> The direct solver should also converge in one iteration. Are you only > assembling an approximation of the Jacobian (e.g. using > -snes_mf_operator)? > If using MFFD, is the system poorly scaled such that the step size is very > low accuracy (maybe try -mat_mffd_type ds)? Are the equations singular? Is > both the Jacobian and residual evaluation correct?
Apparently the equations were singular. I modified the equations describing the Outflow BC by explicitly writing the open boundary condition -mu*(dw/dz)+p=0 (instead of including it in the momentum equation as I was doing before) and the linear solver is now converging within 1 iteration, snes is still diverging though. 0 SNES Function norm 7.128085632250e+00 Linear solve converged due to CONVERGED_RTOL iterations 1 1 SNES Function norm 7.068552744365e+00 Linear solve converged due to CONVERGED_RTOL iterations 1 2 SNES Function norm 7.068535930605e+00 Linear solve converged due to CONVERGED_RTOL iterations 1 3 SNES Function norm 7.068535930605e+00 Linear solve converged due to CONVERGED_RTOL iterations 1 . . (some mumps output here) . Number of Newton iterations = 3 Number of Linear iterations = 4 Average Linear its / Newton = 1.333333e+00 Converged Reason = -6 if I run it with -snes_type tr instead I get 0 SNES Function norm 7.128085632250e+00 Linear solve converged due to CONVERGED_STEP_LENGTH iterations 1 1 SNES Function norm 7.081751494639e+00 Linear solve converged due to CONVERGED_STEP_LENGTH iterations 1 2 SNES Function norm 7.068482944794e+00 Linear solve converged due to CONVERGED_STEP_LENGTH iterations 1 3 SNES Function norm 7.067980457052e+00 Linear solve converged due to CONVERGED_STEP_LENGTH iterations 1 4 SNES Function norm 7.067979237888e+00 Linear solve converged due to CONVERGED_STEP_LENGTH iterations 1 5 SNES Function norm 7.067979237888e+00 . . . Number of Newton iterations = 4 Number of Linear iterations = 5 Average Linear its / Newton = 1.250000e+00 Converged Reason = 4 I don't define the Jacobian myself I'm just calling DMMGSetSNESLocal(dmmg,FormFunctionLocal,0,0,0) I Assumed that the FD evaluation of Jacobian would be exact since the the Function is linear.
