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Visualize those little pictures as the front of vehicles like airplanes and trains. I wonder if I am right about my speculation below, and if so, if somebody could explain the exact reason. Sources are also welcome!

It seems to me that vehicle B and D have relatively less resistance against air molecules attacking them (represented by arrows) as those are the ones common in use. It also seems like A has more - and C even more - resistance against wind as they kind of point in the opposite direction of B.

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marked as duplicate by sammy gerbil, John Rennie, Kyle Kanos, stafusa, Jon Custer Sep 3 '18 at 14:03

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This is hard to answer because the drawings are a little rough, but the general principles can be outlined here.

The main thing determining drag from objects like this is the projected frontal area that faces into the oncoming wind. More frontal area means more drag.

Next is whether or not the air, when trying to flow around a given object, separates from the edges of the object and continues flowing sideways after the object has passed. this is flow separation drag, which makes the object draggier by making it in effect "bigger" than its actual frontal area. You can minimize it by streamlining as in examples B and D.

Next is wetted area drag, which happens because air tends to adhere to the side-facing surfaces of an object. The longer the object, the more drag you get.

Finally there is wake separation drag, when the air flowing around a moving object fails to completely "close up" around the tail surfaces of the object. This can be minimized by gradually tapering the sides of the object back to a point.

  • $\begingroup$ Thanks a lot for your explanation! I got the differences and can search further now. $\endgroup$ – Englishterian Sep 3 '18 at 15:23

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