On Wed, May 27, 2015 at 10:52 PM, Barry Smith <[email protected]> wrote:
> > The built in PETSc LU factorizations have special options to attempt to > "shift" a singular matrix (by adding something to a zero pivot). These > methods are ad hoc but sometimes perform satisfactory inside a > preconditioner. See the manual page for PCLU in particular > -pc_factor_shift_type <shifttype> - Sets shift type or PETSC_DECIDE for the > default; use '-help' for a list of available types See also > http://www.mcs.anl.gov/petsc/documentation/faq.html#zeropivot The other option is to use -pc_type svd for the coarse grid. Matt > > Barry > > > > On May 27, 2015, at 10:40 PM, Amneet Bhalla <[email protected]> > wrote: > > > > Hi Folks, > > > > We are dealing with a singular matrix that arises from Stokes' equation > with Dirichlet velocity BCs. The way we are handling it is to pass > nullspace info to KSP, i.e, vector of {u,p} = {0,1}, and letting the matrix > to be singular. However, we are using ASM as the preconditioner and when > the subpc covers the entire domain (which can be the case in the > preconditioning step of our MG algorithm), LU decomposition fails. We are > thinking of pinning one of the pressure DOFs to zero explicitly, so that we > can avoid singular matrix for the LU decomposition. Is there another method > of doing it, which avoids explicit pinning down of an arbitrary pressure > DOF? > > > > Thanks, > > --Amneet > > > > -- > > --Amneet > > > > > > > > -- 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
