gmsh mesh manipulations with small lengths

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Captain Slow
Posts: 72
Joined: 10 Nov 2009, 19:17

gmsh mesh manipulations with small lengths

Post by Captain Slow » 19 Oct 2011, 19:36

Hi all,

I use gmsh to produce the meshes I use in elmer. I am currently trying to obtain a very small, 10^-6 or less, characteristic length (with box or boundary layer fields) in a specific area of the mesh.

I have managed to control specific areas of the mesh with the box field for test cases of larger structures and characteristic lengths, but these fields seem to be having no effect on the mesh if the lengths are small. So I wonder if anyone with else has experienced this or could suggest anything that could help?

John

tibich72
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Joined: 07 Dec 2009, 05:16

Re: gmsh mesh manipulations with small lengths

Post by tibich72 » 20 Oct 2011, 17:37

I don't have any direct experience with very small characteristic lengths (like you, I've used box fields with reasonably large characteristic lengths and it worked). However, one thing that you might want to try is to scale the model: for example, if the dimensions of your model are in meters, scale the geometry to mm's, and then the characteristic length is 3 times larger (or you can scale the model 5 orders of magnitude, if you need a characteristic length of 10^-6 -- I've noticed that characteristic lengths of 0.1 or so work well). Then, in ElmerGrid, you could use the -scale switch to scale back the mesh by the same factor.

raback
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Re: gmsh mesh manipulations with small lengths

Post by raback » 20 Oct 2011, 18:06

..or you can give the "Coordinate Scaling = Real 1.0e-6" keyword in the Simulation section of Elmer. -Peter

Captain Slow
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Joined: 10 Nov 2009, 19:17

Re: gmsh mesh manipulations with small lengths

Post by Captain Slow » 26 Oct 2011, 18:09

I've now found the solution to my original question.

The points created in gmsh are saved as 3 co-ordinates plus a fourth for the characteristic length (set to 1e+22 by default), it seems that to get the box field to work correctly the ''Vout'' of the box section must match the characteristic length part of the points. After this one has to be careful to select appropriate values for Vin, Vout and the box's dimensions.

tibich72 & Peter, thankyou for your comments.

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