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It looks like a ....................Diffuser

Diffuser (automotive - Wikipedia, the free encyclopedia)

A diffuser, in an automotive context, is a shaped section of the car underbody which improves the car's aerodynamic properties by enhancing the transition between the high-velocity airflow underneath the car and the much slower freestream airflow of the ambient 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. 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 drag).

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 on the car and increasing downforce.

The aft part of a car underbody can be a diffuser. It works to bring the low pressure air below the car back to the ambient atmospheric pressure without inducing turbulence. It uses Bernoulli's principle, such that the pressure increases while the velocity decreases. Since the pressure below the car is lower than on the side and above the car, downforce is produced if implemented correctly. Injecting the exhaust into the rear diffuser can also help extract the air from below the car. The exhaust gasses effectively energize the boundary layer, helping to raise the pressure of the low-pressure, fast-moving airstream back to the ambient atmospheric pressure at the exit of the diffuser. This helps extract the air more efficiently from the underbody.

Note that the front of the car slows down the air without a diffuser making this the ideal place for an inlet. Instead, a splitter is commonly used here. The splitter serves to reduce the pressure below the front of the car and thereby increase the amount of downforce in that region. The airstream is brought to stagnation above the splitter, by an air dam, causing an area of high pressure. Below the splitter, air is accelerated, as stated above, which causes the pressure to drop and create downforce. Some race cars, such as the Toyota GT-One actually use a proper diffuser in this area to help create more downforce.[1]

While cars with a smooth underbody use the air flow below the car to increase downforce without much drag, cars with a rough underbody produce too much turbulence for this to work and instead just try to decrease the air flow to minimizing drag. Their blunt nose starts as low as possible above the street, often the lower part is made of elastic rubber or Acrylonitrile butadiene styrene to survive impacts, and stops as low as possible, that is over the bumper the hood and the lights are inclined backwards. In some cars the lights are set back to stay at a legal height. The stagnant higher pressure air in front of the car makes the oncoming airflow bend upwards somewhat, but basically the nose splits the flow into two halves (i.e. 60% / 40% ), one goes over the top and one goes below the car. To appeal to customers some street legal cars have a nose, which is inclined towards the front (sort of anti splitter) and directs air below the car. As the air flow is squeezed below the car it accelerates much more than the flow over the hood leading to very low pressures. The edges of the nose need to be round (and a lot of costumers say "boring") to avoid turbulence. The lower round edge ends below and in front of the oil sump. If the engine and the front axle are high enough, the round edge can even go upwards again to slow the air flow. After all its main purpose is to avoid air going below the car. The air can also flow to the sides of the car. The round side edges happen to be in front of the tires. Here a splitter can work very well, because the air on the upper side can flow relative easily to side of the car. In the end this reduced the drag due to the tires. The edge extents outside the tires because their upper side moves against the air flow and needs a boundary layer of twice the thickness. The disk and the brake within the tire can get a jet of clean air from the front of the car. The inlet to the radiator is placed as low as possible to reduce the air flow below the car. The inlet does not extent into the edges, because the high pressure air would then leak out. A smaller inlet reduces the chance of foreign object ingestion. A not to small inlet is quite efficient and the radiator does not convert all pressure into heat, thus in some cars with front radiator (and more so in aircrafts) after the radiator the air is re-accelerated in a variable nozzle (for example the Bugatti Veyron closes the nozzle at high speeds and in cold air) and emitted below the car to form a layer with intermediate speed between the rough underbody and the fast airflow. These cars can still use a rear diffuser. There the flow is split into the fast part, which is decelerated just as in the case of the flat underbody, and the slow part, which flows through the rear wheels or back to the front of the car. It flows close the underbody, in trenches around the exhaust pipe or the axle. These cars often further employ a fastback and avoid spoilers to increase the pressure in the wake and thus decrease the drag. Without a rear diffuser this higher-pressure air would flow backwards under the car.

Cheers
Michael