Noise generated from turbulent shear flow

[alchemist]

Hi guys,

its been awhile since i have been back. Right now, i am involved in some research on aeroacoustics and i have been trying to get some of the basic fundamentals on aerodynamic generation of sounds. Unfortunately most of the online literature do not give much details on the physics of such mechanism. I hope you guys could provide me with some guidance.

From my understanding, sounds are perceived by us through changes in air pressure that is picked up by our ears. And most of the time, such changes in air pressure (compressions and rarefractions) are generated by objects vibrating in a medium and the compression waves generated propagates outwards and reaches us.

However, in aeroacoustics, which is sound generated aerodynamically, there do not seem to be a vibrating source here (i think..). Sound is generated by turbulent shear flow instead. What I understand is that when air flows across a surface, and becomes turbulent, vortex shedding occurs and noise is generated. But in this case, how is the noise picked up by our ears if there are no changes in pressure?

Could it be that due to vortex shedding, the source vibrates and hence produces sounds? Or are the vortices the one that generate compression hence sounds here? How is this applicable to aircraft noise generated when air flows over edges of flaps and slats when separation occurs?
Thank you all so much for you help!

 

[boneh3ad]

Aerodynamically generated noise doesn't necessarily have to be turbulent. All it requires are small fluctuations in the flow, which are present in laminar flow as well, though to a much lesser degree. That could directly produce noise because velocity fluctuations necessarily mean pressure fluctuations and therefore sound. Indirectly, those fluctuations mean fluctuating forces on a body, so they can induce vibrations in a body that is in contact with the flow, producing sound.

 

[alchemist]

"velocity fluctuations necessarily mean pressure fluctuations "

thank you boneh3ad, this is the missing link i have been searching for =)

My next question would be the role of frequency in the generation of aerodynamic sounds. Or rather, what affects the frequency, hence wavelength of the sound produced by airflow? And is there any implication from the range of frequencies generated?

 

[boneh3ad]

Well the frequency would define what sound you hear if you are talking the sound that would be directly generated by velocity/pressure fluctuations. Indirectly, it would affect how a structure interacts and that structure may or may not vibrate at the same frequency. Whatever frequency that structure vibrates at will be the sound frequency you hear.

The frequencies generated in the flow are very diverse and their nature is very complicated. It all depends on the pressure gradient along the surface (and therefore the shape of the surface), the velocity/Mach number, the surface finish of the surface, the turbulence characteristics of the free stream and plenty of other factors. Multiple frequencies can and often are present, and the frequency that is most amplified will more than likely change as you change positions in the flow.