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Old   January 22, 2007, 03:13
Default abou the Y Plus
  #1
Eric
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Dear Friends,

I am simulating the distribution of compressible air in cylinder. The value of Y plus should be between 30 and 100. But my result is between 10 and 148. How to do it? Could you give me some advice, thank you in advance.

Eric
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Old   January 22, 2007, 16:11
Default Re: abou the Y Plus
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Glenn Horrocks
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Hi,

Why do you say y+ should be between 30 and 100? For many simulations y+ can higher than 100, and with automatic wall functions y+ can go down to less than one.

The standard way of adjusting y+ is by remeshing with a different thickness for the first inflation layer.

Glenn Horrocks
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Old   January 22, 2007, 20:23
Default Re: abou the Y Plus
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eric
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Dear Mr. Horrocks

Many thanks for your answer.

The person in Ansys (CFX) company told me that Y plus value.

As you said, I am trying the first layer thickness.

Best Regards Eric
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Old   January 23, 2007, 15:54
Default Re: abou the Y Plus
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Glenn Horrocks
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Hi,

Whether that range is suitable or not depends on what you are simulating. In that range you will be using the wall function approach. You should do a mesh sensitivity check to see if your near wall spacing gives reliable results.

Glenn Horrocks
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Old   January 23, 2007, 20:08
Default Re: abou the Y Plus
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Hong
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Hello, Glenn

Could you please provide more information on mesh sensitivity check?

For example, should I keep the same Y+ value for different meshes ( which will be used for sentivity tests)

Regards,

Hong
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Old   January 23, 2007, 21:38
Default Re: abou the Y Plus
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eric
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Maybe Y plus is used for the mesh quality near the wall of the model.

So, Mr. Horrocks could you explain "a mesh sensitivity check to see if your near wall spacing gives reliable results." Thank you very much

Eric
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Old   January 24, 2007, 16:18
Default Re: abou the Y Plus
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Glenn Horrocks
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Hi,

A mesh sensitivity check is where you run a number of different mesh sizes and check the results of important parameters. Assuming the simulation converges to the exact answer at zero mesh spacing, you then can estimate the error of your simulation by comparing to the finest mesh size. You then can select a mesh size which produces an acceptable level of error for your purposes.

A detailed analysis of mesh sensitivity analysis can be found in the textbook "Computational Fluid Dynamics" by Roache. Highly recommended reading for anyone interested in validating CFD results.

The Journal of Fluids Engineering demands all papers have minimum standards of validation and verification. Their policy is well worth a read: journaltool.asme.org/Templates/JFENumAccuracy.pdf

Glenn Horrocks
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Old   January 24, 2007, 22:28
Default Re: abou the Y Plus
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Eric
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Many thanks for Mr.Horrocks answer. Thank you very much.
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Old   January 26, 2007, 07:35
Default Re: abou the Y Plus
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tarek
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Hi Eric, I come cross ur qtion on line, Ive read the answers; I don't agree about that, because the first thing to talk about before look at the Y+, is which turbulence model you use?. Example standard K-E is not longer valid in internal flows, so your y+ doesn't mean any thing, and flowing ur setting , u are in compressible flow, ur y+ should be less than 3 units, no even 5. U need to do very fine mesh near the wall; then u can make a mesh adaptation in the region where ur solution on y+ is still very high .

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