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Compressible Subsonic Flow - Oscillating Solution

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Old   October 19, 2022, 08:33
Default Compressible Subsonic Flow - Oscillating Solution
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Aleksei
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Hi everyone
It is my first time I am simulating compressible flow in a microfluidic device (the chanel size is in micrometers, the fluid is gas Nitrogen).
I am applying pressure inlet and pressure outlet / pressure opening. The solver is CFX. From experiments I know that flow is subsonic in the pressure range I am interested in.
The problem is that with this kind of setup I am getting oscillating velocities of the broad range (and consequentely Re, Ma, mass flows). Does anyone know how to get rid of this oscillations? By oscillations I mean the oscillating monitor of velocity when looking at how solution is converging.
I already tried initialization with flow field based on the velocity inlet solution (velocity prescribed from experimental measurmenets) - but it did not help to get non-oscillating solution with pressure inlet.
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Old   October 19, 2022, 15:58
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If the solution is transient, then there is no easy way out, other than defining symmetry planes. In this way you restrict the flow in 1 direction. If possible add more symmetry planes. This might make things even more stable, but also possibly further away from reality.
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Old   October 19, 2022, 18:10
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Glenn Horrocks
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It is quite possible the oscillations are real and therefore you do not want to damp them out - they are part of what happens so you want to model them to get an accurate result.

But it is surprising that you have got a MEMS device to a Reynolds number high enough that it does this. I do a lot of MEMS simulations and they are all low Reynolds numbers and therefore you do not get transient flow features like this. Can you describe what you are doing and the geometry?
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Old   October 20, 2022, 00:16
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Thank you, Glenn
It seems I managed to get rid of the oscillations by changing the timescale in the Solver Control.
My device is operating with gas (not liquid) at high pressure - and from experiments we know that Ma is high, going up to the transonic flow. Therefore it is at high Re.
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