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March 6, 2012, 09:47 |
Boundary Layer thickness estimation
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#1 |
New Member
Stephen
Join Date: Feb 2012
Location: London
Posts: 6
Rep Power: 14 |
Hi guys and ladies.
I have gotten to grips with the meshing in Ansys finally and just need to be able to justify that the mesh is fine enough around the boundary layer. I am doing a VERY simple 2-d circle. I am just not sure how to work out the thickness. I have been told that I can use whatever velocity I want. Can anyone give me a quick crash course in how to calculate/justify this. Thanks in advance for any help. |
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March 6, 2012, 10:12 |
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#2 |
Senior Member
Andrew
Join Date: Mar 2009
Location: Washington, DC
Posts: 209
Rep Power: 18 |
you can calculate the thickness based on your Re. Any fluids book will show you how.
good luck |
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March 6, 2012, 10:26 |
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#3 |
New Member
Roland Peters
Join Date: Feb 2012
Posts: 3
Rep Power: 14 |
I'm having a similar issue, and was just wondering if it was ok, as an approximation, to use the Blasius solution for an airfoil? Or is that too inaccurate?
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March 6, 2012, 10:32 |
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#4 |
Senior Member
Andrew
Join Date: Mar 2009
Location: Washington, DC
Posts: 209
Rep Power: 18 |
you just need a good estimation in order to make your grid fine enough to capture it in your CFD analysis.
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March 6, 2012, 10:33 |
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#5 |
New Member
Roland Peters
Join Date: Feb 2012
Posts: 3
Rep Power: 14 |
Tidy, I've been going around in circles :s
I'll give it a crack thanks |
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March 6, 2012, 10:38 |
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#6 |
New Member
Stephen
Join Date: Feb 2012
Location: London
Posts: 6
Rep Power: 14 |
Cheers very much mate. Wasn't sure that you were aloud do that I thought I was missing a geometry factor somewhere.
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March 6, 2012, 15:26 |
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#7 |
Administrator
Peter Jones
Join Date: Jan 2009
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March 7, 2012, 04:51 |
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#8 |
New Member
Join Date: Mar 2012
Posts: 4
Rep Power: 14 |
Depending on your Re (flow material, characteristic length, velocity), you will have to use a specific thickness for your first cell to achieve a desired y+ value. For high Re you have to define very small thickness for the first cell. If you have been told you can use any velocity you want, then I would advise to run the calculation for low velocity, where you can have a good result for not so fine mesh.
The desired value of y+ depends on the turbulence model you are using. For example, for SST model y+ value should be around 1-2. The tool given above gives you an easy way to determine all these. The only thing is that in this tool you also have to specify the boundary layer length, which is not easy to be determined in all applications. I hope that helps. Panos |
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March 7, 2012, 05:35 |
Check your yplus value
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#9 |
Member
Join Date: Mar 2011
Posts: 50
Rep Power: 15 |
Your yplus value has to be less than 2 in order to accuratly represent the boundary layer. If it is more than two, refine your mesh.
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May 30, 2012, 11:03 |
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#11 |
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Join Date: Mar 2011
Posts: 50
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