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Ashutosh February 9, 2001 08:51

Gambit mesh & Negative Volumes
Hi Fluent User's I am modelling a complex geometry and meshing it by subdividing it. When I mesh the volumes with coarse size and import the mesh into Fluent, it has negative volumes. What may be the cause of this problem? In Gambit I am checking the volume but it doesn't show any negative volume. Also in Fluent I try to use command : grid>grid-modify>face-handedness, then also the problem remains and grid check fails because of negative volume.The mesh is tetrahedron and the skew angle for coarse mesh case for few ( 10-20) cells is around 0.95.When I refine some volumes the problem disappears. But the no. of cells are more by 20%. What may be the cause and possible remedy for that? Thanks for any suggestions. Ashutosh

John C. Chien February 11, 2001 13:34

Re: Gambit mesh & Negative Volumes
(1). Try to solve a simple geometry problem first, to see whether you still have the negative volume problem. (2). If the problem is still there, call your support engineer. (3). For your complex geometry problem, first ask your support engineer to see whether it can be handled by the code or not. (4). It is not possible to answer your question because I don't know whether a general code can handle a complex geometry of yours. "EACH GEOMETRY AND MESH IS A SPECIFIC CASE". (5). It is likely that the algorithm inside the black box(the code) does not like your complex geometry and your processes to create a mesh. (A company using a commercial code without experienced engineer is likely to die many times.)

Kai Kang February 12, 2001 14:10

Re: Gambit mesh & Negative Volumes
1. sub-divide more

2. you can examine negative volumes (if any) in Gambit by shading the mesh. Becuase the surface normal of the negative volume would be different from the other good ones, there is a slightly difference in shading...

3. gambit check mesh (topology and geometry) works,too.

Ashutosh February 12, 2001 23:37

Re: Gambit mesh & Negative Volumes
Thanks Kang,

I would like to visualize the negative volumes if any in Gambit itself. Why the volume check in Gambit doesn't show negative volumes? It should work the same way we check skew angle, aspect ratio etc.If we put lower limit as negative then it should show that there are some cells ( if any) with negative volumes. Your suggeation of subdividing volumes further is well considered.

regards Ashutosh

Flav February 20, 2001 11:37

Re: Gambit mesh & Negative Volumes
I've just had a look at your .dbs file. You should try a geometry simplification for the meshing, by means of the virtual tool. Merge some of the faces, I mean the ones whose nodes are very close to each other, but not connected. That is the reason why you get some cells with skewness>0.95. The tetra cells generated in places where vertices are very close are flat, and therefore of poor quality.

Ashutosh February 21, 2001 01:59

Re: Gambit mesh & Negative Volumes
Thanks Flav, I also got similar skewness value over which the chances of getting negative volume are more.I may have to merge some nodes to get lower skewness, or use more small volumes. Even if I am successful in creating mesh with high skewness without negative volume, the convergence time would increase. So I may have to optimize between the solution time and no. of cells. Ashutosh

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