/*T Concepts: KSP^solving a system of linear equations Concepts: KSP^Laplacian, 2d Processors: n T*/ /* Laplacian in 2D. Modeled by the partial differential equation div grad u = f, 0 < x,y < 1, with forcing function f = e^{-(1 - x)^2/\nu} e^{-(1 - y)^2/\nu} with pure Neumann boundary conditions The functions are cell-centered This uses multigrid to solve the linear system Contributed by Andrei Draganescu Note the nice multigrid convergence despite the fact it is only using peicewise constant interpolation/restriction. This is because cell-centered multigrid does not need the same rule: polynomial degree(interpolation) + polynomial degree(restriction) + 2 > degree of PDE that vertex based multigrid needs. */ static char help[] = "Solves 2D inhomogeneous Laplacian using multigrid.\n\n"; #include #include #include extern PetscErrorCode ComputeMatrix(KSP,Mat,Mat,void*); extern PetscErrorCode ComputeRHS(KSP,Vec,void*); typedef enum {DIRICHLET, NEUMANN} BCType; typedef struct { PetscScalar nu; BCType bcType; } UserContext; int main(int argc,char **argv) { KSP ksp; DM da; UserContext user; const char *bcTypes[2] = {"dirichlet","neumann"}; PetscErrorCode ierr; PetscInt bc; ierr = PetscInitialize(&argc,&argv,(char*)0,help);if (ierr) return ierr; ierr = KSPCreate(PETSC_COMM_WORLD,&ksp);CHKERRQ(ierr); ierr = DMDACreate2d(PETSC_COMM_WORLD, DM_BOUNDARY_NONE, DM_BOUNDARY_NONE,DMDA_STENCIL_STAR,12,12,PETSC_DECIDE,PETSC_DECIDE,1,1,0,0,&da);CHKERRQ(ierr); ierr = DMSetFromOptions(da);CHKERRQ(ierr); ierr = DMSetUp(da);CHKERRQ(ierr); ierr = DMDASetInterpolationType(da, DMDA_Q0);CHKERRQ(ierr); ierr = KSPSetDM(ksp,da);CHKERRQ(ierr); ierr = PetscOptionsBegin(PETSC_COMM_WORLD, "", "Options for the inhomogeneous Poisson equation", "DM");CHKERRQ(ierr); user.nu = 0.1; ierr = PetscOptionsScalar("-nu", "The width of the Gaussian source", "ex29.c", 0.1, &user.nu, NULL);CHKERRQ(ierr); bc = (PetscInt)NEUMANN; ierr = PetscOptionsEList("-bc_type","Type of boundary condition","ex29.c",bcTypes,2,bcTypes[0],&bc,NULL);CHKERRQ(ierr); user.bcType = (BCType)bc; ierr = PetscOptionsEnd();CHKERRQ(ierr); ierr = KSPSetComputeRHS(ksp,ComputeRHS,&user);CHKERRQ(ierr); ierr = KSPSetComputeOperators(ksp,ComputeMatrix,&user);CHKERRQ(ierr); ierr = KSPSetFromOptions(ksp);CHKERRQ(ierr); ierr = KSPSolve(ksp,NULL,NULL);CHKERRQ(ierr); ierr = KSPDestroy(&ksp);CHKERRQ(ierr); ierr = DMDestroy(&da);CHKERRQ(ierr); ierr = PetscFinalize(); return ierr; } PetscErrorCode ComputeRHS(KSP ksp,Vec b,void *ctx) { UserContext *user = (UserContext*)ctx; PetscErrorCode ierr; PetscInt i,j,mx,my,xm,ym,xs,ys; PetscScalar Hx,Hy; PetscScalar **array; DM da; PetscFunctionBeginUser; ierr = KSPGetDM(ksp,&da);CHKERRQ(ierr); ierr = DMDAGetInfo(da, 0, &mx, &my, 0,0,0,0,0,0,0,0,0,0);CHKERRQ(ierr); Hx = 1.0 / (PetscReal)(mx); Hy = 1.0 / (PetscReal)(my); ierr = DMDAGetCorners(da,&xs,&ys,0,&xm,&ym,0);CHKERRQ(ierr); ierr = DMDAVecGetArray(da, b, &array);CHKERRQ(ierr); for (j=ys; jnu)*PetscExpScalar(-(((PetscReal)j+0.5)*Hy)*(((PetscReal)j+0.5)*Hy)/user->nu)*Hx*Hy; } } ierr = DMDAVecRestoreArray(da, b, &array);CHKERRQ(ierr); ierr = VecAssemblyBegin(b);CHKERRQ(ierr); ierr = VecAssemblyEnd(b);CHKERRQ(ierr); /* force right hand side to be consistent for singular matrix */ /* note this is really a hack, normally the model would provide you with a consistent right handside */ if (user->bcType == NEUMANN) { MatNullSpace nullspace; ierr = MatNullSpaceCreate(PETSC_COMM_WORLD,PETSC_TRUE,0,0,&nullspace);CHKERRQ(ierr); ierr = MatNullSpaceRemove(nullspace,b);CHKERRQ(ierr); ierr = MatNullSpaceDestroy(&nullspace);CHKERRQ(ierr); } PetscFunctionReturn(0); } PetscErrorCode ComputeMatrix(KSP ksp, Mat J,Mat jac, void *ctx) { UserContext *user = (UserContext*)ctx; PetscErrorCode ierr; PetscInt i,j,mx,my,xm,ym,xs,ys,num, numi, numj; PetscScalar v[5],Hx,Hy,HydHx,HxdHy; MatStencil row, col[5]; DM da; PetscFunctionBeginUser; ierr = KSPGetDM(ksp,&da);CHKERRQ(ierr); ierr = DMDAGetInfo(da,0,&mx,&my,0,0,0,0,0,0,0,0,0,0);CHKERRQ(ierr); Hx = 1.0 / (PetscReal)(mx); Hy = 1.0 / (PetscReal)(my); HxdHy = Hx/Hy; HydHx = Hy/Hx; ierr = DMDAGetCorners(da,&xs,&ys,0,&xm,&ym,0);CHKERRQ(ierr); for (j=ys; jbcType == DIRICHLET) { v[0] = 2.0*(HxdHy + HydHx); ierr = MatSetValuesStencil(jac,1,&row,1,&row,v,INSERT_VALUES);CHKERRQ(ierr); SETERRQ(PETSC_COMM_WORLD,PETSC_ERR_SUP,"Dirichlet boundary conditions not supported !\n"); } else if (user->bcType == NEUMANN) { num = 0; numi=0; numj=0; if (j!=0) { v[num] = -HxdHy; col[num].i = i; col[num].j = j-1; num++; numj++; } if (i!=0) { v[num] = -HydHx; col[num].i = i-1; col[num].j = j; num++; numi++; } if (i!=mx-1) { v[num] = -HydHx; col[num].i = i+1; col[num].j = j; num++; numi++; } if (j!=my-1) { v[num] = -HxdHy; col[num].i = i; col[num].j = j+1; num++; numj++; } v[num] = (PetscReal)(numj)*HxdHy + (PetscReal)(numi)*HydHx; col[num].i = i; col[num].j = j; num++; ierr = MatSetValuesStencil(jac,1,&row,num,col,v,INSERT_VALUES);CHKERRQ(ierr); } } else { v[0] = -HxdHy; col[0].i = i; col[0].j = j-1; v[1] = -HydHx; col[1].i = i-1; col[1].j = j; v[2] = 2.0*(HxdHy + HydHx); col[2].i = i; col[2].j = j; v[3] = -HydHx; col[3].i = i+1; col[3].j = j; v[4] = -HxdHy; col[4].i = i; col[4].j = j+1; ierr = MatSetValuesStencil(jac,1,&row,5,col,v,INSERT_VALUES);CHKERRQ(ierr); } } } ierr = MatAssemblyBegin(jac,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr); ierr = MatAssemblyEnd(jac,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr); if (user->bcType == NEUMANN) { MatNullSpace nullspace; ierr = MatNullSpaceCreate(PETSC_COMM_WORLD,PETSC_TRUE,0,0,&nullspace);CHKERRQ(ierr); ierr = MatSetNullSpace(J,nullspace);CHKERRQ(ierr); ierr = MatNullSpaceDestroy(&nullspace);CHKERRQ(ierr); } PetscFunctionReturn(0); } /*TEST test: args: -pc_type mg -pc_mg_type full -ksp_type fgmres -ksp_monitor_short -pc_mg_levels 3 -mg_coarse_pc_factor_shift_type nonzero TEST*/