A diffuser is an aerodynamic device primarily used on automobiles.
A diffuser, in an automotive context, is usually a shaped section of the Car underbody which improves the car's Automotive aerodynamics properties by enhancing the transition between the high-velocity airflow underneath the car and the much slower freestream airflow of the ambient Earth's atmosphere. It works by providing a space for the underbody airflow to decelerate and expand so that the boundary between the car's airflow and "external" airflow is less turbulent, and it also provides a degree of "Wake infill" (the wake being a turbulent area of low pressure that is caused by the passage of the vehicle through the air; this can cause Pressure).
As the air enters towards the front of the car it accelerates and reduces Pressure. There is a second suction peak at the transition of the flat bottom and diffuser. The diffuser then eases this "high velocity" air back to normal velocity and also helps fill in the area behind the car making the whole underbody a more efficient Downforce producing device by reducing Drag (physics) on the car and increasing downforce.
The aft part of a car underbody can be a Diffuser. It tries to connect the underbody to the back without producing turbulents so that Bernoulli's principle applies and the pressure increases while the velocity decreases. The side and the roof end in a sharp edge, so that their pressure does not increase. Because the pressure in the back tends to equilibrate, the pressure below the car is lower than on the side and the roof of the car.
Note that the front of the car slows down the air without a diffuser making this the ideal place for an inlet. A splitter separates the flow into the upper path, which will increase pressure in the diffuser and the lower path which stays as low pressure or even reduce pressure as the combination of an up looking splitter and the street acts as a Venturi nozzle. The height of the splitter is as low as possible without hitting small bumps, for street legal cars there is a minimum of about 100 mm. But a diffuser reduces the pressure in front of the car and thereby the amount of air pressed below the car. Sometimes a diffuser is used to let the wheel-casing also pump down the pressure below the car. Many small engine-exhausts in the back also help pumping. The exhaust from the engine compartment in the underbody can blows the air outwards by means of convergent nozzles, which reduce the pressure from the high pressure in the compartment to the low pressure below the car (the nozzle is the inverse of the diffuser).
- Venturi effect
- Racing car
- Ground effect in cars
- Cold air intake
- Ram-air intake