<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by CameronBeaton:
There is actually a pressure loss in the system, meaning the pressure at the inlet is higher that the exit. (my understanding is it goes high-low-high) </div></BLOCKQUOTE>
This is true because with any viscid and real gas there is irrecoverable pressure loss. Other than restricting mass flow, this is one effect that engine restrictors have. Reducing this pressure loss is something restrictor designers should look at, since you can change this pressure loss using different geometries.
However, this is not the reason that air flows through a diffuser under a car. That is to say a perfect inviscid gas would still flow through the diffuser without a net pressure drop in the system.
Even with a real gas, where do you define the beginning and end of your streamline? It is of course an arbitrary point. You could define the beginning of the streamline 100m in front of the car and 200m behind the car and the static pressure and velocity relative to the car would be the exact same. Similarly, you could say the car is in a closed loop windtunnel, i.e. a volume of gas can take a complete revolution and arrive at the same place with the same pressure and velocity. So why does the air flow at all?
Of course you would say "duh because the car is moving" or the air is moving. There's the answer right there.
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">
Originally posted by Jan_Dressler:
It is indeed a legitimate question, and a very interesting and sometimes controversial one... Bernoulli might not always be enough.
But the thread opener might have looked up "Diffuser (automotive)" in Wikipedia, then read the article about "Lift (force)", and then maybe some research in forums like f1technical before asking that question here... </div></BLOCKQUOTE>
The answer is not controversial, in fact it is as basic as physics gets, and Bernoulli's equation is more than enough. You need to understand Bernoulli's equation and be able to break it down into its conceptual components. That entails some basic knowledge of high school physics, mainly the relationship between velocity, acceleration, and position.
If an object is moving at velocity v, it will continue moving at velocity v unless acted on by another force.
Pressure can be defined as force/area. Let's say pressure P exerts a force F on an object of mass m moving at velocity v. And lets say the pressure changes with distance traveled. If pressure changes as a function of distance (ie a pressure gradient), and let's say it varies linearly, then velocity of the object will change with a v^2 relationship (you can do the calculus). Herein lies the connection with Bernoulli's equation. The object we are talking about is a volume of gas, and the pressure gradient is, well the pressure gradient along the streamline.
See the reason the gas travels through the diffuser at an adverse pressure gradient is because it has velocity wrt the car. Just as any object can continue moving in the same direction with a force applied in the opposite direction, so can air. The air starts at static atmospheric pressure and velocity equal in magnitude to the car's velocity, then as it must pass through a smaller area, the pressure drops/velocity increases (pressure gradient is such that it speeds up the air) then the air emerges from the diffuser. The pressure gradient from the throat to the exit is an adverse one, and the pressure slows down the air (the adverse pressure gradient does not mean the air must flow backwards, it just means the air slows down because the gas pressure exerts a force opposite to the direction of travel).
Not controversial, Newtonian physics. Perhaps interesting, but there are bigger fish to fry. Still, understanding the basic physics is important if you want to design a good diffuser. Understanding allows you to break free from copying. Most diffusers I see have poorly designed entrances for FSAE, basically because they copy F1 or some other racing series which has restrictions on diffuser rules. When net downforce and high cl's are important (as in FSAE), you can modify the nozzle design to help achieve that goal.
@manifold:
There are still a few posts on the forums that are interesting. Take the recent tilt table post for instance, I think everyone can learn something from that. I find it very interesting. All forums have nonsense in them, you just have to sort through it I guess...