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Old   March 16, 2004, 18:11
Default "flat tetrahedral elements"
  #1
Tudor Miron
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Have a following notice: ******Notice******

2638 flat tetrahedral elements have been detected in element set 1 of domain WingSimon1.The coordinates of one of the elements are

( -0.3855E-02, -0.3878E-01, 0.6898E+00 ).

If you experience robustness or accuracy problems, consider improving the mesh

I guess that this is the reason for "old sinusoidal story" here which is happening after E-3 convergence which it reached nicely. As "notice" mentions tetra elements I assume this is not the problem with inflation.... any suggestions? One person (fluent user) suggested me to remove inflation but CFX manual states that I should use it and ensure that I have at list 5 layers in boundary layer to take advantage of SST model. Running a dual element race car rear wing. Any help is greatly appreciated.

Thank you Ted
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Old   March 17, 2004, 01:24
Default Re: "flat tetrahedral elements"
  #2
deLuther
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Hi, Ted

You build your mesh in CFX-Build or ICEM CFD? In ICEM after creation of prism layer you can smooth your tetrahedral elements(if flat tetrahedrals located at the boundary of prism layer). Principial (if you don`t have problems with geometry) you should get only prism elements in boundary layer.
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Old   March 17, 2004, 17:29
Default Re: "flat tetrahedral elements"
  #3
Glenn Horrocks
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Hi Ted,

Here is a posting I put on the CFX-Community web page a little while ago, addressing this issue. These flat elements are quie common after doing the prism layers. This is an extract of what I posted:

It is quite common to get a small number of very poor elements after prism meshing. By poor, I mean minimum element qualities around 1e-5 or worse. This is what I do to fix it (and I am very interested to hear what other people do, this is a very common problem with the default settings in ICEM CFD 4.CFX):

1) Before prism meshing, make the tet mesh as high quality as you can. A good way of doing this is to smooth the tet mesh with the following procedure: a) Set the tet elements to float, the tri elements to smooth, and turn on Laplace smoother. Smooth the mesh. b) Set the tet elements to smooth, the tri elements to freeze, and turn off the Laplace smoother. Smooth the mesh again. You are aiming to get the minimum element quality above around 0.3.

2) Do the prism mesh. Thin prism layers tend to be better quality than thicker ones in complicated geometry so don't generate too thick a prism layer in complicated geometry if you don't have to.

3) Smooth the mesh with the prism elements frozen. Sometimes this lifts the minimum element quality to a reasonable level, but often it does not.

4) If (3) did not improve the minimum element quality above about 0.1, do a further smoothing step with the following options: smooth all element types, quality metric "aspect ratio", and smooth to quality 0.1. What this does is smooth the worst elements in the entire mesh, and usually lifts the worst element to about 0.1, which is OK for most simulations. It will distort your inflation layers, that is why you use 0.1 as the maximum quality, so it only distorts locally around the worst elements. I use the aspect ratio metric as it seems to work better on the flat elements generated by prism.

That is how I generate my meshes for complicated geometries. I manage to get the minimum element quality to about 0.1 by this technique most of the time. Any comments? Any suggestions on how to improve it? Anybody use different techniques?

Glenn
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Old   March 17, 2004, 18:42
Default Re: "flat tetrahedral elements"
  #4
Tudor Miron
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Hi Glenn, I make my grid in Build so I guess that method described above is not my case . So what is left... I guess to try and make the surface mesh as fine as possible i.e. giving higher values to "surface proximity", mesh controls etc?

Thank you Ted
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Old   March 18, 2004, 17:23
Default Re: "flat tetrahedral elements"
  #5
Glenn Horrocks
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Hi Ted,

I never used to get flat elements in CFX-Build, only in ICEM CFD 4.CFX with the default smoothing options. If you are having this problem with build then as you said I would try using mesh seeds at the offending locations to improve mesh quality locally.

Glenn
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