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Old 01-24-2013, 04:27 PM   #408 (permalink)
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The Truck - '02 Nissan GU Patrol ST
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Originally Posted by Sven7 View Post
Freebeard-right. I imagine the boxfish was tuned specifically to equalize pressure on the top and sides, allowing them to have a sharp corner.

Here you see high pressure air trying to move around the rear/side windows. It creates a little snow drift line along the edge of the liftgate.

Airflow! by Tyler Linner, on Flickr

I don't have energy to read the novelette up there but are we asking whether 22* still applies to the sides? I believe you can calculate the correct side taper by overlaying a teardrop on the top blueprint.

Theoretically if your shape is symmetrical like those beard posted, there should be no pressure difference and therefore vortexes right? The air on the top and the air on the sides travel the same distance in the same time so they go the same speed and have the same pressure. (Faster air has lower pressure)

You run into problems when the vehicle is taller than it is wide, or has more resistance one place than another. Then the air is slowed down somewhere and it needs to "catch up", changing its speed and therefore pressure. So, if your vehicle isn't a perfect teardrop you're going to want to dissipate that pressure as well as you can.

And remember, it's not just about the few inches closest to the vehicle- you move many feet of air wherever you go so macro size form development is important. (Ie, VG's won't solve your problems, more frosting won't un-burn your cake, etc)
I'll try to be brief,
Thanks for comments, some things you said about airflow have clicked a few switches in my understanding.

Regarding the 22* if the object is a perfect teardrop, then it will be 22* all round, but a rectangle trying to become a teardrop invariably exceeds this number if you use the smaller dimension as the guide as has been advised.

So you can't use full size tail template to overlay for side view analysis, then in the next breath use two half size templates to overlay for a top view analysis.

If the smaller dimension rule stands, then most of the side view overlays are a pointless exercise, which one is it?
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