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Before I explain what a rear diffuser does, I should note that the above rear diffuser from subaru is not functional and purely aesthetics.
For a car without a rear diffuser:
- Air hits the front of the car causing a high pressure region in front of the car
- Behind the car there is a low pressure region (think of it like a void)
- Pressure tends to go from high pressure to low pressure
- So basically what happens is that the car is forced backwards because of high pressure region in the front and low pressure region at the rear (this is known as drag)
Car with a rear diffuser:
- Air hits the front of the car -> high pressure region in front of car (as per above)
- This time because we have a rear diffuser, air expands at the bottom rear end of the car where rear diffuser is located
- According to fluid mechanics equations, expanding the air causes the air to slow down
- Air that is slower is associated with higher pressure
- So now we have high pressure region at the front, and high pressure at the rear thereby reducing drag. (high pressure at rear pushes the car forward)
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