Racecar Engineering goes back to basics to look at the function of diffusers.

Given the current controversy over the design of the Brawn GP, Toyota and Williams diffusers Racecar Engineering decided it was time to return to the basics of racecar aerodynamics. This will allow us to better understand exactly why their designs are more effective.

Let’s start with the primary function and operation of a diffuser;

‘The key role of the diffuser on a modern racecar is to accelerate the flow of air under the car, creating an area of low pressure, thus increasing downforce.’

What this actually means in real world terms is that the shaped piece of bdywork at the rear of say an F1 car, draws the air out from under the car. This literally sucks the car to onto the track creating much higher grip levels than would otherwise be available simply through the tyres and suspension setup. This is known as aerodynamic grip.

The air passing under the wing has further to travel than the air passing over the top surface. This causes the air under the wing to accelerate, resulting in a drop in air pressure, this creates a difference in pressure between the upper and lower surfaces. This difference essentially means the wing is pushed down by the higer pressure above, generating what is known as downforce.

An extreme application of this theory can be seen on the Chaparral 2J car above. A pair of fans on the rear of the car sucked the air from under the floor, pulling it onto the road, rather like a reversed hovercraft.

With this in mind, the role of the diffuser on a racing car is to speed the airflow up underneath the car, reducing its pressure, creating a greater difference in pressure between the upper and lower surfaces of the car. This means more downforce and aerodyamic grip, allowing the car to corner faster.

Now that we understand the basics of downforce generation we can look at the more detailed operation of a diffuser, and why they have their distinctive form.

The diffuser increases in volume along its length, creating a void that has to be filled by the air passing under the body. This venturi effect means that the flow is accelerated through the throat of the diffuser, creating the desired low pressure, then gradually returned to the same velocity at which it joined the wake (See Fig 1).The angle or slope of the diffuser is also important, the diffuser must have a gradual change of angle to prevent flow separation from its roof and sides. (McBeath, 1998, Competition Car Downforce)

Fig 1

The addition of the vertical ‘fences’ to a diffuser help to optimise the diffusers efficiency by ensuring that the air is only drawn only from the underbody and does not spill in from the upper body surfaces.
Fig 2

Fig 2 shows the pressure coefficient of a generic diffuser design, with blue reperesenting lowest pressure areas and red highest pressure. This clearly illustrates the reduction in pressure at the throat area as the velocity increases and the subsequent reduction in pressure for the underfloor as the diffuser sucks the car to the ground.(Image kindly provided by Symscape, whose software, SymLab and its add-ons combine to form an easy-to-use CAE software system can help you assess the performance of your 3D model.)

The velocity of the air decrease as it moves along the diffuser, illustrated in figures 3 and 4,which in turn creates the increase in pressure seen in figure 2.
Fig 3

Figures 3 and 4 illustrate the flow velocity of the air under the body with red showing high velocity and blue showing low.
Fig 4

(Images courtesy Tudor Mirron and Sports Racer Network)

So why are the ‘Double Decker Diffusers’ more efficient?
Simple; They have a greater volume than the single layer designs and thus draw more air from the underbody increasing downforce levels.

The aim of the 2009 regulations was to limit the volume of diffusers, by constricting their height and width. Last years devices were much longer and taller that allowed in 2009, the problem being that the turbulence they created had a detrimental effect on the downforce levels of following cars.

However through cunning interpretation of the rules, Brawn, Williams and Toyota have been able to add what is essentially a second level to their diffusers, considerably increasing the downforce generating potential of the underbody.