Compressible Subsonic Flow - Oscillating Solution

alexksei · October 19, 2022, 08:33

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.

Gert-Jan · October 19, 2022, 15:58

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.

ghorrocks · October 19, 2022, 18:10

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?

alexksei · October 20, 2022, 00:16

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.

October 19, 2022, 08:33	Compressible Subsonic Flow - Oscillating Solution	#1
alexksei New Member Aleksei Join Date: Jun 2018 Posts: 24 Rep Power: 8	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.

October 19, 2022, 15:58		#2
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,854 Rep Power: 27	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. alexksei likes this.

October 19, 2022, 18:10		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,781 Rep Power: 143	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? __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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October 20, 2022, 00:16		#4
alexksei New Member Aleksei Join Date: Jun 2018 Posts: 24 Rep Power: 8	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.