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Re: [ff3d-users] Re: Re: Reading GMSH hexahedral mesh.

From: Stephane Del Pino
Subject: Re: [ff3d-users] Re: Re: Reading GMSH hexahedral mesh.
Date: Mon, 12 Mar 2007 13:27:42 +0100
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.6) Gecko/20050319


First, I think I have fixed the interpolation problem for unstructured Q1 meshes. It is quite slow but it seems to work in the cvs tree...

Cécile Giorla wrote:
Because FF3D doesn't allow to refined mesh,
Note that I am not sure that there existes *free* adaptive mesh refinement packages in 3D. If you have anyother information about that, I am interested in...

I thought importing one from GMSH would be the best way to deal with this kind of problem.
What do you think it would be best in this case?
Well, it is not easy to answer. I think it depends on what equation you want to solve: - if it is just scalar problems, using unstructured Q1 meshes will be just as fast as unstructured tetra. - if you indent to solve a time dependent problems such as Navier-Stokes, you will be penalized by the cost of the interpolation on unstructured hexahedral meshes: ff3d uses interpolation in many places to improve its genericity, but sometimes it has a cost...

For instance, for NS equations: you will need interpolation for
the convective term, this can be really costly: the problem is that you have to interpolate for each quadrature point of each element, each function of the second member and the viscosity term. Each localization being independent even if the functions leave on the same mesh. This is written in a general way, not in an optimal way. However, if some optimization have already been performed, there are still a lot missing and localization in a Q1 unstructured mesh is *very* expensive.

I think that if you can use a P1 mesh, it is probably the "best" solution...

In order to get ideas fixed, try to solve a time dependent problem using the 3 strategies:
- structured Q1 (more efficient)
- unstructured P1
- unstructured Q1 (less efficient).

I hope that helps,
Best regards,

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