[aspect-devel] "Not converge" problem with variable viscosity.

Wolfgang Bangerth bangerth at tamu.edu
Sun Sep 6 14:36:53 PDT 2015

> I think we had the same number of elements but the linear vs. quadratic
> elements is/was an issue that I'm not sure we factored in correctly.

It has a large effect since for quadratic elements the matrix is so much 
fuller. For example, for Q1xQ1 elements with Stokes in 3d, you typically get 
108 entries per row. For Q2xQ1 elements, you get 402. So just reading once 
through the matrix will cost you 4 times as much time. Given that the matrix 
is also worse conditioned makes the situation even slower.

> In fact I asked Shangxin to set up and run the low Ra l-3,m=2 problem not
> so much to look at time but to begin to look at accuracy.   But I have not
> seen his results yet.  The goal was to try to do this on various grids and
> see 1) how it compares with CitcomS and 2) how the various diagnostics
> compare on different levels of refinement.

I'd be curious to see what you find out with this. I'm not saying that ASPECT 
may not be slower than it should be, but it would indeed be interesting to see 
a comparison with Citcom that takes into account the accuracy you get from the 
result, and both codes using linear solver tolerances that are comparable.


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

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