I'm posting this question here b/c I'm looking for a more rigorous technical explanation on the function of diffusers than I'm likely to find in the more general forums. I'm not referring to the S2000...obviously the "diffuser" in this case is a vestigial, cosmetic thing now. I'm talking about dramatic ones apparent on F1 cars, the Enzo, etc.

Now, when I look at pictures of a diffuser, it appears that they gradually increase in area towards the rear of the car. This seems to be in keeping with diffusers in aircraft engines, which also are of increasing area. As a result, they decelerate the airflow, thus raising it's pressure. I guess the root of my question is: doesn't a diffuser right at the back of the car interfere with the downforce generated by wings, etc? I mean, high pressure right under the back of the car is a bad thing, right? I've seen a number of people state that a diffuser helps downforce, and that statement just doesn't hold water for me. The only conclusion I can come up with then is that the real function of the diffuser is really to get the low pressure air under the car back to ambient conditions without excess separation/turbulence. If nasty, low-pressure, oscillating vortices can be prevented, stability in the rear is increased and pressure drag is decreased. In essence, the diffuser is not intended to favorably alter the vertical pressure component (it in fact does the opposite), but it does favorably alter the horizontal component...the pressure at the front and back of the car are roughly the same; hence less pressure drag.

Is that an accurate assessment of a diffuser's main function?

 

Two starting points:
http://www.mulsannescorner.com/diffuser.htm
http://corner-carvers.com/forums/showthrea...hlight=diffuser

 

Bleah, I'm still not quite getting it. The plot on the Mulsanne site indicates a large peak in negative pressure right at the entrance to the diffuser. What causes this? Is there some vortex generation there or something? I get the impression from that site and the other discussions that perhaps some low-pressure vortices are generated at the start of the diffuser, then the rest of the diffuser "eases" that air back to ambient as smoothly as possible. Am I getting warmer?

 

Made some progress through additional research: apparently, the pressure drop is cause by the increase in volume in the diffuser. Boyle's Law -- pressure * volume = constant. I'm still trying to figure out how this effect ties in with the expansion/deceleration of an airflow though. Seems like there's two opposite effects being caused by the same physical parameter (area change).

 

Mass conservation is the key. There isn't really a lot of room to get additional air into the diffuser as the volume expands, so therefore the air must be at a lower pressure.

UL

 

That would seem to imply a choked condition somewhere under the car...? Maybe there's some complexity due to a moving boundary thrown in here...

And UL, you should change your name...a PPD of 2.5 makes you a lurker no more

 

The diffuser accelerates the flow of air - there's a greater distance to travel along the path of the diffuser compared to the path along the ground. Again, just like an inverted airplane wing. Hence the lower pressure region under the car, hence the downforce that it generates.

 

[QUOTE]Originally posted by Elistan
[B]The diffuser accelerates the flow of air - there's a greater distance to travel along the path of the diffuser compared to the path along the ground.

 

[QUOTE]Originally posted by Elistan
[B]The diffuser accelerates the flow of air - there's a greater distance to travel along the path of the diffuser compared to the path along the ground.

 

Tedow, I don't know if you noticed it when you were here or not but mixed in with the spare wings and other racecar bits taking up space in my garage is a rather large diffuser. The last owner of the car never mounted it and I haven't tried either. You're making me wonder if I should try it out at Roebling Road. I don't think I'll need more downforce since I'm going to dial some out of the rear wing for a higher top speed. I am thinking the diffuser might help by smoothing the airflow under the car. Just like with most things, I don't have a clue.