Aerodynamic Effects of the Salient Flow Features in Grand Prix Car Wakes

Abstract

Grand Prix cars are the fastest circuit racing cars in production, a large part of this is due to the high downforce generated by the car's aerodynamic surfaces, in excess of the car's own weight above 150kph. It is well known that a race-car operating in the wake of an upstream vehicle experiences a reduction of aerodynamic drag, and a corresponding increase of ultimate straight line speed. There is also a loss of aerodynamic downforce, predominately from surfaces acting on the front axle. The effect of the reduced downforce is an increase of lap-time and degraded handling characteristics, thereby reducing tyre life and the ability to follow the lead car or affect an overtake.

The wake of a generic Formula 1 car is shown to be characterized by a counter-rotating vortex pair, with centreline up-wash and a region of total pressure deficit, which is predominately a dynamic pressure deficit, with Cpo < 0. The streamwise vorticity is dominated by the tip vortex pair emanating from the rear wing, which merges with other vortices, forming a coherent structure by just half a car length behind the rear of the car. The vortices have an influence on the location and strength of the total pressure deficit, sweeping the loss to the centreline, and upwards to surround the vortex cores, forming a 'mushroom' shaped wake.

The effect of an upstream vehicle wake has been measured in the wind tunnel and computationally, with downforce and drag losses of up to 67% and 29% respectively. The use of a short axial length bluff-bodied wake generator allows for a longer axial separation to be achieved with a complete downstream vehicle, in a conventional length wind tunnel working section, without further compromising the downstream model scale.

The sensitivity of the downstream car to the various salient flow features in the upstream wake have been investigated using the method of imposing the wake on the inlet of a CFD simulation. Imposing the wake has meant that the wake can be altered without the need to modify the upstream vehicle surfaces. The key wake feature has been shown to be the axial velocity deficit, which accounts for up to 90% of the downforce loss experienced by the following vehicle. While secondary flows in the wake do result in downforce loss for the following vehicle, they are also beneficial in diverting the dynamic pressure deficit over the following vehicle, thereby introducing higher energy flow onto the following vehicle.