Hi Wolfgang, So this was why I couldn't figure it out.
The mesh has over 100000 cells in it, it is super refined. And oddly, when the refinement level is less, it doesn't blow up. It's only after a certain point. it is an even global refinement, starting from a hyper divided rectangle. no fancy refinement. I only realised this as I wasn't getting the right convergence rates for v at the lower refinement levels so I just kept going up to see if it eventually did and it blew up. Until this point, at the refinement levels that work, the pressure converges correctly at two consecutive degrees, and so does v at the higher degree. but at the lower degree, the convergence rates for v were decreasing with refinement. That;s why I continued to refine then came across this situation. Seeing that it is only at the bottom, and noticing that if I used Dirichlet condition on that boundary, then the solution doesn't blow up and converges correctly, I can only assume it's to do with the normal flux condition there, which i think I'm imposing correctly. On Monday, March 25, 2019 at 11:36:20 PM UTC, Wolfgang Bangerth wrote: > > On 3/25/19 10:46 AM, [email protected] <javascript:> wrote: > > > > I have v = (0,-y^3) so just vertical, and p = x^3(1-y), easy enough. > > > > on a rectangular domain: > > first set of BCs, no flux on sides (boundary id 0), some top rock stress > > function (boundary id 1 for top), and Dirichlet at bottom boundary > > (boundary id 2 for bottom). > > second set of BCs, no flux on sides, some top rock stress function, and > > no flux on bottom boundary. > > > > So the only difference is the bottom boundary - one has Dirichlet, the > > other has no flux. > > > > I do a convergence test: > > using first set of BCs, refining the grid, get correct convergence rates > > etc. > > > > second set of BCs, blows up - solution on bottom boundary looks like the > > attached picture. blows up along the bottom boundary. > > > > Why does this not work?????!!!!! > > I don't know, but there are great clues that should help you find out > what the problem is. First, what is the mesh? Do you just have a mesh > with 4 cells along the bottom? If so, then each of the spikes would > correspond to one node. But looking at the shape of the spikes, I think > that you actually have many more cells along the bottom edge, so what > distinguishes the three points where there is a spike? If you can find > out what is special about these three points, you might be able to > identify what is going wrong there! > > Best > W. > > -- > ------------------------------------------------------------------------ > Wolfgang Bangerth email: [email protected] > <javascript:> > www: http://www.math.colostate.edu/~bangerth/ > -- The deal.II project is located at http://www.dealii.org/ For mailing list/forum options, see https://groups.google.com/d/forum/dealii?hl=en --- You received this message because you are subscribed to the Google Groups "deal.II User Group" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
