Diffuser question,

[ro284]

What does it do for the performance of the car? i know they look nice, but do they do anything? i noticed on an MR2 there is a small black thing under rear bumper, but it\'s more like a grill, a line of slots, which i thought would diffuse airflow, or am i looking at it the wrong way, but with no slots or air vents (Re-Am street ) what\'s it do, how does it work?
Cheers

[matt p]

think it has something to do with aiding downforce at high speeds mate.

[Jit]

A diffuser is a section of gradually increasing area at the back end of the car. Its purpose is to create a low pressure area at the underside of the car.
If you take a point say on the underside...call it for arguments sake point X and a point at the rear at the diffuser section and call it point Y.

Then apply the continuity equation:

density(X) x Velocity(X) x Area(X) = density(Y) x Velocity(Y) x Area(Y)

We can assume that density stays constant.

then if you re arrange that

Velocity(X) = Velocity(Y) x Area(Y) / Area(X)

now we know that Area(X) is less than Area (Y) from the definition of a diffuser (gradually increasing area..Y is further back than X) which when applied to that equation above means that Velocity(X) is greater than Velocity(Y)

Apply Bernoullis equation to that.

You got     P(X) + (1/2 x Density x V(X))   =   P(Y) + (1/2 x Density x V(Y))

Which when you stick in the terms whould tell you that P(X) is lower than P(Y)

And that lower pressure at the underside essentially sucks the car to the floor...

god im bored

[Jit]

cant we just say it looks pretty lol...

edit..i think what ive written above is right...someone care to proof read it for me

[weejimmy]

is a diffuser like the re street not more to do with stabilising it at high speed. like the evo fq400 has on its roof type jobber?

[marco]

whilst jit is totaly correct in what he has said, this phenomenon comes from the flow of air under the car, which is why careful design of the under body on the car can aid down force.  for an incompressible flow (which means subsonic) the stagnation pressure is going to be a constant. this stagnation pressure can be though of to comprise 2 parts, static pressure, and dynamic pressure (ignoring height differences as they are negligible in this case). at rest
stagnation pressure =static pressure
and so as the velocity is increased the static pressure will reduce. therefore in the case of the car, the upstream stagnation pressure will be constant, and so if you can get the air to flow faster under the car than over the top, there will be a difference in the static pressures above and below the car, which will give downforce.

the trick is to get the air to flow faster. for this it wants the lowest resistance path possible as other wise the air will slow down and also create drag. this is where a diffuser comes in. fluid in general do not like sudden changes in area/velocity/pressure (which are linked as can be seen in the equations above by jit) the diffuser is there to help guide the air smoothly from the underside of the car to rejoin the airflow from above the car. this is to help mainatian the ari velocity under the car (to give downforce) and also reduce turbulence arising from the sudden expansion in area (which would increase drag).

thats the theory. in practice i doubt the rear diffusers on our cars actually do much as the car does not have a smooth underbody and so i doubt the air flow under the car is that great.

[Jit]

yup...sorry hud have mentioned thats for race cars...i doubt any street diffusers do SFA