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Old 04-19-2012, 08:17 AM   #350 (permalink)
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Originally Posted by NeilBlanchard View Post
I'd say that the first one is lower drag, and here's why: the shape of the front accelerates the air out and around the high point on the roof, and this creates a localized high pressure, which then help the air "spring" back and it helps maintain attached flow. So, it adds to the atmospheric pressure and this improves the air flow a bit.

If the rear of the shape is most important, then the front is next most important -- it matters less, but it matters. At the very least the second one has more surface drag.
Yes, this is the reason why boat tailing a train does not improve aerodynamics (well that and a lot of other things...). The ideal shape of the tail will change as the length of the forebody changes. When we are practical about it (on a car, on an airplane) it doesn't matter so much, but when taken to an extreme (a train, some test article in a wind tunnel) we will find that new aft end shapes are needed. It's for exactly the "springieness" reason you callout (actually a high pressure zone that is created that pushes back down). Given enough time, that high pressure zone is absorbed back into the atmosphere, and has nothing to give back to the vehicle. In some cases we will find that there is too much turbulence from things upwind, or there is too thick of a boundary layer built by all that constant diameter, that boat tailing has no effect anymore, regardless of the shape.
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NeilBlanchard (04-19-2012), Vekke (04-19-2012)