Jet Noise

[Nickul18]

Ok , we've been testing a simple convergent nozzle operating at pressure ratios of approxomately 3-4 (To, approx 200C)exhausting into static free air, and we are experiecing some pretty elevated levels of noise. We expected this but the question really is what is the source of the noise? Is it the shearing of the jet boundary ?? or some have said the shock structure itself generates pressure perturbations. Anybody have any ideas ??

 

[btrueblood]

Shear/mixing/turbulence along the jet boundary.

The shocks are pressure peturbations, they don't cause them. And, in general, the shock structure is stationary, therefore can't produce noise. If the upstream pressure fluctuates, then those perturbations add to the jet noise (e.g. rocket engine exhaust). The shear forces, and resulting turbulence, are the dominant noise source.

 

[GregLocock]

There's a good paper on this from ISVR in the UK. Jet noise is proportional to the 8th power of (something or other) - regularly held up by New Scientist as the highest power relationship known.



Cheers

Greg Locock

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