of f is usually less than ten whereas the number of design variables can be greater than ten f .…”

“…This behavior can be anticipated as stated by different investigations of underbody diffusers. 4,5,7 This is explained by the fact that if the angle of the underbody diffuser is greater than the critical value, flow detaches, because of which the separation bubbles form bringing the diffuser to a stall state. As for the downforce, both increments of diffuser angle and length help to reduce C L .…”

“…6 Nevertheless, the effectiveness of an underbody diffuser is so sensitive to design parameters and an underbody diffuser may result in drag rise and stall in some cases. 7 It has been stated in different studies that the diffuser angle should not be too much or too little. Accordingly, if the diffuser angle is too low, the pressure recovery potential of the diffuser may remain low.…”

“…A diffuser provides a space in the back region for the underbody airflow to decelerate which enables momentum transfer to the wake and a substantial pressure recovery at the rear back 6 . Nevertheless, the effectiveness of an underbody diffuser is so sensitive to design parameters and an underbody diffuser may result in drag rise and stall in some cases 7 . It has been stated in different studies that the diffuser angle should not be too much or too little.…”

A Numerical Aerodynamic Analysis on the Effect of Rear Underbody Diffusers on Road Cars

“…In recent years, the easy access to CFD solvers [8] and the generalisation of wind tunnel testing [9] has allowed interesting research and publications. Generalised geometries such as the Ahmed Body have been extensively evaluated.…”