Bumper Vent Having Negative Effects on Rear Diffuser?

[tiale11]

MY UNDERSTANDING

The way I understand rear diffusers is that it allows you to slow down exiting air from under the car by quickly expanding it. The closer in velocity the exiting air is to the outside air, the less turbulence there will be behind the car thus, resulting in less drag and better air flow. The improved air flow will also allow air to move faster under the car creating a lower pressure (more downforce).

TO THE POINT

Now, I have seen some vehicles having rear diffusers but they also have some vents on top of them ( My guess is either looks or to minimize the parachute effect of the rear bumper). Assuming we are talking about a car with a front mounted engine not having a full belly pan, wouldnt having those vents greatly reduce the function and benefits of the rear diffuser as a large portion of air under the car will exit from those vents but will not be "slowed down" ? Should there be any vents back there at all when running a diffuser or its acceptable to run a very small one like in the second picture below?

PICTURES FOR REFERENCE

The location/placement of the diffuser :

The vents in the bumper:

Thanks in advance

 

[MMMotorsports]

Hey Buddy,
To be honest I think these vents re there for looks, all of the images you have shown are body kits rather than earo kits, they look like they are designed to look racey.

In theroy, tho the vents would only cause turbulance in the rear end earo if there was air flowing into them some how, do if there was some airflow goign to there. If it is just a hole in the rear bar, then there might not be much airflow traveling up through the under car to get there, bearing in mind most cars onthe road, which these look like they are dressed up road racers, the rear reo intrusion bar, and boot lining whill be jst there.

In a built race car, they could be exits for diff cooling ducting or brakes. But I doubt it.

The reallity is none of these rear diffusers are doing anything towards downforce through ground effects, the chassis is way to high off the floor, all they "could" not "would' be doing is cleaning up the rear turbuance a little.

M

 

[Tmoose]

"Generally, for theoretical and experimental purposes, gases are assumed to be incompressible when they are moving at low speeds--under approximately 220 miles per hour."

http://centennialofflight.gov/essay/Dictionary/Compressibility/DI136.htm

If, at my vehicle's sub 200 mph speed, airflow stuff is assumed nearly incompressible, how much can it be "expanded" emerging at the back of the car?

 

[moon161]

IIRC If you write continuity through a stream tube or duct, Rho1A1V1 = Rho2A2V2= RhonAnVn. The stream tube or duct can expand and keep the density constant.

 

[Tmoose]

Hi Moon161,

Maybe I misunderstood tiale11.

When he said the air slowed down as a result of "expanding it" I took that to mean the air, not just the duct dimensions, were expanding.

It looks like he has not been back since October, anyhow.

Dan T