On Thu, Mar 6, 2014 at 5:24 PM, Mohammad Mirzadeh <[email protected]>wrote:
> Hi guys, > > I have a discretization of Poisson equation with Neumann bc for embedded > boundary grids in such a way that that nullspace is not the usual constant > vector. Instead the nullspace is constant in the domain of interest and > zero elsewhere. > > I compute this nullspace myself and have checked it against MATLAB by > dumping the matrix and computing the nullspace explicitly using null > function -- they match and there is only this single vector. Then I take > this calculated vector and subtract it off the matrix and rhs. > "subtract it off the matrix" does not make sense to me. Are you calling KSPSetNullSpace()? Matt > However, I am having convergence issues. For instance this is the output > of ksp_monitor_true_residual for one particular run: > > 0 KSP preconditioned resid norm 3.033840960250e+02 true resid norm > 2.332886580745e-01 ||r(i)||/||b|| 1.000000000000e+00 > 1 KSP preconditioned resid norm 1.018974811826e+01 true resid norm > 1.941629896918e-02 ||r(i)||/||b|| 8.322864527335e-02 > 2 KSP preconditioned resid norm 5.450493684941e-02 true resid norm > 1.029339589324e-02 ||r(i)||/||b|| 4.412300185615e-02 > 3 KSP preconditioned resid norm 3.944064039516e-02 true resid norm > 1.030277925024e-02 ||r(i)||/||b|| 4.416322394443e-02 > 4 KSP preconditioned resid norm 6.286181172600e-05 true resid norm > 1.030243055045e-02 ||r(i)||/||b|| 4.416172923059e-02 > 5 KSP preconditioned resid norm 4.349133658643e-06 true resid norm > 1.030239080406e-02 ||r(i)||/||b|| 4.416155885630e-02 > 6 KSP preconditioned resid norm 9.279429568232e-08 true resid norm > 1.030239169298e-02 ||r(i)||/||b|| 4.416156266666e-02 > 7 KSP preconditioned resid norm 3.032522248740e-09 true resid norm > 1.030239175066e-02 ||r(i)||/||b|| 4.416156291393e-02 > 8 KSP preconditioned resid norm 6.533747246875e-09 true resid norm > 1.030239175718e-02 ||r(i)||/||b|| 4.416156294184e-02 > 9 KSP preconditioned resid norm 6.083185162500e-12 true resid norm > 1.030239175259e-02 ||r(i)||/||b|| 4.416156292220e-02 > 10 KSP preconditioned resid norm 5.510319622225e-12 true resid norm > 1.030239175259e-02 ||r(i)||/||b|| 4.416156292221e-02 > 11 KSP preconditioned resid norm 5.456758524534e-12 true resid norm > 1.030239175259e-02 ||r(i)||/||b|| 4.416156292221e-02 > 12 KSP preconditioned resid norm 5.456756081783e-12 true resid norm > 1.030239175259e-02 ||r(i)||/||b|| 4.416156292221e-02 > 13 KSP preconditioned resid norm 5.456755930952e-12 true resid norm > 1.030239175259e-02 ||r(i)||/||b|| 4.416156292221e-02 > 14 KSP preconditioned resid norm 5.456755930949e-12 true resid norm > 1.030239175259e-02 ||r(i)||/||b|| 4.416156292221e-02 > 15 KSP preconditioned resid norm 5.456755930949e-12 true resid norm > 1.030239175259e-02 ||r(i)||/||b|| 4.416156292221e-02 > > > As you can see, the true residual is quite large and moreover it does not > reduce beyond a certain point. This is using hypre as preconditioner, but the > situation is equally bad with several other preconditioner (ilu, sor, jacobi, > or even none). As for the solution itself, the error has poor to none > convergence under grid refinement. All this suggests that the linear system > is not converging in my case. > > > Do you have any idea/suggestions why this is happening and how I can avoid it? > > > Thanks > > -- What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead. -- Norbert Wiener
