[aspect-devel] Duplicate Vertices

[Wolfgang Bangerth]

Hi Eric,

> While working on visualizing some large datasets resulting from
> Aspect runs, I noticed something odd in the output files.  Large
> numbers of the vertices in the output mesh are duplicates and it's
> not clear to me why this is.  The temperature, pressure and velocity
> are also all identical for equivalent vertices, though the error
> indicator is not.

Yes. This is a frequently asked question :-) I'm writing this offline 
but take a look at the deal.II FAQs for the definite answer. There 
really isn't much we can do about it given the file formats we typically 
use as soon as at least one quantity we output is discontinuous :-(

> Although I'm working with HDF5 output, I believe this also applies to
> the other output formats as well.  Can someone explain why duplicate
> vertices (and associated values) exist in Aspect?  Is this an
> artifact of how the mesh is constructed or is there another reason?
> Can we merge these in the output and/or simulation to improve
> performance or are they fundamentally required?

The question is what you intend to do if you have a discontinuous field 
to visualize. (E.g., when using the conservative pressure element, or if 
you want to output the error estimator which is a cellwise quantity.) 
All formats I'm familiar with associate data with vertices but that 
automatically means that a field can only have a single value at each 
vertex. For discontinuous fields we need multiple vertices. 
Unfortunately, you can't choose to have unique vertices for u,p,T and a 
separate set of duplicated vertices for the error estimator, for example.

Best
  W.


-- 
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Wolfgang Bangerth               email:            bangerth at math.tamu.edu
                                 www: http://www.math.tamu.edu/~bangerth/