It is well documented that when a Formula 1 car attempts to overtake a competitor, the ‘dirty air’ of the lead car will normally result in the following car losing a significant proportion of its aerodynamic downforce. Numerous studies have been conducted to quantify the performance losses but the precise mechanisms to explain these changes are often overlooked.

Common phrases such as ‘dirty’ or ‘turbulent’ air are often thrown around since fuller explanations generally require specialist aerodynamic knowledge. In this brief article, we attempt to introduce some key aerodynamic terms commonly used in industry to help provide greater insight. So firstly, what exactly is ‘dirty air’ and why is it so influential in reducing the downforce of the overtaking car?

“The way in which a vehicle disturbs the air it passes through heavily impacts the amount of  downforce a following car can generate, in turn affecting its overtaking potential”

An overtaking scenario modelled in CFD by Catesby Projects visualising the ‘dirty air’ generated by the lead car.

Firstly, dirty air is just an alternative term for aerodynamic wake – this is the region of disturbed air generated as a vehicle passes through the stationary air itself. However, if we change our frame of reference, we can consider the vehicle as stationary and the air particles moving instead (this is the premise of a wind tunnel). The amount of energy associated with a moving fluid (in this case the air) can be quantified with a variable known as the total pressure coefficient (CpT).

It is the sum of static and dynamic pressure, relative to the freestream flow (the moving but undisturbed air). As the fluid passes around the lead car, numerous aerodynamic mechanisms occur which act to reduce the total pressure coefficient downstream (i.e. the amount of energy available to the following car). A fundamental mechanism is the growth and subsequent detachment of the boundary layer – a thin region of flow adjacent to the surface in which viscous effects dominate and frictional losses occur.

An overtaking scenario modelled in CFD by Catesby Projects visualising the ‘dirty air’ generated by the lead car.

Wake structures are formed when the boundary layer detaches from the surface of the vehicle leading to unsteady, low pressure, low velocity regions. The reduced flow energies associated with the wake mixes with the freestream flow thus lowering the average energy received by the following car. A lower energy flow is less capable of keeping the boundary layer attached to a curved surface (which is fundamental in downforce generation).

Thus, aerodynamic components that are optimised to work in undisturbed flow, suddenly find themselves in sub-optimal conditions.

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