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Well, yeah, I meant in the explanation of why an airplane flies. Sure, in a venturi you have Bernoulli effect with no downwash, but the fluid is instead changing speed. The effect is still there, it's just operating in a different direction. Any time you have a pressure change, you must have an acceleration. In the venturi, the pressure is lower where the speed is higher, that's a fact. But is the pressure lower because the speed is higher, or is the speed higher because the pressure is lower? That question makes no sense, because it depends on how you think about it. On the one hand, you can say: mass conservation dictates that the fluid must go faster in the narrow part of the tube. If the fluid is to go faster, it must accelerate, so there must be a pressure gradient. Hence, the pressure in the narrow part must be lower. On the other hand, you can say: Since the fluid goes faster in the narrow part of the tube, the pressure is lower there. Since the pressure is lower, there's a pressure gradient, and that's what causes the fluid to speed up. Neither of these explanations make sense, because there is no cause and effect in the problem, it's just that one state globally obeys all constraints on the fluid and that's the state with higher speed and lower pressure in the narrow part. |