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The fluid moves faster above the objects because it has a smaller space to travel through. Air passing by an object generally can't go through the object, but rather must move around it. The curve above the airfoil and the angle that it travels at limit the space the fluid can travel through. If the object is a flat plate, then just the angle limits the space. Since the same amount of fluid has to get through this small space in the same amount of time due to fluid dynamics, it has to speed up in order to completely get through, just like a river that gets smaller in the middle.