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Sound generated by turbulence.
I would like some understanding of what features of turbulence create the noise we can hear at a distance. To simplify, lets pick one frequency in the audible range, say 1000hz. I imagine there are two cases of hearing turbulence.
1) The listener in inside the turbulent stream and is subject to the pressure variation of the turbulence. Inside a vortex, there is a pressure difference between the center of the vortex and the extremities due to the difference in velocity. If a vortex moves past the listener in 1/1000th second, the listener would hear noise at 1000hz 2) The listener is outside of the turbulent stream and a longitudinal sound wave radiates from the turbulent volume. I imagine that a stable vortex produces no sound. With a stable pressure distribution, no sound will radiate out. However, turbulent vortexes have the urge to break up into N smaller vortexes. Is it the rapid division of a vortex in 1/1000th second that radiates sound at 1000hz ? I can’t find any clear explanation on the web, so would be grateful for any insights. |
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Vibration produced due to violent mixing of the turbulent fluid + formation and popping of air bubbles. - Turbulent water causes more sound than turbulent air of same momentum. Ex : rivers vs wind. - Turbulent water + air mixture causes more sound than turbulent water and turbulent air, due to formation and popping of air bubbles. Example : pouring tea. Don't know what happens at more extreme scenarios like waterfalls, rain, thunderstorm etc. |
I am not expert in the aeroacustic field but the general topic is nothing but a result of the compressible form of the NSE. In other words, by resolving the full problem you can simply observe the distribution of the spectral content of the pressure field that is a result of the complex interaction between density, momentum and energy variables.
Sound is not related simply to a turbulent vortex. You can think about the sound of a shock wave as well as the sound due to some music from a speaker. Or simply due to our voice. Note that the propagation lenght of the sound depends on the energy dissipation, that is high frequencies are dissipated faster. |
The mit notes on the web and their video series is an oldie but a goldie. Typical definitions divides sound generated by flow into pseudosound and actual/true sound (which we call just sound). pseudosound decays rapidly with distance and doesn't radiate in waves like sound. See Lighthill's analogy for some good reading.
Another quick way to differentiate is that sound radiates with the sound speed. A moving vortex moves around at the local convective flow speed, not the sound speed. And so, although you can certainly pick up a pressure variation at 1000 Hz, it is not sound, it's just flow. |
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For a recent literature, I suggest to check the papers at the CTR site.
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Thanks for the links and comments guys. You have to admire Lighthill’s enthusiasm for the subject in his video :) Fluid dynamics does not disappoint in becoming more complex, the more you learn about it.
A search using the terms in your links resulted in some very illustrative simulation examples in Star-CCM+ https://www.youtube.com/watch?v=gt-R5nfu4rQ The modes of sound generation that seem relevant to my application are turbulence encountering solid surfaces and unsteady shear stresses (eg jet exhaust) |
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