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coefficient calculation (a_P, a_nb) in unstructured meshes vs. structured meshes(FVM) |
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#1 |
Senior Member
Tom-Robin Teschner
Join Date: Dec 2011
Location: Cranfield, UK
Posts: 211
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I am currently trying to understand unstructured meshes and am implementing the diffusion equation on a simple 2D rectangular domain with an unstructured mesh. I did this a while back for structured meshes and I was calculating the neighbor coefficient as
![]() where ![]() ![]() ![]() ![]() ![]() ![]() The index 0 refers to the current cell and 1 to one of its neighbor. ![]() ![]() Hence, if ![]() ![]() ![]() ![]() ![]() For unstructured meshes, the neighbor coefficients are calculated as ![]() where ![]() and ![]() If I expand ![]() ![]() Since both ![]() ![]() ![]() ![]() Now I am not sure if that is to be expected (as for the structured code I get solely positive values) but I am having the feeling that this i causing some problems for my code. So the question is probably more, is there any definition for face/unit vector of ![]() I am using a face based approach where I solve over each face in the interior (followed by the boundary faces) where I add contribution to cell 0 (current cell I am in) and substract them from cell C1 (neighbor cell). If anyone with experience with unstructured meshes and has a hunch what I am doing wrong I would appreciate the help. |
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#2 | |
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Michael Prinkey
Join Date: Mar 2009
Location: Pittsburgh PA
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Quote:
![]() ![]() Remember that the coefficients you are computing are really just approximating the summation of the gradient of the unknown quantity at the face dotted with the face area vector. The direction of the area vector assumes that flux is exiting cell0 and entering cell1. Here is how it is done in ANSYS Fluent for reference--note Figure 3.2.2: http://aerojet.engr.ucdavis.edu/flue...udf/node92.htm |
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#3 |
Senior Member
Tom-Robin Teschner
Join Date: Dec 2011
Location: Cranfield, UK
Posts: 211
Rep Power: 17 ![]() |
thanks, my area and unit vector where in fact pointing in different directions for some cells, problem is solved now and results look as expected. my cross diffusion term is making something odd but without it its ok. will try to track that bug down. but thanks again.
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