Diffuser Myths Begone

[reflexx]

It seems all kinds of people are slapping diffusers, canards, slats, etc on their car. If you're doing it for looks, that's one thing. Here's the scoop on what it is, what it does, and whether or not you need it.

The Venturi Principle
This principle states that as a fluid (which air is) flows through a constricted area, its velocity increases. Think of putting your finger over a water hose.

The Bernoulli Principle
As a fluid's speed increases, pressure decreases.






Application in Race Cars

Venturi Tunnel
The car has a shaped underbody. The first third of the car underside slopes towards the ground, the middle third is flat, and the last third slopes away from the ground. The front compression section accelerates the air by constricting it (nozzle effect). The middle part contains the air at a rapid speed and low pressure, creating suction. The final part (diffuser) slows the air back down to ambient.

Flat Underbody Plus Air Dam
The car has a flat underbody. The car also has a low front air dam. The air dam forces incoming air under the car, which then flows at a high speed/low pressure over the flat underbody. The diffuser section at the end slows the air back to ambient.






Factors essential to venturi principle operation

1. Measurement B must be A/3 or less. This translates to about 1-2 inches for a low-sprung race car.
2. The tunnel must be adequately shielded from spilling out on the sides. This means, fairly close to the center of the car.
3. The compression and diffusion section must be at pitched 17 degrees or less, otherwise the flow will detach and become turbulent.
4. For angle T to be 17 degrees or less, and C to be B*3, F must be at least 1/3 the length of the car.


What purpose does the diffuser serve?
The diffuser decelerates the air under the car and returns it to ambient speed and pressure. If the air isn't decelerated, it continues at a negative pressure past the end of the car. This causes drag.


Is this practical in my street car?

Probably not. In order to achieve a low enough center section ("B"), the car would need incredibly harsh suspension to control the ride height. This would be most unpleasant, and the amount of normal body roll may ground out parts of the car around corners.
The center section, if it generates enough suction to make a difference, will probably be ripped from the bottom of the car, unless it's built as an integral component and/or welded on. Then, you'll have a real fun time working on your car - with the bottom welded shut! If you hit anything or went over a speedbump, your painstakingly-attached carbon or aluminum underbody work would probably shatter or gash.

The diffuser's job is to slow down and increase the pressure of the air. If the air is already moving at ambient, slowing it down below ambient will immediately cause drag.



Does the diffuser I saw for sale help me?
Unless you have bodywork which accelerates air to faster-than-ambient, probably not. Unless the diffuser is a real 10-17 deg diffuser, probably not. Unless the lowest point on your car is 2 inches off the ground, probably not.


Here is an example of a real diffuser:


Here is an example of a gimmick diffuser:

[PHOENIX]

Wow, Very impressive write-up, i had no idea!

Please share more knowledge with us

[Mister2.2]

Great write up, it's something I didn't know a lot about, thank you!

[trdny]

reflexx I'm glad you're sharing your knowledge with us. i do have some questions that I haven't found the answers to yet.

1) For the tunnels to be effective is there a % of the cars width it should be? ALso since we are increasing velocity would 3 1" tunnels would be better than 1 3"? Also should the top of the tunnel be B+ as much as possible?

2)Which "type" of diffuser is better? The curved up to the bumber type or the straight back like F1 cars (assuming they both were done correctly)?

[Raptor13x]

Very interesting information... thanks for the post!

[reflexx]

1) The tunnel should be as wide as possible. However, two factors limit the width.
a) in F1, you have a bunch of crap in the middle of the car - the driver pod, engine, etc. This limits how close to the center the two tunnels can come.
b) air spillage. If the tunnels are too close to the outside of the car, the air spills out and causes vorticity. This is bad. F1 guys solved this by using sliding skirts (like the rubber piece on the bottom of a house door), and making full-width tunnels.

Designers put vertical slats in the tunnel to keep the flow laminar and moving upwards along the tunnel contour, instead of sideways. Making multiple tunnels (3) is basically the same general thing. The slats can be increased up to a practical limit of maybe 1-2" spacing between them, wherein a slot/edge effect actually impedes air going into the slots.


2) the f1 ones are actually curved. The curvature helps the tunnel get higher in the bumper faster, so therefore is a good thing if you have a limited amount of room for the diffuser section. Obviously, this curvature has an upper limit, beyond that and the air detaches. A gentle curve like in the F1 picture I provided is generally a good compromise.


I'm working on a similar write-up for the aerodynamics of wings, turbo wheels, and a few other things. Thanks for the positive commentary!

-D

[trdny]

this info is great. keep it coming

[trdny]

Do you think it would be worth while to put a small "lip" behind the front wheels to help with keeping the flow from spilling under the car? I'm thinking like a dive plane but with a little curve.

[reflexx]

if you can get away with creating ridges or something that go from the front of the car to the back, creating a division between the middle 'tunnel' and the outside of the car.... maybe running a foot inside the edge of the car just inside the wheel-well line, yeah, that would help//

[Mister2.2]

Originally Posted by reflexx

I'm working on a similar write-up for the aerodynamics of wings, turbo wheels, and a few other things. Thanks for the positive commentary!

That should be interesting.

[trdny]

http://www.f1-country.com/f1-enginee...odynamics.html

[arashi858]

That's cool

[Dance_Business]

Originally Posted by reflexx

if you can get away with creating ridges or something that go from the front of the car to the back, creating a division between the middle 'tunnel' and the outside of the car.... maybe running a foot inside the edge of the car just inside the wheel-well line, yeah, that would help//

Any chance you can put up an illustration of this?

[reflexx]

red are the 'ridges'
blue is the venturi profile/aluminum underbodywork

[trdny]

I just sent you an email. wanna help with actual design

[Dance_Business]

What happens if you only have one or the other?

[reflexx]

one or the other what?

[G3aR]

great stuff man, i see that yr new to the boards. WELCOME! I think we are all going to love having you here. This write-up was absolutely fantastic. Cant wait for the next one.

[reflexx]

i will try to get one out this weekend while i am drugged and unable to work on my car due to wisdom teeth removal on saturday.

[feral4mr2]

would the 'gimmick' rear diffuser i made for the feral serve any purpose as a diffuser once i ghetto rig a flat bottom etc..?

hrmm.. for some reason i cannot get onto my imageshack site to get thumbnail links.
there are some pics of it on here, though not real close up.
http://www.sounddomain.com/ride/237058/4