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Cd simply put is the "smoothness" of your shape. so a tear shape is going to have the best Cd because only at a very few places does either the direct impact of resistance take place(the front) or the concurrent drag resist the flow of the object. The airfoil shape allows the fluid to be pushed out of the way. . .smoothly.
If you've taken calculus there are no points where it is not continuous.
If you haven't then there is no place where the object forces the fluid to do something at a different angle than the angle that preceeded it. so if its going to force it away faster it does it gradually and not with a flat edge, or if its going to allow it past(the tail section) it allows the drag to avoid pulling the shape and expand against the surface.
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