Matching mesh for cyclic symmetry

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jampotp
Posts: 3
Joined: Thu Feb 18, 2021 7:13 pm

Matching mesh for cyclic symmetry

Post by jampotp »

Hi,
I'm very much enjoying using GiD - thanks for developing it.
I'm using GiD with ERMES as a problem type, and I'm using a 'periodic boundary condition' within ERMES to set up cyclic periodicity. The model geometry is a sector/wedge of the overall cylindrical volume, and the periodic boundary condition is applied to the two side faces of the cylindrical sector. I've attached an image for clarity. Under the hood of the solver, the periodic boundary condition means that nodal values on one face are copied, rotated and applied to the equivalent node on the opposite face.

Ideally the mesh on the opposite faces should be identical, so that each node has a corresponding node on the opposite face. At the moment I'm using unstructured mesh so the mesh is not identical, and the solver has to apply a large 'tolerance' to find the closest equivalent node on the opposite face. At the moment this tolerance has to be set quite large and introduces some inaccuracy to the solver.

I'm wondering if there's a way to ensure that the mesh on the opposite faces of the sector are equivalent? I have to use tetrahedral elements - other elements are not supported by ERMES.

Many thanks
James
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escolano
Posts: 1915
Joined: Sun Sep 05, 1982 10:51 pm

Re: Matching mesh for cyclic symmetry

Post by escolano »

Hi James,

About your question:

1- for this simple geometry, where both 'cyclic' surfaces are rectangles, you can set structured mesh for these surfaces, setting the same number of divisions in both surfaces. The final volume mesh will be unstructured tetrahedra, only both surfaces are structured, and you will have a 'periodic mesh'

2-In a more general case, both 'cyclic' surfaces cannot be meshed structured. You must to create an extra 'contact volume' linking both surfaces
Geometry->Create->Contact->Separated volume
and select the two 'equivalent' surfaces (must have the same number of lines)
This 'contact volume' could be considered like a 'periodicity mesh criteria', that force the meshes of both surface to have a 1-1 relationship.
This pseudo-geometric volume will generate also volume elements (by default prisms between the triangles of both surfaces), these element can aid your solver to identify paired faces or nodes (also line elements can be generated). If you don't want to have these extra elements simple set this contact volume entity to not be meshed (Mesh->Mesh criteria->No mesh->Volumes and select it)

Enrique
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