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transient analysis of a free surface

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Old   June 1, 2007, 10:06
Default transient analysis of a free surface
  #1
Juan Martin Catelén
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What will happen if I set an adaptive mesh on a transient running? Will it last forever? I imagine the mesh adapting in each zone where the surface is flowing. What should I do to have a good result? Should I make a fine mesh in all the zones of the geometry?
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Old   June 1, 2007, 11:55
Default Re: transient analysis of a free surface
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Joe
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CFX doesnt allow mesh adaption in transient analyses.
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Old   June 1, 2007, 12:59
Default Re: transient analysis of a free surface
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Alex
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This may not be the group to ask, but I will anyways since I have been wondering about this topic. Anyone have any idea why CFX doesn't do this?

On the one hand, I can see requiring the steady state simulation for "mesh convergence," but at the same time you can use the same procedure and gradients in a transient analysis to converge the mesh for each time step in a dual time stepping scheme. I could use this feature myself in the same application: a transient free surface bubble column simulation. Also, I have seen other solvers that do utilize mesh adaption of transient solutions (Craftech/ Sandy Dash touts this as one of their competitive advantages.)

A very smart CFD engineer with many many years of experience once cautioned me against automatic mesh adaption altogether on the principle that if you begin with a coarse grid and get a poor solution, then refine the mesh based on that poor solution, you may be forcing the final result towards something that is incorrect or unphysical. I open the floor for comments...

-Alex

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Old   June 1, 2007, 13:08
Default Re: transient analysis of a free surface
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brunoc
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Well, actually CFX does it, but the GUI won't let you turn it on. It has to be done through CCL. I believe this is not allowed because in mesh adaption variables are linearly interpolated between old and new meshes. This does not guarantee conservation. On steady state it doesn't really matter since intermediate results may not be conservative, but in a transient simulation it can be a big problem. I believe this is the same reason why CFX does not re-mesh (as Fluent).
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Old   June 1, 2007, 13:52
Default Re: transient analysis of a free surface
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Joe
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Fluent does offer MA for transient flows. CFX, annoyingly, does not.
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Old   June 4, 2007, 17:19
Default Re: transient analysis of a free surface
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Lee Axon (Point Zero Consulting Ltd)
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Mesh adaption is available for CFX transient calculations. I have a document from support regarding this, drop me a mail if you want a copy.

Cheers,

Lee.

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