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Old   November 6, 2007, 10:59
Default ICEM - problems with prism mesh
João Lourenço
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I am using ICEM to generate a mesh around an aircraft. I generated a good quality surface mesh and after this began with the prism mesh for the BL. I assigned a new name for the prism part as well as the side and top surfaces of the prisms but these surface parts are never created when the prism mesh is done. Does anyone know why this happens?

I tried proceeding with the volume mesh using robust method with the existing mesh, but it changes the prisms into tetras. I believe if I had a part for the top surface of the BL this problem might be solved.

I also tried using the delaunay and advancing front but they terminate abnormally.

Does anyone have any suggestions?

Thank you very much for your attention and I'm sorry for the long post.


Joo Loureno
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Old   November 6, 2007, 15:41
Default Re: ICEM - problems with prism mesh
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Prism mesh generation in ICEM has long been a challenge. ICEM was around and functioning when many persons believed CFD could never predict pressure levels right, so few ever dreamed of considered boundary layers. ICEM therefore meshed the entire volume with tets, then tries to insert prism layers after. This is different than many more recent mesh generation programs, which tend to mesh the surface and prims layers, then fill the remaining volume with tets....for example.

The following may not exactly address your issue, but following is the 2004 text from an ICEM support engineer concerning prism meshing in ICEM. I hope it will be helpful for many.... (reference to version 5.0 is now outdated)...

The tech writes:

"I promised you some instructions on improving prism layer generation. Prism generates prism layers by extruding the surface mesh into the volume. As it marches into the mesh, it uses the existing Octree mesh to find it's way. After the last prism layer is generated, the neighboring tet mesh is split, if necessary and the prisms and surrounding tet mesh are smoothed.

Most of the problems encountered with prism generation are due to this last step. If you have a refined surface mesh, but the volume mesh expands quickly away from the surface, the neighboring tets after the prism layer may be 4 to 16 times larger than the triangular face of the last prism. This makes it very difficult to split and smooth the mesh effectively. Furthermore, since the prisms are being extruded through progressively larger tets, the smoothness of the prism layers may suffer as well (although this has been improved in version 5.0).

With this in mind, I suggest using the 'width' parameter when defining the surface element sizes. The width parameter is an integer value that tells Tetra to refine the elements in the volume next to a surface to be the same size as the surface mesh, up to 'width' elements deep. In other words, if you specify width as 4, Tetra will make the first 4 elements away from the surface the same size as the surface element. To achieve a smooth transition from the last prism layer into the volume, you should specify about 1/2 as many width layers as you want prism layers. So if you want 20 prism layers, use a width of 10. You may refine this based on your own experience as well.

A good prism layer has two main qualities; it should have a constant first node height (at least along a given surface) and it should provide a low volume ratio between the last prism element and the first tet. Prism allows you to ensure both of these criteria. The "Initial Height" parameter specifies the distance to the first node. The "Max height over base" parameter specifies the maximum ratio of thickness of the extruded dimension (height) to the edge length of the triangle being extruded (base). Generally, if you assume the first tet is nearly isotropic, a base/height ratio of about .8 will give you a volume ratio of nearly one.

If you follow these instructions, you should be able to generate a pretty good prism mesh. The three parameters to remember are "width", "initial height" and "max base over height". Please feel free to forward this to your colleagues and provide feedback to me on this advice."

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