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Unread 09-01-2012, 12:00
Ian Curtis Ian Curtis is offline
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Re: Drag coefficient of 2012 Game Piece

Quote:
Originally Posted by Michael Hill View Post
I just did some back of the envelope aerodynamic calculations for the game piece this year and realized we're RIGHT on a near asymptote when trying to predict drag on a ball. (Hey, I'm bored at work with nothing else to do.)

I estimated the Reynolds number to be around 120000 (assuming around 10 m/s) and the relative roughness factor to be around 4.9 *10^-3 (I assume 1 mm dimples with an 8 inch ball, best I can estimate without having the game piece in my posession).

Then I found this little jewel (see attachment):

As you can see, the roughness (epsilon/D) line for 5*10^-3 has a nice little vertical right around Re=120000.

This doesn't take into account any spin, mind you (and I really don't feel like getting into Magnus Effect forces right now).

Enjoy!

P.S. In my opinion, the dodgeball couldn't be any more imperfectly designed if consistancy is desired.
I knew someone would bring up the drag crisis, that's what I get for keeping it simple I suppose...

To remain at the low spot requires pretty spectacular attention to surface roughness. We tunnel tested several models as part of one of my aero labs, and indistinguishable (by fingers) differences in surface roughness will knock you out of the crisis. Seeing as these are low quality foam balls produced in the thousands by robots, I don't think there is much to worry about.

(Where did that chart come from? I've never seen Cd v. Re with how the drag crisis moves as a function of roughness before, that's a nice chart!)
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