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Old   October 13, 2005, 11:15
Default Improving mesh resolution
Vidya Raja
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Hi, I have a geometry imported from ProE into Gambit. I have created a cylinder (in GAMBIT) around the solid (the idea is that the solid model is put into a tube). Then I attached another cylinder to the distal part of the first cylinder - meaning thefirst cylinder is connected to the second one. For this should I use the UNION function in GAMBIT? Or is it OK if I just move/ align the two so that the distal face of the first coincides with the proximal face of the first and thus make them appear as if they are connected?

Now,my idea is to mesh the mesh the flow field within and around the solid. For this, I subtracted the volume of the solid from the volume of the first cylinder and then meshed all the volumes. Is this procedure OK? Now, my problem is that the mesh is triangular and very very coarse. Is there any way to improve its resolution and if so, how can I do it? Do I have to use sizing function? I do not know how exactly to apply them. Can anyone help me out with this?

Thanks, Vidya
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Old   October 13, 2005, 13:52
Default Re: Improving mesh resolution
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First, about the two cylinders: No, simply aligning the two cylinders will not do you any good. You have two options. The first option is to use the UNION function in Gambit. This will create a single volume that you can mesh. The second option is to use the CONNECT function to connect the two faces that are coincidental (the CONNECT function is in the Face commands). When you connect the two faces, you will end up with two volumes sharing a single face. Now you can mesh the two volumes and when you export the mesh Gambit will recognize this as an internal feature and will not attach any boundary condition to it and will export a single continuous volume mesh for both volumes.

Now about the meshing and sizing functions: There are a few types of sizing functions (you can find the sizing functions under the tools commands... the hammer and screwdriver icon on the top right of the screen, and then it's the yellow grid that looks kind of like a yellow radar screen). The most commonly used sizing function is Fixed. Here you pick a source, an attachemnt, a start size, a growth rate, and a size limit. The source face is where you want the sizing function to start from, it can be a vertex, edge or volume and doesn't even have to be part of the geometry you are meshing. The attachment is where you want the sizing function to act. The start size, growth rate, and size limit are pretty self explanatory. So lets say you have a small curved surface, and you want a 0.1 size mesh on that face, and you want the mesh to grow from this surface across the volume at 5% until it gets to 1.0. So your source is the small face, your attachment is the volume, your start size is 0.1, your growth rate is 1.05, and your size limit is 1.0. A "meshed" sizing function is similar, except that you will have already meshed the source using whatever technique you want, and then all you have to specify is the source, attachment, growth rate, and size limit. For curvature you specify an angle (which is the maximum allowable angle between any two face normal vectors) instead of a start size.

Sizing functions are described even better in the documentation (section 5.2 in Gambit's Modeling Guide). Tutorial 6 ("Sedan Geometry - Tolerant Modeling") actually uses sizing functions. I highly recommend you going through the different tutorials. I can try and describe stuff as much as I want, but it won't set in like working through the tutorials.

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