The first thing you need to understand is the Bernoulli principle. The Bernoulli principle states that if you follow an air particle along its path within a flow, the energy density remains constant.
Using the Bernoulli equation, for the density to remain constant, if one term increases, the other must decrease. In other words, if the velocity increases, the pressure decreases and if velocity decreases, then the pressure increases.
So how does the Bernoulli principle relate to roadcars and racecars? Consider a car moving through air, as it does so, the airflow collides with the front of the car, creating an area of high local pressure. However, at the rear, the resulting turbulent wake effectively pulls the car backwards; creating an area of low local pressure.
Air naturally moves from areas of high pressure to areas of low pressure. Therefore, this air pressure differential between the front and rear of the car, drives airflow underneath the car. Furthermore, as the section between the car’s underbody and the ground narrows, this also helps to accelerate the air further.
This is where the Bernoulli effect comes in. High velocity means low pressure. Consequently, this area of lower pressure creates a suction force which effectively pulls the car towards the ground.
This results in an increase in downforce and therefore grip, which helps to improve the car's overall vehicle dynamics. Yet, this effect can be further exploited by an efficiently designed rear diffuser.
A diffuser is the section of the rear underfloor which has been shaped with an upward angle. As air travels underneath the car, it then expands through this diffuser section. Here, the Venturi effect takes place.
The Venturi effect states that as air is expanded from a narrow cross section to a wider cross section, air velocity decreases and pressure increases. Therefore, the air under the car has high velocity and low pressure. As this air moves through the expansion chamber, created by the diffuser, the velocity then decreases which results in an increase in pressure. Once again, this difference in pressure helps to draw the air out from underneath the car, creating more downforce.
The shape of the diffuser also improves the transition of air from underneath the car to the surrounding air as well as fill the wake.
When designing a diffuser, the main question you should ask is - what should the angle of the diffuser be? If the angle is too shallow, then you will not benefit from the full effect. But if the angle is too high, then flow separation can occur and the air will no longer follow the geometry of the diffuser, losing downforce.
Of course, the optimum diffuser angle will depend on the specific dimensions of your car as well as the relevant aspect ratios and the characteristics of the local airflow. However, a diffuser angle between 7 – 10 degrees is usually a good starting point.
To analyse the performance of your diffuser and discover areas to improve, why not run a 3D model of your car through an AirShaper simulation.