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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 |

Hello.

`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,
Stephane.