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Optimal mesh for moving boundary problem

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Old   April 11, 2007, 13:59
Default Optimal mesh for moving boundary problem
  #1
Luk_Fiz
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Hello all, I simulate very simple arrangement consisted on box (one side inflow, opposite outflow). Through the centerline of the box, perpendiculary to the flow direction there is a cylinder that slowly puts into the box and stops the flow. The problem is: I meshed the arrangement with tetras and notices negative volume elements with certain position of the cylinder no matter about timesteps and other. I want to discuss-are there any limitations of CFX 10 in mesh characterizations for moving walls? If so how to mesh more complicated arrangement f.e. cylinder of the IC engine with the nozzles and valves??

A second: is that true that in CFX 10 other than "wall" BC's cannot be moving walls?

With best regards, Luk
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Old   April 12, 2007, 02:48
Default Re: Optimal mesh for moving boundary problem
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Glenn Horrocks
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Hi,

The moving mesh in CFX can stretch meshes reliably but squashing a mesh up can lead to negative volume elements. Try to start the simulation in the squashed up position if possible.

Glenn Horrocks
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Old   April 12, 2007, 03:04
Default Re: Optimal mesh for moving boundary problem
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Luk
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I guess that the best possible solution is to use hexas? But what with complicated geometry and inflation layer? I'll try to start steady state from the moment in which error appears.

Luk
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Old   April 12, 2007, 18:59
Default Re: Optimal mesh for moving boundary problem
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Glenn Horrocks
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Hi,

Hexas work better as high aspect ratio elements so that can help but you can still get negative volume elements with hexas so it will not stop the problem.

There are also some controls where you can weight the smoothing function and this can help, but no guarantees.

Glenn
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Old   April 13, 2007, 02:30
Default Re: Optimal mesh for moving boundary problem
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Luk
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You mean mesh stiffnes?

Thanks for help. Luk
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Old   April 14, 2007, 06:21
Default Re: Optimal mesh for moving boundary problem
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Glenn Horrocks
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Hi,

Yes, that's correct. I have not found it to be very useful but it might help.

Glenn Horrocks
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