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What is the reason behind the noise created by fan blades while moving? Some paper says that it is because of the Kármán vortex street but it can be formed in static bodies only. Most of the researchers settle on tip vortices and vortex shedding but the main part is that how do vortices make the noise. How does turbulence in flow result in noise?

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    $\begingroup$ How does turbulence in flow result in noise? Total guess on my part, but turbulence causes pressure differences and pressure differences are detected by the ear as noise. Wikipedia says a Kármán vortex street is responsible for the vibration of a car antenna at certain speeds, and that is no more a static body than a fan blade. Is it really as straightforward as that? It's not my field....so am I being naive here? $\endgroup$
    – user81619
    Commented Aug 23, 2015 at 18:13
  • $\begingroup$ Thank you for your response . Can you tell me more quite functioning of our ear towards pressure difference.Besides it, it would be quite generous of you if you forward this question to someone who is specialized in this field. $\endgroup$
    – tyler
    Commented Aug 25, 2015 at 12:27

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Acid Jazz is quite right. Generally any instability in air medium can result in sound waves radiation. Vortices causes local pressure instabilities and the medium is "able to propagate the image of it" (well, the Wave equation).

Kármán vortex street is nice (and most of all solvable!) example of vortex shedding. But in certain domain of the Reynolds number the shedding can occur anyway.

If you are interested in quantitative approach, start e.g. with the Lighthill's equation:

$$ \Box p = \rho_0\frac{\partial^2}{\partial x_i \partial x_j}\left(v_iv_j \right) $$

where the source term is based on Reynolds stress only (there are more general cases).

I can recommend Theory of Vortex Sound by M. C. Howe for more details.

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