<div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On Tue, Oct 22, 2013 at 3:37 PM, Matthew Knepley <span dir="ltr"><<a href="mailto:knepley@gmail.com" target="_blank">knepley@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div dir="ltr"><div>On Tue, Oct 22, 2013 at 8:29 AM, Christophe Ortiz <span dir="ltr"><<a href="mailto:christophe.ortiz@ciemat.es" target="_blank">christophe.ortiz@ciemat.es</a>></span> wrote:<br>
</div><div class="gmail_extra">
<div class="gmail_quote"><div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div dir="ltr">Hi Barry,<div>
<br></div><div>Thanks for suggestions. Here is the output.</div><div>
Before using the options you suggest I get:</div><div><br></div><div><div> TSAdapt 'basic': step 182 accepted t=0.341561 + 3.416e-02 wlte=1.1e-09 family='arkimex' scheme=0:'3' dt=3.757e-02 </div>
<div> TSAdapt 'basic': step 183 accepted t=0.375717 + 3.757e-02 wlte=9.05e-14 family='arkimex' scheme=0:'3' dt=4.133e-02 </div><div> TSAdapt 'basic': step 184 stage rejected t=0.413289 + 4.133e-02 retrying with dt=1.033e-02 </div>
<div> TSAdapt 'basic': step 184 stage rejected t=0.413289 + 1.033e-02 retrying with dt=2.583e-03 </div></div><div>...</div><div><div> TSAdapt 'basic': step 184 stage rejected t=0.413289 +2.212e-198 retrying with dt=5.529e-199</div>
<div> TSAdapt 'basic': step 184 stage rejected t=0.413289 +5.529e-199 retrying with dt=1.382e-199</div><div>[0]PETSC ERROR: --------------------- Error Message ------------------------------------</div><div>
[0]PETSC ERROR: Floating point exception!</div><div>[0]PETSC ERROR: Vec entry at local location 12 is not-a-number or infinite at end of function: Parameter number 3!</div></div><div><br></div><div><br></div><div> I re-checked my Jacobian several times, and I found few errors. But now I think it is ok.<br>
</div><div>Then, with the options you suggest, it stops immediatly. Not even one timestep:</div></div></blockquote><div><br></div></div><div>Something is very wrong here. The Jacobian should at least have something on the diagonal from the time derivative,</div>
<div>but the Laplacian (u, v) and q (w) also have diagonals.</div><div><br></div><div> Matt</div><div><br></div></div></div></div></blockquote><div><br></div><div>I found out that it works up to a certain time. Then, for intermediate times, solution exhibits artificial oscillations. For larger times, complete divergence.<br>
</div><div><br></div><div>In case it could matter, I start for u and v with two initial gaussian distributions with a peak at 1e20. They are very close to surface (i=0). For w, uniform value of 1. Do you think it could be due to a combination of very small/very large values ?</div>
<div><br></div><div>Anyway, I tried one of the hints given in the link you sent me (Why is Newton's method (SNES) not converging, or converges slowly?). I multiplied all the F() and the Jacobian by hx (the mesh size) in order to rescale, as it is suggested (<i>Change the units (nondimensionalization), boundary condition scaling, or formulation so that the Jacobian is better conditioned</i>). This trick is used in ex25.c but I removed it at the beginning since I thought it was not useful...</div>
<div> </div><div>Now it works better for larger times. The solution seems correct. However, for longer times, it gets into trouble again. The solution presents artificial oscillations again. And for longer times, complete divergence:</div>
<div><br></div><div><div> TSAdapt 'basic': step 322 accepted t=17.8389 + 5.000e-01 wlte= 0 family='arkimex' scheme=0:'1bee' dt=5.000e-01 </div><div> 0 SNES Function norm 7.451002297320e+57 </div>
<div>[0]PETSC ERROR: --------------------- Error Message ------------------------------------</div><div>[0]PETSC ERROR: Floating point exception!</div><div>[0]PETSC ERROR: Vec entry at local location 0 is not-a-number or infinite at end of function: Parameter number 3!</div>
<div>.... ....</div><div>[0]PETSC ERROR: ------------------------------------------------------------------------</div><div>[0]PETSC ERROR: VecValidValues() line 30 in src/vec/vec/interface/rvector.c</div><div>[0]PETSC ERROR: PCApply() line 444 in src/ksp/pc/interface/precon.c</div>
<div>[0]PETSC ERROR: KSP_PCApply() line 227 in src/ksp/ksp/interface//home/u5751/petsc-3.4.1/include/petsc-private/kspimpl.h</div><div>[0]PETSC ERROR: KSPInitialResidual() line 64 in src/ksp/ksp/interface/itres.c</div><div>
[0]PETSC ERROR: KSPSolve_GMRES() line 239 in src/ksp/ksp/impls/gmres/gmres.c</div><div>[0]PETSC ERROR: KSPSolve() line 441 in src/ksp/ksp/interface/itfunc.c</div><div>[0]PETSC ERROR: SNESSolve_NEWTONLS() line 220 in src/snes/impls/ls/ls.c</div>
<div>[0]PETSC ERROR: SNESSolve() line 3636 in src/snes/interface/snes.c</div><div>[0]PETSC ERROR: TSStep_ARKIMEX() line 765 in src/ts/impls/arkimex/arkimex.c</div><div>[0]PETSC ERROR: TSStep() line 2458 in src/ts/interface/ts.c</div>
<div>[0]PETSC ERROR: TSSolve() line 2583 in src/ts/interface/ts.c</div><div>[0]PETSC ERROR: main() line 457 in src/ts/examples/tutorials/diffusion.c</div></div><div><br></div><div>Other findings:</div><div><br></div><div>
- Limiting the max timestep allowed seems helping and allows to go to larger times without divergence.</div><div>- Increasing number of nodes seems to help too.</div><div><br></div><div>Does it have something to do with explicit stage of IMEX method ?</div>
<div><br></div><div><br></div><div>Christophe</div>
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