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problem meshing with low y+

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Old   February 11, 2009, 08:19
Default problem meshing with low y+
  #1
cesco
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Hello all, I hope someone of you can help me with this problem, I really don't know wath else I can do.

I have to mesh an area around an airfoil but the first points of the boundary layer must be really close to the airfoil

I solved the problem of the negative volumes that Gambit usually creates when the distances between the vertices are so small, creaing a large number of areas to be mesh around the airfoil.

Now the problem is the zones handedness. Typing /grid/modify-zones/repair-face-handedness I solve the problem but the .cas that I write is not readble again in fluent because f the error:

Cell Centroid is xc -0.041689 yc 1.003255 WARNING: no face with given nodes. Thread 11, cell 24868 Error: Build_Grid: grid error.

The mesh is the result of a tmerge, it will be a moving mesh.

Please help me if yo can. Thank you in advance
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Old   February 11, 2009, 09:55
Default Re: problem meshing with low y+
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Alagesanj
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Hi Cesco, I usually create single element 2D airfoil models (y+=1) with around 120,000 cells. But till now I didn't face this kind of problems. So I can not help on your problem. But my suggestion is rather than trying to modify the mesh in Fluent, create a better mesh in Gambit. If you are using single element airfoil with quad mesh then i can provide some help in creating better mesh.

cheers, Alagesan.
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Old   February 11, 2009, 11:11
Default Re: problem meshing with low y+
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cesco
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thank you very much for your interest Alagesanj, the problem is that I really do not know how to improve my mesh in Gambit. I am about sure that the problem is on the airfoil what of help would you give me? Do you think you will be able to give a lok to my mesh if I will send it to you?
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Old   February 11, 2009, 11:46
Default Re: problem meshing with low y+
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Alagesanj
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Can you provide some more information like the airfoil you are using, 2D or 3D, velocity, Y+, turbulence model you are going to use.

By the mean time refer the following links. for first cell height calculations http://geolab.larc.nasa.gov/APPS/YPlus/ , For gambit modeling http://instruct1.cit.cornell.edu/cou...tep1.htm,which will help you to create a coarse mesh. Based on this i'll give some modifications which will help to create a better model. From ther you can proceed to create a mesh for your requirement.

Cheers, Alagesan.

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Old   February 11, 2009, 12:06
Default Re: problem meshing with low y+
  #5
cesco
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Dear Alagesanj, thank you again for your interest but I already know both the sites you indicated. Anyway my problem is not to calculate the right y+, but to solve the error that the grid check gives me back:

Cell Centroid is xc -0.041689 yc 1.003255 WARNING: no face with given nodes. Thread 11, cell 24868 Error: Build_Grid: grid error.

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Old   February 11, 2009, 12:12
Default Re: problem meshing with low y+
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Alagesanj
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I never faced this type of error. So I am not sure. Try using the search function if any one faced this type of error.

Cheers, Alagesan.
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Old   February 13, 2009, 07:28
Default Re: problem meshing with low y+
  #7
bubua
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if you use single precission then you may use double precission! It helps if problem not in mesh quolity.

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Old   February 13, 2009, 11:14
Default Re: problem meshing with low y+
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cesco
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sorry but I really do not understand you suggestion. What does it mean "use single precision" When I start fluent <i can chose a version 2d or 2ddp. Do you mean I have to start the programas 2d and write the case and then reopen the same case as 2ddp?
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Old   February 23, 2009, 06:12
Default Re: problem meshing with low y+
  #9
Ralf Schmidt
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Hi!

how do you create your mesh in gambit? Du you use boundary layer option?? You can set the hight of the first cell very fine without any problems!!

Another option would be, to adapt your grid in fluent! When the simulation is finished, and you see, that your yplus is to low, adapt the mesh (adapt->yplus)

That will create hanging notes, but be careful. Fluent does only adapt the mesh, where the yplus is out of the given range - so you might end up having a jump in your cell size.

My suggestion: addapt all cell by setting the yplus range very low, e.g. min allowed = 0.0001, max allowed = 0.001.

Fluent will only half each cell once, so you halve our yplus everywhere! It is not possible in gambit to create hanging notes!

best wishes! Ralf

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