dynamics of air flow over / under a car body 1

[crashercox]

Is there a book(s), software or rule, that I can use to model air flow over / under a car body for the purpose of determining high and low pressure zones at different speeds. I'm investigating the placement of openings and type / style of opening for component cooling. Additionally any information to determine the type or style of opening is important to attain the proper cooling air volume in a given ambient temperature range.

 

[ivymike]

If you want a simple rule, try the following:

Any place where the airflow is turning away from the car (where the windshield meets the hood, for example) will have a high pressure region associated with it.

Any place where the airflow is turning towards the car (where the windshield meets the roof, for example) will have a low pressure region associated with it.

 

[evelrod]

That's a pretty good rule of thumb. Since a large percentage of aero drag is from the engine compartment and the underside of the chassis the 'rule' becomes a bit more complicated, wouldn't you say? Ground effect?

Rod

 

[ivymike]

If I'm not mistaken, undercarriage drag is often in the neighborhood of 8% of total aero drag. I don't know about the engine compartment contribution.

 

[evelrod]

I don't have any referance material on aero for cars aside the airfoil book but, I know that a great deal of attention goes into engine compartment openings and undercarriage aero on the land speed cars. For qualifying runs NASCAR crews tape closed all openings save a tiny one for the rad.
I have done a couple of simple pressure tests to see what areas were high or low . The obvious of course and some not so obvious. Low press areas were at the differential rear end hump, at the top of the rear bumper, just aft of the front airdam (but high there sans airdam) just rear of the leading edge of the hood to about 3/4 way back to windscreen on the Lotus but I suspect this would not be typical to some of the newer designs. Simple to do. Just some plastic tubing, duct tape, a simple manometer and a friend to ride along to read it. Tape the open end of the tubing where you wish to test. Not up to aircraft standards but very similar to what goes on in the wind tunnel, I suspect. One test involved running down the front straight at the old Ontario Motor Speedway. At about 125mph the pressure would rise a bit (on the side of the car closest to the wall) as I neared the wall up to about 6 inches where it suddenly took a drop. Perhaps that is the real reason Richard Petty always stayed so close to the wall(he said it was so he didn't have so far to slide when he screwd up). For me the best was mid course, away from the wall.
I was allways doing screwball deals just to find out "why" or "why not".
If Greg or someone else has some figures on engine compartment or underside drag I would be interested.
I keep reminding myself that aerodynamics in relation to race cars is NOT an intuitive science. Often what one would think works well, in fact does not.

Rod

PS---Isaac, in racing the mini, all this is just academic. It has the aero of a large brick!

 

[GregLocock]

I have consulted the guru. She says rough breakdown is:

Underbody 1-7%
Engine 4-14%
Brakes cooling 2-5%

Then, confirming her guru status, she said: I'm reading these out of the Bosch Blue Book. So we know those figures are good!
Cheers

Greg Locock

 

[crashercox]

Thanks everyone you've all been a great help. And you basically confirmed the inevitable that I was trying to avoid. Wind tunnel and field testing. Just trying to save design time. As usual behind schedule, but not over budget. Yet.

 

[TMSengineer]

Hey guys...I should have put my question in this thread but take a look it you have the time......

thread87-59141

 

[modesign]

Had the same questions re body design. Try a book Automotive Aerodynamics by Car Styling Vol.142 a Japanese publication but dual laguage.It has even a chapter on engine compartment drag. External accesories such as wing mirrors and drain channels are also detailed. Simple to understand and answers those question other books do not.
Cheers

 

[sean01]

Crashercox,

I have seen some demonstrations of modelling this kind of problem using STAR-CD, an FEA package. The application was based around predicting airflow and the effect on cabin pressure of a car when being overtaken in different directions by a car and a lorry. Sorry I can't be more detailed about this, but if you are looking for a non-prototype based method - you may find this gets the ball rolling (bearing in mind that you will need correllation at some point !!)

sean

 

[Kamikaze]

Another trick to try for feild testing is Marking ribbon <Kind of ribbon used for wrapping around a tree or whatever> taped in different locations on the car.
Use a video camera for review later to really see what is going on. also food dye in water sprayed from a washer type outlet in front of area for testing is cheap and effective, be aware if it sprays on lexan windows it may be hard ot get off.
Sorry I have a real cheap solution but I can not afford the big buck approach.

 

[NavierStokesEq]

I am a CFD engineer and have produced a basic model (in fluent, but can use CFX and Star-CD as well) on a SCCR race car if you are interested