Date: 2025-03-28T21:10:54+00:00

Actual source code: ex66.c

      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:        Neumann 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;

     40:   PetscFunctionBeginUser;
     41:   PetscCall(PetscInitialize(&argc, &argv, NULL, help));
     42:   PetscCall(KSPCreate(PETSC_COMM_WORLD, &ksp));
     43:   PetscCall(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:   PetscCall(DMSetFromOptions(da));
     45:   PetscCall(DMSetUp(da));
     46:   PetscCall(KSPSetDM(ksp, (DM)da));
     47:   PetscCall(DMSetApplicationContext(da, &user));

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

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

     57:   PetscCall(DMDestroy(&da));
     58:   PetscCall(KSPDestroy(&ksp));
     59:   PetscCall(PetscFinalize());
     60:   return 0;
     61: }

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

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

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

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

     98: PetscErrorCode ComputeJacobian(KSP ksp, Mat J, Mat jac, void *ctx)
     99: {
    100:   PetscInt     i, j, M, N, xm, ym, xs, ys;
    101:   PetscScalar  v[5], Hx, Hy, HydHx, HxdHy;
    102:   MatStencil   row, col[5];
    103:   DM           da;
    104:   MatNullSpace nullspace;

    106:   PetscFunctionBeginUser;
    107:   PetscCall(KSPGetDM(ksp, &da));
    108:   PetscCall(DMDAGetInfo(da, 0, &M, &N, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0));
    109:   Hx    = 1.0 / (PetscReal)M;
    110:   Hy    = 1.0 / (PetscReal)N;
    111:   HxdHy = Hx / Hy;
    112:   HydHx = Hy / Hx;
    113:   PetscCall(DMDAGetCorners(da, &xs, &ys, 0, &xm, &ym, 0));
    114:   for (j = ys; j < ys + ym; j++) {
    115:     for (i = xs; i < xs + xm; i++) {
    116:       row.i    = i;
    117:       row.j    = j;
    118:       v[0]     = -HxdHy;
    119:       col[0].i = i;
    120:       col[0].j = j - 1;
    121:       v[1]     = -HydHx;
    122:       col[1].i = i - 1;
    123:       col[1].j = j;
    124:       v[2]     = 2.0 * (HxdHy + HydHx);
    125:       col[2].i = i;
    126:       col[2].j = j;
    127:       v[3]     = -HydHx;
    128:       col[3].i = i + 1;
    129:       col[3].j = j;
    130:       v[4]     = -HxdHy;
    131:       col[4].i = i;
    132:       col[4].j = j + 1;
    133:       PetscCall(MatSetValuesStencil(jac, 1, &row, 5, col, v, ADD_VALUES));
    134:     }
    135:   }
    136:   PetscCall(MatAssemblyBegin(jac, MAT_FINAL_ASSEMBLY));
    137:   PetscCall(MatAssemblyEnd(jac, MAT_FINAL_ASSEMBLY));

    139:   PetscCall(MatNullSpaceCreate(PETSC_COMM_WORLD, PETSC_TRUE, 0, 0, &nullspace));
    140:   PetscCall(MatSetNullSpace(J, nullspace));
    141:   PetscCall(MatNullSpaceDestroy(&nullspace));
    142:   PetscFunctionReturn(PETSC_SUCCESS);
    143: }

    145: /*TEST

    147:    build:
    148:       requires: !complex !single

    150:    test:
    151:       args: -pc_type mg -pc_mg_type full -ksp_monitor_short -da_refine 3 -mg_coarse_pc_type svd -ksp_view

    153:    test:
    154:       suffix: 2
    155:       nsize: 4
    156:       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

    158: TEST*/