Amateur Aerodynamics

Nearly a year ago, I was sitting in an incompressible flows class when the professor said something I had never considered before. Loss of lift over an airfoil after stall, she claimed, is caused by a pressure increase due to flow separation over the upper surface.   This was contrary to everything I thought I knew about separated and attached flow. Keep flow attached, the conventional wisdom goes, over a tapered shape for greatest pressure recovery and lowest drag (and “zero lift” on a ground vehicle, if you believe some people online, which you shouldn’t); if the flow separates, the pressure drops behind the separation point. Yet in this case, it must be true that separation increases pressure—otherwise, airfoils would not lose lift in stall.   Conventional wisdom, as is so often the case, does not tell us the whole story. The effect of separation on surface static pressures and, consequently, lift and drag can be complicated. To investigate, I decided to run some tests. Yo...

The drag force cars create as they move through air is generated largely by a pressure imbalance. Cars are bluff bodies as opposed to streamlined; that is, airflow separates over them and, in modern cars, this separation usually occurs most prominently at the very back and creates a large wake (even fully streamlined shapes like wings have wakes, too, but they tend to be a lot smaller). This large wake results in negative gauge pressure acting on whatever body surface is exposed to it which, since it’s the back of the car, tends to be a large vertical surface (we call this the base )—so the low pressure ( base pressure ) results in a force acting almost directly backward.   One way to reduce the drag of cars is to make this wake area and its resulting pressure drag smaller by tapering the rear part of the car in a long tail; this allows the flow to recover pressure so long as it does not detach prematurely and reduces the vertical surface area exposed to low gauge pressure. Alt...