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Implementing an Inlet with time dependent length in Fluid flow module

Posted 28 avr. 2023, 16:33 UTC−4 Computational Fluid Dynamics (CFD), Structural Mechanics, Physics Interfaces 4 Replies

Fabian Henn
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Hey everyone!

I am currently working on a fluid flow in a tube whose walls are moving with the form of a propagating wave using the solid Mechanics module with the prescribed displacement option.

I deployed a pressure boundary condition on the inlet and the outlet (5Pa and 0Pa). The problem is, that in order for the simulation to work I need to fix the end points of the moving wall which leads to discontinuities near the wall (see attached image) and thereby affects the pressure near the wall.

I already read in many different posts that it is possible to have time dependent boundary conditions but I wasnt able to change the inlet size with time. I appreciate any ideas that come to your mind!

4 Replies Last Post 3 mai 2023, 02:14 UTC−4
Jeff Hiller COMSOL Employee

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Posted: 2 years ago 1 mai 2023, 14:36 UTC−4

Hello Fabian,

Can you explain why you are not also prescribing the displacements on the left edge? You say "in order for the simulation to work I need to fix the end points of the moving wall" but I don't understand what you mean by that. Why do you need to do that?

Jeff

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Jeff Hiller
Hello Fabian, Can you explain why you are not also prescribing the displacements on the left edge? You say "in order for the simulation to work I need to fix the end points of the moving wall" but I don't understand what you mean by that. Why do you need to do that? Jeff
Fabian Henn
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Posted: 2 years ago 1 mai 2023, 16:00 UTC−4

Can you explain why you are not also prescribing the displacements on the left edge?

Hey Jeff, thank you very much for you answer! I tried to prescribe the left edge aswell however this leads to wrong results during the simulation and after a short period to an error where the Nonlinear solver does not converge.

I attached two images to show what I'm referring to. I hope you can see that the solid material overlaps with the fluid domain and somehow a flow starts to form in the solid domain.

Do I need to specify something in the Fluid module for it to understand that there are changes of size at the inlet aswell?

Fabian

>Can you explain why you are not also prescribing the displacements on the left edge? Hey Jeff, thank you very much for you answer! I tried to prescribe the left edge aswell however this leads to wrong results during the simulation and after a short period to an error where the Nonlinear solver does not converge. I attached two images to show what I'm referring to. I hope you can see that the solid material overlaps with the fluid domain and somehow a flow starts to form in the solid domain. Do I need to specify something in the Fluid module for it to understand that there are changes of size at the inlet aswell? Fabian

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Jeff Hiller COMSOL Employee

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Posted: 2 years ago 1 mai 2023, 16:29 UTC−4

Hello Fabian,

I have not set up an FSI model in quite some time, so I am rusty on this, but yes, my idea is that you would specify a displacement of the mesh nodes on the left edge of the fluid domain. Let's call "point A" the point on the left edge where the fluid meets the pipe, and let's call "vi" the vertical displacement of point A. Then you could assign a vertical mesh displacement along the left edge of the fluid domain (your inlet) as vi*(1-s) where s is the curve parameter (or vi*s depending on the direction of the parametrization) See Reference Manual for COMSOL Multiphysics, version 6.1, page 325 if you have not dealt with curve parameters in COMSOL before.

Best,

Jeff

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Jeff Hiller
Hello Fabian, I have not set up an FSI model in quite some time, so I am rusty on this, but yes, my idea is that you would specify a displacement of the mesh nodes on the left edge of the fluid domain. Let's call "point A" the point on the left edge where the fluid meets the pipe, and let's call "vi" the vertical displacement of point A. Then you could assign a vertical mesh displacement along the left edge of the fluid domain (your inlet) as vi\*(1-s) where s is the curve parameter (or vi\*s depending on the direction of the parametrization) See Reference Manual for COMSOL Multiphysics, version 6.1, page 325 if you have not dealt with curve parameters in COMSOL before. Best, Jeff
Fabian Henn
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Posted: 2 years ago 3 mai 2023, 02:14 UTC−4

Then you could assign a vertical mesh displacement along the left edge of the fluid domain (your inlet) as vi*(1-s) where s is the curve parameter (or vi*s depending on the direction of the parametrization)

It worked! Thank you very much :)

> Then you could assign a vertical mesh displacement along the left edge of the fluid domain (your inlet) as vi\*(1-s) where s is the curve parameter (or vi\*s depending on the direction of the parametrization) It worked! Thank you very much :)

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