[petsc-users] convergence problem- 3D Cahn Hillard

[Sepideh Kavousi]

Dear Petsc Users
I wrote a code in 2D solving PF equations for solidification of alloys. I have two variables in the equations 1-non-dimensional concentration (U) 2- order parameter (p). Everything is fine with this code.
When I change the code to 3D, the code has convergence problems which I cannot solve it.
I know the term which is causing problem is a term in the equation for U:
(dp/dt)*(Pxx+Pyy+Pzz)
where Pxx,Pyy,Pzz are the second derivative of order parameter in x,y,z, respectively.

When I eliminate this term, the results of 2D and 3D are close to each other. But adding that term the time step should be reduced a lot to obtain convergence, otherwise the timestep does not converge. I have no idea how I should solve this problem.
 I solve the equations considering (I chose based on suggestions of Moose for solving Cahn-hillard)
 -ts_type bdf -ts_bdf_adapt -pc_type bjacobi -snes_linesearch_type l2 -snes_type newtontr -ksp_type gmres -ksp_gmres_restart 1001 -sub_pc_type ilu -sub_ksp_type preonly

Would you please help me solve this issue?
Best,
sepideh

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