Hi Wolfgang Thanks for your reply.
That was actually what I had done previously. I've tried all sorts by looking at the values on different boundaries, indicators, tried multiple problems, etc before I had asked the question, which is why I came to thinking it had something with the implementation. Thanks for your suggestions - I had another thought, that perhaps it might be a mathematical issue. On a no normal flux condition in the Stokes case, am I correct in assuming that for the reamining components (1- nxn)(n.[pI-2e(u)]) for the boundary ocndition, if homogenous (=0) then it disappears within the weak form, and if non zero, it needs to be imposed weakly? or does something else need to take place for this? On Monday, April 15, 2019 at 3:33:36 PM UTC+1, Wolfgang Bangerth wrote: > > > > 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. > > Jane -- I don't think anyone other than you is in a position to figure > this > out. I've learned to debug problems with a mind set of assuming that > everything I believe is correct may in fact not. So when you say > "I can only assume it's to do with the normal flux condition there, > which > i think I'm imposing correctly" > then my approach is to assume that they are not imposed correctly, and to > write code that helps me to *verify* that it is. > > As an example, even though you think that you've set the boundary > indicators > correctly at all cells on the bottom, this may not be the case for > whatever > reason. Write a loop over all cells and all boundary faces and *output* > location and boundary indicator, to make sure it is. Go through this sort > of > procedure for everything you *believe* should be true to make sure that it > *is* true. > > I'm sorry I have no other suggestions for what to do. We've all been in > your > situation where stuff doesn't work for reasons that seem completely > mysterious. The successful among us are the ones who have built the mental > and > computer skills to figure out what the cause is. Among the mental skills > is > the ability to assume that what one believes to be true (because one has > written the code) may not be so. The computer skills is then to use a > debugger > or print statements in the right places to verify that something is indeed > the > case or not, and to narrow down the possible root causes. > > 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.
