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Re: [ff3d-users] how to use an external mesh

From: Pierre Michaud
Subject: Re: [ff3d-users] how to use an external mesh
Date: Thu, 19 Jan 2006 09:02:36 +0100
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.6) Gecko/20050323 Fedora/1.7.6-1.3.2

I Stephane,
thanks for your answer

Stephane Del Pino wrote:

Hello Pierre,

At first reading I did not noticed any mistake in your ff3d's file ...

This might be a bug in ff3d, but I am not sure about that (this is a quite basic test case, but one never knows ...). What does happen when your mesh is more refined along the Z-axis? Is it converging? The difference with the structured case could be explained by the fact that structured discretization uses Q1 (hexahedra) finite element, which is exact for tri-affine functions ((a+x)*(b+y)*(d+z)+e) and P1 is only exact for affine functions (ax+by+cz+d) functions ...

Please let me know if this converges.

I have tried with a much finer mesh (>200000 tetrahedra), but I obtain the same wrong value
as with the coarse mesh.

By the way I have a few remarks concerning your ff3d's command file:
- you should use cg instead of bicgstab: your problem is symetric, so it is more efficient, and you can use the ichol preconditionner if you use the sparse matrix storage (default is no preconditionner); - since you are consumming to much memory you could use the 'memory(matrix=none)' option. It is a bit slower but requires few memory.

Actually, in the more complex (and asymetric) problems I am studying, I am using the fictitious domain method because my geometry is not a simple box, and I use the "extract" command to define boundaries. I reach the memory limit during the initial phase, before the solver launches.


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