xref: /petsc/src/snes/tutorials/ex9.c (revision d71ae5a4db6382e7f06317b8d368875286fe9008)

static const char help[] = "Solves obstacle problem in 2D as a variational inequality\n\
or nonlinear complementarity problem.  This is a form of the Laplace equation in\n\
which the solution u is constrained to be above a given function psi.  In the\n\
problem here an exact solution is known.\n";

/*  On a square S = {-2<x<2,-2<y<2}, the PDE
    u_{xx} + u_{yy} = 0
is solved on the set where membrane is above obstacle (u(x,y) >= psi(x,y)).
Here psi is the upper hemisphere of the unit ball.  On the boundary of S
we have Dirichlet boundary conditions from the exact solution.  Uses centered
FD scheme.  This example contributed by Ed Bueler.

Example usage:
  * get help:
    ./ex9 -help
  * monitor run:
    ./ex9 -da_refine 2 -snes_vi_monitor
  * use other SNESVI type (default is SNESVINEWTONRSLS):
    ./ex9 -da_refine 2 -snes_vi_monitor -snes_type vinewtonssls
  * use FD evaluation of Jacobian by coloring, instead of analytical:
    ./ex9 -da_refine 2 -snes_fd_color
  * X windows visualizations:
    ./ex9 -snes_monitor_solution draw -draw_pause 1 -da_refine 4
    ./ex9 -snes_vi_monitor_residual -draw_pause 1 -da_refine 4
  * full-cycle multigrid:
    ./ex9 -snes_converged_reason -snes_grid_sequence 4 -pc_type mg
  * serial convergence evidence:
    for M in 3 4 5 6 7; do ./ex9 -snes_grid_sequence $M -pc_type mg; done
  * FIXME sporadic parallel bug:
    mpiexec -n 4 ./ex9 -snes_converged_reason -snes_grid_sequence 4 -pc_type mg
*/

#include <petsc.h>

/* z = psi(x,y) is the hemispherical obstacle, but made C^1 with "skirt" at r=r0 */
PetscReal psi(PetscReal x, PetscReal y)
{
  const PetscReal r = x * x + y * y, r0 = 0.9, psi0 = PetscSqrtReal(1.0 - r0 * r0), dpsi0 = -r0 / psi0;
  if (r <= r0) {
    return PetscSqrtReal(1.0 - r);
  } else {
    return psi0 + dpsi0 * (r - r0);
  }
}

/*  This exact solution solves a 1D radial free-boundary problem for the
Laplace equation, on the interval 0 < r < 2, with above obstacle psi(x,y).
The Laplace equation applies where u(r) > psi(r),
    u''(r) + r^-1 u'(r) = 0
with boundary conditions including free b.c.s at an unknown location r = a:
    u(a) = psi(a),  u'(a) = psi'(a),  u(2) = 0
The solution is  u(r) = - A log(r) + B   on  r > a.  The boundary conditions
can then be reduced to a root-finding problem for a:
    a^2 (log(2) - log(a)) = 1 - a^2
The solution is a = 0.697965148223374 (giving residual 1.5e-15).  Then
A = a^2*(1-a^2)^(-0.5) and B = A*log(2) are as given below in the code.  */
PetscReal u_exact(PetscReal x, PetscReal y)
{
  const PetscReal afree = 0.697965148223374, A = 0.680259411891719, B = 0.471519893402112;
  PetscReal       r;
  r = PetscSqrtReal(x * x + y * y);
  return (r <= afree) ? psi(x, y)                 /* active set; on the obstacle */
                      : -A * PetscLogReal(r) + B; /* solves laplace eqn */
}

extern PetscErrorCode FormExactSolution(DMDALocalInfo *, Vec);
extern PetscErrorCode FormBounds(SNES, Vec, Vec);
extern PetscErrorCode FormFunctionLocal(DMDALocalInfo *, PetscReal **, PetscReal **, void *);
extern PetscErrorCode FormJacobianLocal(DMDALocalInfo *, PetscReal **, Mat, Mat, void *);

int main(int argc, char **argv)
{
  SNES          snes;
  DM            da, da_after;
  Vec           u, u_exact;
  DMDALocalInfo info;
  PetscReal     error1, errorinf;

  PetscFunctionBeginUser;
  PetscCall(PetscInitialize(&argc, &argv, (char *)0, help));

  PetscCall(DMDACreate2d(PETSC_COMM_WORLD, DM_BOUNDARY_NONE, DM_BOUNDARY_NONE, DMDA_STENCIL_STAR, 5, 5, /* 5x5 coarse grid; override with -da_grid_x,_y */
                         PETSC_DECIDE, PETSC_DECIDE, 1, 1,                                              /* dof=1 and s = 1 (stencil extends out one cell) */
                         NULL, NULL, &da));
  PetscCall(DMSetFromOptions(da));
  PetscCall(DMSetUp(da));
  PetscCall(DMDASetUniformCoordinates(da, -2.0, 2.0, -2.0, 2.0, 0.0, 1.0));

  PetscCall(DMCreateGlobalVector(da, &u));
  PetscCall(VecSet(u, 0.0));

  PetscCall(SNESCreate(PETSC_COMM_WORLD, &snes));
  PetscCall(SNESSetDM(snes, da));
  PetscCall(SNESSetType(snes, SNESVINEWTONRSLS));
  PetscCall(SNESVISetComputeVariableBounds(snes, &FormBounds));
  PetscCall(DMDASNESSetFunctionLocal(da, INSERT_VALUES, (DMDASNESFunction)FormFunctionLocal, NULL));
  PetscCall(DMDASNESSetJacobianLocal(da, (DMDASNESJacobian)FormJacobianLocal, NULL));
  PetscCall(SNESSetFromOptions(snes));

  /* solve nonlinear system */
  PetscCall(SNESSolve(snes, NULL, u));
  PetscCall(VecDestroy(&u));
  PetscCall(DMDestroy(&da));
  /* DMDA after solve may be different, e.g. with -snes_grid_sequence */
  PetscCall(SNESGetDM(snes, &da_after));
  PetscCall(SNESGetSolution(snes, &u)); /* do not destroy u */
  PetscCall(DMDAGetLocalInfo(da_after, &info));
  PetscCall(VecDuplicate(u, &u_exact));
  PetscCall(FormExactSolution(&info, u_exact));
  PetscCall(VecAXPY(u, -1.0, u_exact)); /* u <-- u - u_exact */
  PetscCall(VecNorm(u, NORM_1, &error1));
  error1 /= (PetscReal)info.mx * (PetscReal)info.my; /* average error */
  PetscCall(VecNorm(u, NORM_INFINITY, &errorinf));
  PetscCall(PetscPrintf(PETSC_COMM_WORLD, "errors on %" PetscInt_FMT " x %" PetscInt_FMT " grid:  av |u-uexact|  = %.3e,  |u-uexact|_inf = %.3e\n", info.mx, info.my, (double)error1, (double)errorinf));
  PetscCall(VecDestroy(&u_exact));
  PetscCall(SNESDestroy(&snes));
  PetscCall(DMDestroy(&da));
  PetscCall(PetscFinalize());
  return 0;
}

PetscErrorCode FormExactSolution(DMDALocalInfo *info, Vec u)
{
  PetscInt    i, j;
  PetscReal **au, dx, dy, x, y;
  dx = 4.0 / (PetscReal)(info->mx - 1);
  dy = 4.0 / (PetscReal)(info->my - 1);
  PetscCall(DMDAVecGetArray(info->da, u, &au));
  for (j = info->ys; j < info->ys + info->ym; j++) {
    y = -2.0 + j * dy;
    for (i = info->xs; i < info->xs + info->xm; i++) {
      x        = -2.0 + i * dx;
      au[j][i] = u_exact(x, y);
    }
  }
  PetscCall(DMDAVecRestoreArray(info->da, u, &au));
  return 0;
}

PetscErrorCode FormBounds(SNES snes, Vec Xl, Vec Xu)
{
  DM            da;
  DMDALocalInfo info;
  PetscInt      i, j;
  PetscReal   **aXl, dx, dy, x, y;

  PetscCall(SNESGetDM(snes, &da));
  PetscCall(DMDAGetLocalInfo(da, &info));
  dx = 4.0 / (PetscReal)(info.mx - 1);
  dy = 4.0 / (PetscReal)(info.my - 1);
  PetscCall(DMDAVecGetArray(da, Xl, &aXl));
  for (j = info.ys; j < info.ys + info.ym; j++) {
    y = -2.0 + j * dy;
    for (i = info.xs; i < info.xs + info.xm; i++) {
      x         = -2.0 + i * dx;
      aXl[j][i] = psi(x, y);
    }
  }
  PetscCall(DMDAVecRestoreArray(da, Xl, &aXl));
  PetscCall(VecSet(Xu, PETSC_INFINITY));
  return 0;
}

PetscErrorCode FormFunctionLocal(DMDALocalInfo *info, PetscScalar **au, PetscScalar **af, void *user)
{
  PetscInt  i, j;
  PetscReal dx, dy, x, y, ue, un, us, uw;

  PetscFunctionBeginUser;
  dx = 4.0 / (PetscReal)(info->mx - 1);
  dy = 4.0 / (PetscReal)(info->my - 1);
  for (j = info->ys; j < info->ys + info->ym; j++) {
    y = -2.0 + j * dy;
    for (i = info->xs; i < info->xs + info->xm; i++) {
      x = -2.0 + i * dx;
      if (i == 0 || j == 0 || i == info->mx - 1 || j == info->my - 1) {
        af[j][i] = 4.0 * (au[j][i] - u_exact(x, y));
      } else {
        uw       = (i - 1 == 0) ? u_exact(x - dx, y) : au[j][i - 1];
        ue       = (i + 1 == info->mx - 1) ? u_exact(x + dx, y) : au[j][i + 1];
        us       = (j - 1 == 0) ? u_exact(x, y - dy) : au[j - 1][i];
        un       = (j + 1 == info->my - 1) ? u_exact(x, y + dy) : au[j + 1][i];
        af[j][i] = -(dy / dx) * (uw - 2.0 * au[j][i] + ue) - (dx / dy) * (us - 2.0 * au[j][i] + un);
      }
    }
  }
  PetscCall(PetscLogFlops(12.0 * info->ym * info->xm));
  PetscFunctionReturn(0);
}

PetscErrorCode FormJacobianLocal(DMDALocalInfo *info, PetscScalar **au, Mat A, Mat jac, void *user)
{
  PetscInt   i, j, n;
  MatStencil col[5], row;
  PetscReal  v[5], dx, dy, oxx, oyy;

  PetscFunctionBeginUser;
  dx  = 4.0 / (PetscReal)(info->mx - 1);
  dy  = 4.0 / (PetscReal)(info->my - 1);
  oxx = dy / dx;
  oyy = dx / dy;
  for (j = info->ys; j < info->ys + info->ym; j++) {
    for (i = info->xs; i < info->xs + info->xm; i++) {
      row.j = j;
      row.i = i;
      if (i == 0 || j == 0 || i == info->mx - 1 || j == info->my - 1) { /* boundary */
        v[0] = 4.0;
        PetscCall(MatSetValuesStencil(jac, 1, &row, 1, &row, v, INSERT_VALUES));
      } else { /* interior grid points */
        v[0]     = 2.0 * (oxx + oyy);
        col[0].j = j;
        col[0].i = i;
        n        = 1;
        if (i - 1 > 0) {
          v[n]       = -oxx;
          col[n].j   = j;
          col[n++].i = i - 1;
        }
        if (i + 1 < info->mx - 1) {
          v[n]       = -oxx;
          col[n].j   = j;
          col[n++].i = i + 1;
        }
        if (j - 1 > 0) {
          v[n]       = -oyy;
          col[n].j   = j - 1;
          col[n++].i = i;
        }
        if (j + 1 < info->my - 1) {
          v[n]       = -oyy;
          col[n].j   = j + 1;
          col[n++].i = i;
        }
        PetscCall(MatSetValuesStencil(jac, 1, &row, n, col, v, INSERT_VALUES));
      }
    }
  }

  /* Assemble matrix, using the 2-step process: */
  PetscCall(MatAssemblyBegin(jac, MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(jac, MAT_FINAL_ASSEMBLY));
  if (A != jac) {
    PetscCall(MatAssemblyBegin(A, MAT_FINAL_ASSEMBLY));
    PetscCall(MatAssemblyEnd(A, MAT_FINAL_ASSEMBLY));
  }
  PetscCall(PetscLogFlops(2.0 * info->ym * info->xm));
  PetscFunctionReturn(0);
}

/*TEST

   build:
      requires: !complex

   test:
      suffix: 1
      requires: !single
      nsize: 1
      args: -da_refine 1 -snes_monitor_short -snes_type vinewtonrsls

   test:
      suffix: 2
      requires: !single
      nsize: 2
      args: -da_refine 1 -snes_monitor_short -snes_type vinewtonssls

   test:
      suffix: 3
      requires: !single
      nsize: 2
      args: -snes_grid_sequence 2 -snes_vi_monitor -snes_type vinewtonrsls

   test:
      suffix: mg
      requires: !single
      nsize: 4
      args: -snes_grid_sequence 3 -snes_converged_reason -pc_type mg

   test:
      suffix: 4
      nsize: 1
      args: -mat_is_symmetric

   test:
      suffix: 5
      nsize: 1
      args: -ksp_converged_reason -snes_fd_color

   test:
      suffix: 6
      requires: !single
      nsize: 2
      args: -snes_grid_sequence 2 -pc_type mg -snes_monitor_short -ksp_converged_reason

   test:
      suffix: 7
      nsize: 2
      args: -da_refine 1 -snes_monitor_short -snes_type composite -snes_composite_type multiplicative -snes_composite_sneses vinewtonrsls,vinewtonssls -sub_0_snes_vi_monitor -sub_1_snes_vi_monitor
      TODO: fix nasty memory leak in SNESCOMPOSITE

   test:
      suffix: 8
      nsize: 2
      args: -da_refine 1 -snes_monitor_short -snes_type composite -snes_composite_type additive -snes_composite_sneses vinewtonrsls -sub_0_snes_vi_monitor
      TODO: fix nasty memory leak in SNESCOMPOSITE

   test:
      suffix: 9
      nsize: 2
      args: -da_refine 1 -snes_monitor_short -snes_type composite -snes_composite_type additiveoptimal -snes_composite_sneses vinewtonrsls -sub_0_snes_vi_monitor
      TODO: fix nasty memory leak in SNESCOMPOSITE

TEST*/