From: Derek Gaston <friedmud@gm...>  20080602 13:45:53

Oops... you're right about the Neumann condition... I forgot that I get a boundary term out of the integration by parts that I can just use for this purpose... I still don't quite know about the Dirichlet condition though. Yes, I am enforcing it essentially as a penalty... but that penalty just goes into the residual and is finite differenced like the rest of it. If the residual goes to zero... then it shouldn't matter if there is a multiplier on the penalty or not. The only question is of convergence. Or do you think it's the rest of the normal stuff in the residual for a boundary DOF that's screwing it up... and that's why I need the multiplier? Hmmmm.... I guess that makes sense. So, any guidance on choosing a multiplier? The problem is obviously sensitive to it... because if I make it too large I get the wrong answer just as easily as making it too small (and I do get good answers when I get the multiplier in the right range BTW). Thanks, Derek On Jun 1, 2008, at 10:39 PM, John Peterson wrote: > On Sun, Jun 1, 2008 at 10:51 PM, Derek Gaston <friedmud@...> > wrote: >> So... I've been working with the NonlinearImplicit system... solving >> jacobian free problems using Petsc SNES. Everything is going pretty >> well (Ben's interface is great!)... but I've hit a bit of a snag on >> the boundary conditions. I would really like to impose both >> Dirichlet >> and Neumann conditions similarly... by integrating their residual >> contributions over the boundary. So for instance for a Dirichlet >> condition I have something like this: >> >> Intg_boundary( u  BC ) > > Hey Derek, > > Are you doing a penalty formulation for the Dirichlet BC? You'll need > to multiply by a large penalty for this condition to be enforced. > >> and similarly for Neumann: >> >> Intg_boundary( grad_u  BC ) > > Here I don't think you subtract... you just replace du/dn with > whatever the BC is (it may depend on u, it may be nonlinear) and then > add that term to your weighted residual statement. > > J > >> The trouble that I'm running into is that these residual >> contributions >> seem to be too small... and are only very loosely satisfied by SNES. >> For instance, solving u''=0 on [0,1]x[0,1] with u=0 at x=0 and u=1 at >> x=1... I end up with u actually being .3333 at x=0 and .6666 at x=1. >> Of course I can dial down the residual tolerances for snes... but >> this >> only gets me so much closer. The other thing I've tried is to put a >> multiplier in front of the boundary integral (like 1e7 or so)... this >> ends up working well for satisfying the boundary conditions... but >> then the interior residuals are swamped out... meaning they aren't >> well resolved. >> >> So, I'm wondering if anyone has any better ideas? Is there a good >> way >> to choose the correct multiplier? Am I going about the whole thing >> incorrectly? >> >> Thanks for any help... >> >> Derek >> >>  >> This SF.net email is sponsored by: Microsoft >> Defy all challenges. Microsoft(R) Visual Studio 2008. >> http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ >> _______________________________________________ >> Libmeshusers mailing list >> Libmeshusers@... >> https://lists.sourceforge.net/lists/listinfo/libmeshusers >> 