[petsc-dev] The question about TS^pseudo-timestepping

[Lulu Liu]

Hi,

I looked at the example /petsc/src/ts/examples/tutorials/ex1.c, which is to
solve a 2D Bratu nonlinear PDE  with pseudo-timestepping.

  ierr = TSSetRHSFunction(ts,NULL,*FormFunction*,&user);CHKERRQ(ierr);

*Q1:In the FormFunction(), I do not understand why this example uses 1D
array, not 2D array x[j][i]? Is it necessary to use 1D array when we use
the pseudo-timestepping method?*

#undef __FUNCT__
#define __FUNCT__ "FormFunction"
PetscErrorCode FormFunction(TS ts,PetscReal t,Vec X,Vec F,void *ptr)
{

…….
  for (j=0; j<my; j++) {
    for (i=0; i<mx; i++) {
      row = i + j*mx;
      if (i == 0 || j == 0 || i == mx-1 || j == my-1) {
        f[row] = x[row];
        continue;
      }
      u      = x[row];
      ub     = x[row - mx];
      ul     = x[row - 1];
      ut     = x[row + mx];
      ur     = x[row + 1];
      uxx    = (-ur + two*u - ul)*hydhx;
      uyy    = (-ut + two*u - ub)*hxdhy;
      f[row] = -uxx + -uyy + sc*lambda*PetscExpScalar(u);
    }
  }
……..
}

*Q2: I try to modify this function  using local 2D array x[j][i], but got
some errors:*
[0]PETSC ERROR:
[0]PETSC ERROR: Must call DMShellSetLocalVector() or
DMShellSetCreateLocalVector()
[0]PETSC ERROR: See http://www.mcs.anl.gov/petsc/documentation/faq.html for
trouble shooting.
[0]PETSC ERROR: Petsc Release Version 3.5.1, Jul, 24, 2014
[0]PETSC ERROR: ./test on a arch-darwin-c-debug named kl-12681.local by
liul Sun Sep 14 18:42:38 2014
[0]PETSC ERROR: Configure options --with-cc=gcc --with-cxx=g++
--with-fc=gfortran --download-fblaslapack --download-mpich
[0]PETSC ERROR: #1 DMCreateLocalVector_Shell() line 271 in
/Users/liul/soft/petsc-3.5.1/petsc/src/dm/impls/shell/dmshell.c
[0]PETSC ERROR: #2 DMCreateLocalVector() line 709 in
/Users/liul/soft/petsc-3.5.1/petsc/src/dm/interface/dm.c
[0]PETSC ERROR: #3 DMGetLocalVector() line 49 in
/Users/liul/soft/petsc-3.5.1/petsc/src/dm/interface/dmget.c
[0]PETSC ERROR: #4 FormFunction() line 253 in
/Users/liul/Desktop/Combustion/test.c
[0]PETSC ERROR: #5 TSComputeRHSFunction() line 475 in
/Users/liul/soft/petsc-3.5.1/petsc/src/ts/interface/ts.c
[0]PETSC ERROR: #6 TSComputeIFunction() line 688 in
/Users/liul/soft/petsc-3.5.1/petsc/src/ts/interface/ts.c
[0]PETSC ERROR: #7 TSPseudoMonitorDefault() line 330 in
/Users/liul/soft/petsc-3.5.1/petsc/src/ts/impls/pseudo/posindep.c
[0]PETSC ERROR: #8 TSMonitor() line 2809 in
/Users/liul/soft/petsc-3.5.1/petsc/src/ts/interface/ts.c
[0]PETSC ERROR: #9 TSSolve() line 2751 in
/Users/liul/soft/petsc-3.5.1/petsc/src/ts/interface/ts.c
[0]PETSC ERROR: #10 main() line 162 in /Users/liul/Desktop/Combustion/test.c

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