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Old 09-17-2012, 10:24 PM   #10 (permalink)
ERTW
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box fish

dang you! I came on here to post my pics of my latest CFD testing. I'm trying to replicate the box fish, which MB used to shape the bionic car. My very first iteration, which was 80" long from peak to the kamm back, came to 0.143. My second iteration is 120" long, and came to 0.101. MB said the box fish came to around 0.005 with excellent yaw stability.

I used a convex rectangle section with rounded corners, approximating Morelli's later designs, instead of the "X" type shape of the box fish (I actually took a video of one at the local aquarium today). I was surprised how easily it obtained such a low Cd, considering how much more work I did to get Aero's template to 0.092. The body falls away from the streamlines much quicker, yet there's no delamination.

This answers my question of whether we "need" the tear drop shape for a low Cd. A box is certainly roomier, and easier to fit/hide wheels, and it's much shorter than the template.

Note that any body will work better with a rounded bottom - not flat - and generous fillets on the rocker panels, and a raised tail. Anything to equalise pressure around the perimetre of the body helps prevent vortices. Morelli discusses elliptical shapes to minimise wetted area, hence, skin drag.

discuss amongst yourselves
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