Multigrid with 3D implicit line agglomeration

[bigfooted]

Proposed Changes

fix implicit line agglomeration

When MG_IMPLICIT_LINE_AGGLOMERATION=YES, we try to create structured coarser meshes by first agglomerating the boundary, then use a paving method, also known as an advancing front method. We advance the entire front layer by layer, so all viscous walls advance one layer at a time, until the end, until an edge direction deviates more than 30 degrees or until a collision. When paving for a single cell fails, paving for the entire marker stops.

After agglomeration of the viscous walls, the other boundaries are agglomerated in a structured way if their first cell aspect ratio is higher than 2. For structured quadrilateral meshes with inflation layers this method leads to very high quality multigrids.
With MPI, the layers try to continue through the mpi interface. When the mpi interface created by parmetis is not very 'straight', this method often terminates and quality deteriorates. However, even the fallback to regular agglomeration still leads to high quality meshes.

Related Work

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