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Old   February 11, 2009, 07:54
Default Moving Wall
  #1
Smagmon
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Hi there,

Does anybody knows how to prescribe a displacement for a wall? I would like to impose a sinusoidal motion to a boundary to observe how the fluid around this wall behaves. I am trying to use the option "specified displacement" with a CEL sinusoidal function, but I dont know if there is something wrong. What happens is, when I post-process the results, for some steps I have some distortions in the wire frame, exactly in the region where the mesh motion was specified.

Thanks in advance
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Old   February 16, 2009, 15:08
Default Re: Moving Wall
  #2
Gustavo
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Hi, I had this problem too, I donīt know if itīs exactly the same as yours, but maybe this will help you. The problem was that I specified the motion of a piston inside a cilinder, but I forgot to set the cillinder wall as "Unspecified" instead of "Stationary", so the piston moved but the cilinder wall stood still, and then the mesh became really weird a few steps after I startes simulating. Maybe your problem is that you specified the motion of the wall but forgot to set the mesh that is in contact with this wall as "Unsepecified".
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Old   February 17, 2009, 05:10
Default Re: Moving Wall
  #3
Smagmon
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Hi, I also thought that, but I changed all the walls around to unspecified motion and nothing changed. About this approach, when I configure a wall with unspecified motion, what does it means physically? Is it a closed wall with some compliance?

thanks in advance
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Old   February 17, 2009, 13:09
Default Re: Moving Wall
  #4
Gustavo
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Sorry, I didnīt understand what youīre trying to say really well, but I guess what youīre asking is if when you set unspecified motion for a wall, and it moves, if the wall is being comprised or it diminishes. If youīre dealing with fluid the wall simply diminishes, some part of it cease to exist, or is created, according to the movement of the specified movement part. Sorry if I didnīt help much, but I really didnīt understand what you tried to say with "closed wall with some compliance".
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Old   February 17, 2009, 18:08
Default Re: Moving Wall
  #5
Glenn Horrocks
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Hi,

Mesh movement is done by defining the motion of the wall boundaries and using a Laplacian mesh smoother to generate the position of the internal nodes. When you set a wall to "unspecified" then rather than explicitly defining the location of the wall then the positions of the nodes in this wall will also be found using the mesh smoother.

Glenn Horrocks
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Old   February 17, 2009, 19:10
Default Re: Moving Wall
  #6
Smagmon
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Ok, I understood how it works, but if I simulate a closed tube, where the closing part is set to wall with unspecified motion, what am I doing? Is it working like a tube rigidly closed, a closing with some compliance or what?

I tried to simulate a closed tube where the input is a sinusoidal prescribed mesh motion. I would expect some wave behavior because of the fluid compressibility, but I couldn't understand how this wall at the end works physically. In a real situation, depending on the frequency of the input displacement I would have different pressure waves propagation. Also according to the end stiffness.

Can you help with this question?

Thanks..
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Old   February 18, 2009, 16:28
Default Re: Moving Wall
  #7
Gustavo
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ok, now I see where youīre trying to get. In your tube case, even if you set the closing part as unspecified motion, it will not move! Iīm simulating a similar case, itīs a compressor cilinder, and the valves open accordingly to the increasing or decreasing pressure, caused by the sinusoidal movement of the piston, but for that to happen I had to set the valves as specified motion, accordingly to a CEL that calculates the fluid force on the valves and diminishes it from the spring force acting on the valve. So if you want your closing part to move youīll have to write a CEL telling it when to move according to the pressure acting on it, but it will move as a rigid part, if you want it to have some elasticity itīs more complicated and unfortunatelly I canīt help. Thereīs a really good tutorial on CFX that you can read, itīs the fluid-structure interaction. Hope I helped....
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Old   February 24, 2009, 11:45
Default Re: Moving Wall
  #8
Smagmon
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Hi Gustavo,

Sorry for the long time without answer. I was not here in the last days. About your last answer, I did exactly what you said. About rigid body motion, it is not a problem, because is my first approach.

when I use this idea of force equilibrium, something doesn't work as well. I used the idea of the tutorial 20 but it doesn't work.

I set the mesh region to "specified displacement", and used as CEL expression for the displacement of this mesh the equation:

displ = force_x()piston / k

where piston is the region where the fluid forces act, and k is the elasticity of the spring.

thanks in advance
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Old   March 2, 2009, 16:45
Default Re: Moving Wall
  #9
Gustavo
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hummm ok. I really donīt know whatīs happening there, but let me ask you something that maybe will solve this problem, did you remember to put in the cel expression a "max" funtion? for example: displ = max (0, force_x()piston / k ) so that the top of the cilinder canīt go down, because of the spring force. Hope I helped.

Gustavo
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Old   March 2, 2009, 20:10
Default Re: Moving Wall
  #10
Smagmon
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Ahhh... no! I will try it... thanks!
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