Calculating CD - Fuel Economy, Hypermiling, EcoModding News and Forum

hendan · 06-22-2010, 12:27 AM

Is there any way to calculate the CD on a digital model if I already have it in .DXF format in 3D?

hendan · 06-22-2010, 01:25 PM

No clue?

3-Wheeler · 06-22-2010, 01:36 PM

I would think you would need access to a CFD program to get a close approximation.

Jim.

Daox · 06-22-2010, 01:37 PM

With the proper CFD software you might be able to get close. Your dxf would have to include every component in the engine bay, suspension components and everything on the car. Solve time would be crazy for something like that.

Wind tunnel testing is really the only way to get solid info.

There is a DIY approach though. Someone made a spreadsheet that you logged coast down tests on to get Cd.

hendan · 06-22-2010, 01:40 PM

But that requires already having the car. I am working on a design, weather I ever build it or not, and I want it to have a good CD before I get started.
So, is it possible that anyone here has a CFD program?

Daox · 06-22-2010, 01:45 PM

OpenFOAM&#174 - The Open Source Computational Fluid Dynamics (CFD) Toolbox is the only one I know of. Everything else you're gonna have to pay a few thousand for.

hendan · 06-22-2010, 05:00 PM

I looked, and found a couple that, as it turns out, may even run on my computer (a Mac) IF I was a programmer, and wanted to take the time to configure them and the other programs necessary to support them. Same if it were windows, though, perhaps even more daunting. LOL. Not easy.

Daox · 06-22-2010, 06:40 PM

Nope, usually you go to training/school to use CFD.

hendan · 06-22-2010, 11:55 PM

If I even knew of someone I could send my file and have them analyze it, that would be nice.

aerohead · 06-23-2010, 05:24 PM

Quote:

Originally Posted by hendan

If I even knew of someone I could send my file and have them analyze it, that would be nice.

I think it's only been a few years since MIT cracked the Navier-Stokes equation with spherical coordinate system,necessary to accurately solve the turbulent wake.
And this would be with a supercomputer.Really big-buck stuff.Over time,something may trickle down the pipeline.I wouldn't know how to guess.
The Solar-Challenge cars will give you an idea of actual real world low drag forms.
The Shell Mileage Marathon vehicles are real tasty,but they never make it into turbulent boundary layer,so we can't expect their kinds of Cds at real road speeds.
If you can give us a Cd target,we may be able to put up some examples that have achieved that.