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Old   October 18, 2006, 08:51
Default aspect ratio
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boe
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How can I control the aspect ratio of my mesh in CFX-mesher?I need of an inflated boundary to obtain y+ values for k-epsilon turbolence model: how high aspect ratio must be to optimize my analisys? Thanks!
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Old   October 18, 2006, 17:01
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Glenn Horrocks
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Hi,

If you are referring to the inflation layers at the boundaries: You can control them with the properties tab. Have a look at the documentation.

Optimum aspect ratio: There is no general optimum, it problem dependant. You should do a sensitivity analysis and determine what the best aspect ratio to use for your simulation.

Glenn Horrocks
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Old   October 19, 2006, 02:22
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boe
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Thank you Glenn. I tried several discretization by forcing either y+ value and first element high.The problem is that I obtained very small elements that are not suitable for k epsilon turbulence model.Calculation fails with a "fatal overflow error".Now I'm trying with k omega and double precision: calculation goes, but it is very very slow! Do you think this problems could be related? Do you haev some hint about this? Thanks again!
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Old   October 19, 2006, 16:54
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Glenn Horrocks
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Hi,

Your error does not mean your elements are too small for the k-epsilon turbulence model. It means the solution has diverged. You would have to describe what you are modelling better if you want us to help you.

Glenn Horrocks
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Old   October 20, 2006, 00:14
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TB
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Carrying out a mesh dependency test will give you lots of insight about the discretisation error. A balancing approach is always needed. Is it worth the effort to gain the 1% accuracy if it takes a month to produce a solution? Errors such as assumptions made in turbulence model could easily overwhelm the discretisation error, as the mesh is refining.
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Old   October 20, 2006, 02:13
Default Re: aspect ratio
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boe
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Ok I'll describe my problem. I have to calculate a multiphase flow field. My domain is entirely closed and contains 60% in volume fraction of liquid and 40% of compressible fluid (Air Ideal Gas). Domain oscillates around a stable position so calculation is unsteady calculation with a time step of about 10E-5. How can I set the problem?How can I fix the pressure level for the compressible fluid since the domain is closed? Last calculation failed and gave a fatal bounds error (Absolute Pressure). Have you some hint?

Thank you!
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