Are there harmonic patterns in turbulence?

17 October 2010

Question

Are there harmonic patterns in turbulence?

Answer

We posed this question to Tobias Schneider from Harvard University...

Tobias - Typically there are not. Turbulence - one of the features of turbulence is that it has structure on all scales. So typically, you have vortices, and a large vortex creates a smaller vortex, and so on and so forth. So you typically don't have regular patterns. However, if you're looking at theoretical work done right now which tries to understand turbulence, then there are these regular patterns, harmonic patterns which you observe in solutions of the underlying equations which are computed with big computer programs, and they seem to be very important. At pipe flow, they have even been observed in 2004, so yeah, there are these patterns, but the patterns are extremely special. Typically, turbulence, doesn't have any structure. Dave - I guess the one place where you can sort of use some of the noise and these vibrations from turbulence is something like a flute where you've got a resonator attached to something which is producing a turbulent airflow, and that turbulent airflow drives the resonator in the flute to the air, so it rushes in and back, in and out, and that connects to the turbulence being produced as you blow over the top of the flute. And so, you can get a big sort of resonance, big airflow, in and out and vibrations which you hear as sound, but I guess that's a property of the resonator more than the actual turbulence itself. Ben - It's that effect related to noisy pipe syndrome? Is it something similar going where you turn your hot water on and you get this horrendous noise or buzzing or a low hum for awhile? Dave - I think that again, yes something very similar. You get sort of turbulence in the pipe which produces lots of different frequencies of vibration. It's quite low level and then if you've got some air trapped inside the pipe somewhere, that can compress and expand, and it acts like a spring. You've got a mass and a spring so that will vibrate and resonate, and build up to quite a large vibration even if the original turbulence with lots of different frequencies to start off with and not very large.

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