View Single Post
Old 02-17-2012, 09:18 AM   #253 (permalink)
KamperBob
Recreation Engineer
 
KamperBob's Avatar
 
Join Date: Dec 2009
Location: Somewhere USA
Posts: 521

Black Stallion - '02 Toyota Tundra 4WD xCab

Half Pint - '06 Yamaha XT225
Thanks: 311
Thanked 136 Times in 101 Posts
Quote:
Originally Posted by ERTW View Post
Kamperbob, you are correct. I wanted to investigate the prismatic shape because firstly I don't see anybody using the revolved shape. The template works for a revolved shape, or even a high aspect ratio wing. However, cars are narrow boxes. Flow around such a body is very different. That's why I'm modeling a prism. Perhaps Aerohead can comment.

You could argue that the box fish is the slipperiest shape found in nature. Boxes are also much more pragmatic than footballs; they offer more usable space and are easier to build than compound curves.

Aerohead, I put the template up against a 2010 GMC Sierra, and found that it only goes out to 45%. What is the minimum Cd at that point?

My template approximation is good to about 60%. The zebra stripes should be concentric circles - as they are at the peak. You can see the broad space and the reverse stripe on the tail which confirms a discontinuity.
Quote:
Originally Posted by ERTW View Post
Aerohead, I used an equation based spline for the back. It makes it really easy to shape the curve as I like.

As a confirmation of the hard work you've put in, the revolved solid shows smooth contours all the way out to the tail. So the boxed shape requires a different approach as I'd hypothesized. I hope to have some CFD analysis done this weekend.
ETRW, I hear you. And I'm delighted to see your revolved model drawn up in solid works. Everyone I know who does simulation professionally validates the model first. If CFD on a highly developed template form confirms attachment that's good. If a slightly stubbier/steeper version (ie, violates template rules) shows separation that's better. Because then you can play with other geometries (ie, prismatic) with higher confidence in computed results. Of course wind tunnel, tufts and/or field tested MPG would wrap it all up with a bow on top. Filling an analysis hole is still an excellent contribution to our collective body of knowledge.
__________________
Recreation Engineer
  Reply With Quote