Actual source code: ex66.c
petsc-3.13.6 2020-09-29
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      1: /*   DMDA/KSP solving a system of linear equations.
      2:      Poisson equation in 2D:

      4:      div(grad p) = f,  0 < x,y < 1
      5:      with
      6:        forcing function f = -cos(m*pi*x)*cos(n*pi*y),
      7:        Periodic boundary conditions
      8:          p(x=0) = p(x=1)
      9:        Neuman boundary conditions
     10:          dp/dy = 0 for y = 0, y = 1.

     12:        This example uses the DM_BOUNDARY_MIRROR to implement the Neumann boundary conditions, see the manual pages for DMBoundaryType

     14:        Compare to ex50.c
     15: */

     17: static char help[] = "Solves 2D Poisson equation,\n\
     18:                       using mirrored boundaries to implement Neumann boundary conditions,\n\
     19:                       using multigrid.\n\n";

     21:  #include <petscdm.h>
     22:  #include <petscdmda.h>
     23:  #include <petscksp.h>
     24:  #include <petscsys.h>
     25:  #include <petscvec.h>

     27: extern PetscErrorCode ComputeJacobian(KSP,Mat,Mat,void*);
     28: extern PetscErrorCode ComputeRHS(KSP,Vec,void*);

     30: typedef struct {
     31:   PetscScalar uu, tt;
     32: } UserContext;

     34: int main(int argc,char **argv)
     35: {
     36:   KSP            ksp;
     37:   DM             da;
     38:   UserContext    user;

     41:   PetscInitialize(&argc,&argv,(char*)0,help);if (ierr) return ierr;
     42:   KSPCreate(PETSC_COMM_WORLD,&ksp);
     43:   DMDACreate2d(PETSC_COMM_WORLD, DM_BOUNDARY_PERIODIC, DM_BOUNDARY_MIRROR,DMDA_STENCIL_STAR,11,11,PETSC_DECIDE,PETSC_DECIDE,1,1,NULL,NULL,&da);
     44:   DMSetFromOptions(da);
     45:   DMSetUp(da);
     46:   KSPSetDM(ksp,(DM)da);
     47:   DMSetApplicationContext(da,&user);

     49:   user.uu     = 1.0;
     50:   user.tt     = 1.0;

     52:   KSPSetComputeRHS(ksp,ComputeRHS,&user);
     53:   KSPSetComputeOperators(ksp,ComputeJacobian,&user);
     54:   KSPSetFromOptions(ksp);
     55:   KSPSolve(ksp,NULL,NULL);

     57:   DMDestroy(&da);
     58:   KSPDestroy(&ksp);
     59:   PetscFinalize();
     60:   return ierr;
     61: }

     63: PetscErrorCode ComputeRHS(KSP ksp,Vec b,void *ctx)
     64: {
     65:   UserContext    *user = (UserContext*)ctx;
     67:   PetscInt       i,j,M,N,xm,ym,xs,ys;
     68:   PetscScalar    Hx,Hy,pi,uu,tt;
     69:   PetscScalar    **array;
     70:   DM             da;
     71:   MatNullSpace   nullspace;

     74:   KSPGetDM(ksp,&da);
     75:   DMDAGetInfo(da, 0, &M, &N, 0,0,0,0,0,0,0,0,0,0);
     76:   uu   = user->uu; tt = user->tt;
     77:   pi   = 4*PetscAtanReal(1.0);
     78:   Hx   = 1.0/(PetscReal)(M);
     79:   Hy   = 1.0/(PetscReal)(N);

     81:   DMDAGetCorners(da,&xs,&ys,0,&xm,&ym,0); /* Fine grid */
     82:   DMDAVecGetArray(da, b, &array);
     83:   for (j=ys; j<ys+ym; j++) {
     84:     for (i=xs; i<xs+xm; i++) {
     85:       array[j][i] = -PetscCosScalar(uu*pi*((PetscReal)i+0.5)*Hx)*PetscCosScalar(tt*pi*((PetscReal)j+0.5)*Hy)*Hx*Hy;
     86:     }
     87:   }
     88:   DMDAVecRestoreArray(da, b, &array);
     89:   VecAssemblyBegin(b);
     90:   VecAssemblyEnd(b);

     92:   /* force right hand side to be consistent for singular matrix */
     93:   /* note this is really a hack, normally the model would provide you with a consistent right handside */
     94:   MatNullSpaceCreate(PETSC_COMM_WORLD,PETSC_TRUE,0,0,&nullspace);
     95:   MatNullSpaceRemove(nullspace,b);
     96:   MatNullSpaceDestroy(&nullspace);
     97:   return(0);
     98: }

    100: PetscErrorCode ComputeJacobian(KSP ksp,Mat J, Mat jac,void *ctx)
    101: {
    103:   PetscInt       i, j, M, N, xm, ym, xs, ys;
    104:   PetscScalar    v[5], Hx, Hy, HydHx, HxdHy;
    105:   MatStencil     row, col[5];
    106:   DM             da;
    107:   MatNullSpace   nullspace;

    110:   KSPGetDM(ksp,&da);
    111:   DMDAGetInfo(da,0,&M,&N,0,0,0,0,0,0,0,0,0,0);
    112:   Hx    = 1.0 / (PetscReal)(M);
    113:   Hy    = 1.0 / (PetscReal)(N);
    114:   HxdHy = Hx/Hy;
    115:   HydHx = Hy/Hx;
    116:   DMDAGetCorners(da,&xs,&ys,0,&xm,&ym,0);
    117:   for (j=ys; j<ys+ym; j++) {
    118:     for (i=xs; i<xs+xm; i++) {
    119:       row.i = i; row.j = j;
    120:       v[0] = -HxdHy;              col[0].i = i;   col[0].j = j-1;
    121:       v[1] = -HydHx;              col[1].i = i-1; col[1].j = j;
    122:       v[2] = 2.0*(HxdHy + HydHx); col[2].i = i;   col[2].j = j;
    123:       v[3] = -HydHx;              col[3].i = i+1; col[3].j = j;
    124:       v[4] = -HxdHy;              col[4].i = i;   col[4].j = j+1;
    125:       MatSetValuesStencil(jac,1,&row,5,col,v,ADD_VALUES);
    126:     }
    127:   }
    128:   MatAssemblyBegin(jac,MAT_FINAL_ASSEMBLY);
    129:   MatAssemblyEnd(jac,MAT_FINAL_ASSEMBLY);

    131:   MatNullSpaceCreate(PETSC_COMM_WORLD,PETSC_TRUE,0,0,&nullspace);
    132:   MatSetNullSpace(J,nullspace);
    133:   MatNullSpaceDestroy(&nullspace);
    134:   return(0);
    135: }

    139: /*TEST

    141:    build:
    142:       requires: !complex !single

    144:    test:
    145:       args: -pc_type mg -pc_mg_type full -ksp_monitor_short -da_refine 3 -mg_coarse_pc_type svd -ksp_view

    147:    test:
    148:       suffix: 2
    149:       nsize: 4
    150:       args: -pc_type mg -pc_mg_type full -ksp_monitor_short -da_refine 3 -mg_coarse_pc_type redundant -mg_coarse_redundant_pc_type svd -ksp_view

    152: TEST*/