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Whenever a fluid has a nonzero vorticity, it looses some of its energy in a form of sound wave. Formally is this mechanism described by Lighthill's equation or some related model (like e.g. Curle's theory). But I have a lack of physical intuition on this process. Derivation of the Lighthill's model is based on disturbances of momentum flux tensor in a stokesian fluid, so nothing straightforward. I am fully capable of understanding this derivation with all the maths, but I can't "explain that to my grandmother in plain english".

Could anybody provide that sort of explanation?

Note 1: I am mainly interested in generating noise in turbulent fluid states. Tone-like sounds generated by periodical emerging of a macroscopic vortex eddies (Strouhal's Aeolian sound) are clear to me.

Note 2: I am aware of brilliant M. C. Howe's publication on the Theory of Vortex Sound. I have read it. But it is mainly on applied maths - that's why I am asking this question.

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  • $\begingroup$ I have not read Howe's book, but his article "Vorticity and the theory of aerodynamic sound" from J. Eng. Math. (2001) is very good. $\endgroup$ – Nick P Nov 13 '15 at 11:14
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Good question. My Problem is that I can't give you a "Mainstream answer". The reason is that Turbulence is considered happening In three space dimensions and time, given an initial velocity field, there exists a vector velocity and a scalar pressure field, which are both smooth and globally defined, that solve the Navier–Stokes equations.

But with these premise you are not able to produce sound, which needs vibration. Why would a single object start to ever vibrate through internal stresses and tensions? There simply must be something else, as the vibration allways needs some kind of a shock, or sudden release.

My explanation is, that the Turbulence is collision and friction, instead of viscous forces. And these collisions are able to produce vibrations, aka Sound. A Turbulent flow is a fluid cutted in peaces, internal surface tensions inside the fluid.

But as said, this is not "Mainstream physics", the mainstream physics just haven't explained this yet. This forces me to make another approach; Vortex tube The German language version of this wiki explains how the whistling sound is crucial for the Functionality. If the sound is removed, the temperature differences which can be produced drops from 40 Kelvin to just few Kelvin's. This supports the idea of the separating surfaces inside the fluid. The whistling sound comes from these surfaces, and when this sound disappears the fluids are contacted and the temperature is transferred.

Answer; The noise is caused by the collisions of the separated Fluid components.

I hope this helps. But also feel free to vote this down. Our thoughts or votes doesn't really change anything in nature.

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  • $\begingroup$ @VictorPira Wow! After having this idea in my desk since 2012, I haven't been able to get any such acceptance before! Thanks. I just Yesterday Talked with Guido, the man who showed me how to play Trumpet. And I told him about this question; There actually isn't any instrument which is not based on vortex shedding. About NS-problem definition; The turbulence is n x 3d-space instead of one 3d-space. Here's link to optical aspect of this; physics.stackexchange.com/questions/190414/… in edit, there is more good pics in there. $\endgroup$ – Jokela Nov 14 '15 at 8:02
  • $\begingroup$ Sorry for late replying to this comment. I beg to differ. There are plenty of instruments (even aerophones) which are not based on vortex shedding phenomena. $\endgroup$ – Victor Pira Nov 23 '15 at 10:45
  • $\begingroup$ @VictorPira I am in no need to argue. Maybe I just haven't thought this troughly. At least loudspeakers aren't working through this priciple. But as I see it, even the drums are. Which aerophone or instrument you had in mind? $\endgroup$ – Jokela Nov 23 '15 at 12:56

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