The slipstreaming effect

Slipstreaming (or drafting), is a well-recognised overtaking strategy in motorsport racing in which the following car will pursue directly behind the lead car in order to gain a speed advantage. It is also an effective strategy in qualifying whereby teammates will cooperate to create a ‘tow’ as a way of reducing lap times.  In this article, we will attempt to explain the precise aerodynamic mechanisms of slipstreaming and introduce key terminology used in industry.

The strict definition of stagnation pressure is beyond the scope of the article, but in the context of race car aerodynamics, generally refers to the maximum static pressures generated at stagnation points, locations in which a moving fluid has been brought to rest (i.e. zero velocity).

“As a way to better understand stagnation and slipstreaming, let us consider that instead of the car moving through air, the air moves around the car just as in a wind tunnel.”

Figure 1. Flow visualisation showing the normalised fluid velocity close to the surface of the vehicle. Note where the ‘streamlines’ appear to split (e.g. front of the nose) which coincide with the stagnation points.

At the stagnation point, the static pressure is at local maximum since all of the kinetic energy has been converted. This increase in static pressure of the fluid, exerts the resistive force that is ‘pushing you back’ and hence contributes to the aerodynamic drag. If we can limit the increase in static pressure (essentially reduce the stagnation pressure), we can lower the aerodynamic drag.

Figure 2. An overtaking scenario modelled in CFD by Catesby Projects visualising the static pressure changes around the lead and following car. Note the reduction in static pressure acting on the following car which helps to reduce aerodynamic drag.

Figure 4. Following Car static pressure distribution

But how can we do this? In part 1 of the following car article series, we explain that an aerodynamic wake is generated through boundary layer separations and that this region of disturbed airflow is associated with low total pressures (i.e. low velocity and low static pressure). Where there is less total pressure available, there is a reduction in stagnation pressure since a) the static pressure is ‘low’ to begin with and b) less kinetic energy is converted to raise the static pressure. Therefore, the driver in the following car can strategically position themselves in the aerodynamic wake shed by the lead car in order to reduce the stagnation pressures, and therefore aerodynamic drag.