Thank you. Hailong
On Mon, Dec 5, 2011 at 10:14 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote: > Ah, okay. With petsc-dev, just call MatSetNullSpace(). With earlier > versions, SNESGetKSP, KSPSetNullSpace. > On Dec 5, 2011 8:10 PM, "Hailong Xiao" <xiaohl1986 at gmail.com> wrote: > >> Hi Jed >> >> I am not going to set Dirichlet conditions. >> I just look at the webpage (maybe you wrote ) someone recommended >> >> >> http://www.cfd-online.com/Forums/main/15709-singular-linear-system-pressure-poisson-equation.html >> >> "You need to remove the null space. The best way is to tell the Krylov >> solver about this (see KSPSetNullSpace(...) in PETSc) but you can also fix >> the pressure at one point. That is, pick any row of your matrix and set it >> to a row of the identity and put 0 on the right hand side. This should also >> make the matrix nonsingular, but it disrupts the spectrum so may effect >> convergence rates (highly preconditioner dependent). " >> >> Since I am studying the performance of the preconditioners, it would be >> great for me that the spectrum would not be disrupted. >> It seems like KSPSetNullSpace(...) could better handle it. >> My code only has the application of the matrix not forming the matrix >> explicitly. >> >> Maybe I just fix one value is 1. >> Hailong >> >> >> On Mon, Dec 5, 2011 at 9:36 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote: >> >>> There is not a PETSc function to set Dirichlet conditions. There are >>> many ways to do it, you should pick a way they works well for your problem. >>> >>> I think you might misunderstand KSPSetNullSpace. >>> On Dec 5, 2011 6:59 PM, "Hailong Xiao" <xiaohl1986 at gmail.com> wrote: >>> >>>> Hi >>>> >>>> I am solving an elliptic problem with Neumann boundary condition >>>> with dmda routines by snes solver. >>>> I set up the residue by DMDASetLocalFunction. >>>> Besides the way like fix the first value to be one by myself, is there >>>> way to do it by petsc routine? like the KSPSetNullSpace() rountine >>>> does? >>>> >>>> -- >>>> Hailong >>>> >>> >> >> >> -- >> Hailong >> > -- Hailong -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20111205/f3134626/attachment.htm>
