On Jun 17, 2014, at 2:30 PM, Sang pham van <[email protected]> wrote:
> Thanks Barry, > > To refine the grid, I just put more points in the direction. For both fine > and coarse meshes, I used simple initial guess (say constant values in whole > domain for all variables). > > By using grid sequencing, is the finest mesh is the one I first input the > solver? No, you pass in the coarse one. > > Can you let me know what options should I use to have pure Newton method? Not sure what you mean by pure Newton method, maybe without grid sequencing? It simply may not be possible to get convergence from a “poor” initial guess. One should always use grid sequencing if possible. Barry > > S. > > > On Tue, Jun 17, 2014 at 3:20 PM, Barry Smith <[email protected]> wrote: > > On Jun 17, 2014, at 2:12 PM, Sang pham van <[email protected]> wrote: > > > Hi, > > > > I am using DM structure and SNES to solve a 3D problem. In the problem I > > have 3 variables. > > > > I got SNES converged with a grid. Obtain result are physically right. > > However when I refine the grid, SNES does not always converge, > > With the refined grid how are you starting the solution? Do you use the > interpolated solution from the coarser grid (called grid sequencing) or just > some “not good” initial guess? > > > the reason of divergence is line search fail or linear solver failed. (I > > also tried other type of SNES, line search seems to be the one best fits my > > problem) > > > > Can you please give me a suggestion to figure out the problem with my > > solver? What options should I use to have pure Newton method in SNES? Is > > there any advance option I can use with line search to improve SNES > > convergence. > > > > You should use grid sequencing, not only does it get convergence when you > may not otherwise get it but it will also solve the problem faster. With > PETSc DM you can use -snes_grid_sequence n or SNESSetGridSequence() in the > code to do n levels of grid sequencing. > > Barry > > > Thank you very much. > > > > Minh. > > > >
