Pressure Wave Speed

prb43 · August 18, 2017, 06:32

I am attempting to model pressure waves and resonance within a cavity with flow.

There is an unsteady heat source oscillating at 3.7kHz producing pressure waves. When I run to completion, I observe resonance which tallies with experiment.

Through the use of monitor points I can see the initialising pressure wave as it travels down the length of the cavity and by measuring the difference between arrival times I calculate the average wave speed between monitor points.

What is odd is that the wave speed 384m/s is well in excess of the local acoustic speed which is a constant 346m/s between the monitor points. Even allowing for the local flow velocity(~14m/s), this wave would appear to be travelling at supersonic speeds.

Is this an inaccuracy of the CFD simulation and if so what can I do to correct this?

FMDenaro · August 18, 2017, 09:25

I am not sure to understand your problem...
the sound wave velocity depends on the temperature field, being a=sqr(gamma*R*T), therefore I am not surprised that a heat source can increase the velocity propagation...

Maybe you should give the full details of the simulation

prb43 · August 18, 2017, 09:38

The temperature around the source does increase, however over the region that I am examining the temperature is a constant 25C, so the acoustic speed is fixed. There has not been enough time for the heat to propagate.

prb43 · August 18, 2017, 09:52

The cavity in question, with monitor points marked in yellow, looks something like this

transient_LES_3_7kHz_HighRes.jpg

whilst the acoustic speed for the cavity looks like this

acoustic_speed.png

FMDenaro · August 18, 2017, 09:53

Well, so assume you have 298 K of temperature, consider R and gamma for your fluid (I don't know if you have air) and compute the sound velocity a.
Then, the propagation follows the eigenvalues (for simplicity I am considering an inviscid flow) of the Euler equation |u|+a and |u|-a.
Try to make an estimation for the local advection velocity and check if the result is acceptable.

August 18, 2017, 06:32	Pressure Wave Speed	#1
prb43 New Member Patrick Boyle Join Date: Aug 2017 Posts: 3 Rep Power: 8	I am attempting to model pressure waves and resonance within a cavity with flow. There is an unsteady heat source oscillating at 3.7kHz producing pressure waves. When I run to completion, I observe resonance which tallies with experiment. Through the use of monitor points I can see the initialising pressure wave as it travels down the length of the cavity and by measuring the difference between arrival times I calculate the average wave speed between monitor points. What is odd is that the wave speed 384m/s is well in excess of the local acoustic speed which is a constant 346m/s between the monitor points. Even allowing for the local flow velocity(~14m/s), this wave would appear to be travelling at supersonic speeds. Is this an inaccuracy of the CFD simulation and if so what can I do to correct this?

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August 18, 2017, 09:25		#2
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,768 Rep Power: 71	I am not sure to understand your problem... the sound wave velocity depends on the temperature field, being a=sqr(gammaRT), therefore I am not surprised that a heat source can increase the velocity propagation... Maybe you should give the full details of the simulation

August 18, 2017, 09:38		#3
prb43 New Member Patrick Boyle Join Date: Aug 2017 Posts: 3 Rep Power: 8	The temperature around the source does increase, however over the region that I am examining the temperature is a constant 25C, so the acoustic speed is fixed. There has not been enough time for the heat to propagate.

August 18, 2017, 09:52		#4
prb43 New Member Patrick Boyle Join Date: Aug 2017 Posts: 3 Rep Power: 8	The cavity in question, with monitor points marked in yellow, looks something like this transient_LES_3_7kHz_HighRes.jpg whilst the acoustic speed for the cavity looks like this acoustic_speed.png

August 18, 2017, 09:53		#5
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,768 Rep Power: 71	Well, so assume you have 298 K of temperature, consider R and gamma for your fluid (I don't know if you have air) and compute the sound velocity a. Then, the propagation follows the eigenvalues (for simplicity I am considering an inviscid flow) of the Euler equation \|u\|+a and \|u\|-a. Try to make an estimation for the local advection velocity and check if the result is acceptable.