Getting the actual higher order mesh

[Cherithl]

Hi,
I have tried to output the solution using Paraview and although I set it HigherOrderOutput to true what it gave was a grid with the same refinement level of the order of the mesh used for the solution but the nodes are equispaced in the output and not the actual grid used for the simulation. Is there a way we can get the actual quadrature grid with connectivity and the solution at those points during simulation?

[v-dobrev]

I'm not 100% sure but this may be a limitation of the ParaView format -- only uniformly-spaced (in reference space) nodes are supported.

@pazner or @mlstowell, do you know if that is actually the case?

[v-dobrev]

Alternatively, if you export your data with a VisItDataCollection, then in the latest VisIt version, 3.5, you should be able to select how to subdivide the high-order elements in the MultiresControl Operator -- either uniform or Gauss-Lobatto. See https://visit-dav.github.io/visit-website/ and https://visit-dav.github.io/visit-website/releases/VisIt-3-5-0-Release/.

[Cherithl]

Thanks for the reply. I will look into it.

I was unsure whether the ASCII solution files written by VisItDataCollection contain the actual nodal solution values. I suspect they do, since VisIt allows the visualization points to be placed either uniformly or at Gauss–Lobatto locations. To perform that interpolation accurately, VisIt would presumably need access to the original nodal data.

My main goal is to post-process very large datasets. Ideally, I would like a Python-readable format that directly provides the element connectivity, nodal coordinates, and solution values. I could then construct the reference GLL differentiation operators, metric terms, and mass matrices myself for gradient calculations and volume integrals.

Is there a Python interface or VisIt plugin that can generate the high-order GLL mesh that VisIt constructs internally from the low-order skeleton mesh?

In any case, this was very helpful. Thanks so much! :)

[pazner]

With ParaView's high-order output, only equispaced nodes are supported. This is a limitation of the VTK Lagrange functionality.

If you want to output exactly the solution values at the high-order nodes, the way to achieve this is to set the levels of detail corresponding to your polynomial degree, disable high-order output, and call GlobGeometryRefiner.SetType(quad_type), where quad_type corresponds to your nodal points. This will output a "low-order-refined" solution, where the vertices of the refined mesh coincide exactly with the locations of the high-order nodes.

[Cherithl]

Thanks for the reply, this is helpful.