If wind has zero resistance, does it make a sound?

17 October 2010

Question

If wind has zero resistance, does it make a sound? Since we can't see wind, only the things it moves, I would think we could not hear wind, unless it his some kind of mater which causes turblence.

Answer

Dave - It certainly won't cause any sound if there's nothing to cause any turbulence, if there's no vibrations because sound is just a vibration of air. So, if the wind is flowing past something and that thing makes it vibrate, like if you get some swirling when it blows across the edge of a surface or something, you get some turbulence or that turbulence can drive vibrations in a bottle or something when you blow across the top of a bottle, you produce very loud sounds. If you're in a middle of an air column, so if you're up high in the air where all the air is moving the same direction very fast, there is no relative movement, so you won't get any vibrations at all. It would be very quiet. So people hot air ballooning is very, very still, even if the wind is quite high because they're moving with the wind. However, there's various interpretations of what he means by resistance. One of them is how gloopy it is, how viscous the fluid is. Ben - So that's sort of the internal resistance of the fluid itself, rather than it interacting with say, a wall. This is resistance inside the fluid. Dave - That's right. Basically, how much friction there is inside the fluid itself and fluids which are very, very viscous, things like treacle will move very, very smoothly, and you won't get any vibrations and it'll essentially be very, very quiet. However, if there's very, very low viscosity, it can't lose energy by viscous flow by friction. It can only lose it by turbulence. airflows being incredibly turbulent and very, very noisy. Ben - So that goes back to what Fred was saying about how you have to put energy in to create turbulence and then it actually loses the energy back to this internal friction. Dave - That's right and the more viscosity there is, the quicker the turbulence will die away, and if there's enough it won't form in the first place.

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