Date: 2021-03-03T22:27:55+00:00

Actual source code: ex1.c
petsc-3.14.5 2021-03-03
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      2: static char help[] ="Solves the time independent Bratu problem using pseudo-timestepping.";

      4: /*
      5:    Concepts: TS^pseudo-timestepping
      6:    Concepts: TS^pseudo-timestepping
      7:    Concepts: TS^nonlinear problems
      8:    Processors: 1

     10: */

     12: /* ------------------------------------------------------------------------

     14:     This code demonstrates how one may solve a nonlinear problem
     15:     with pseudo-timestepping. In this simple example, the pseudo-timestep
     16:     is the same for all grid points, i.e., this is equivalent to using
     17:     the backward Euler method with a variable timestep.

     19:     Note: This example does not require pseudo-timestepping since it
     20:     is an easy nonlinear problem, but it is included to demonstrate how
     21:     the pseudo-timestepping may be done.

     23:     See snes/tutorials/ex4.c[ex4f.F] and
     24:     snes/tutorials/ex5.c[ex5f.F] where the problem is described
     25:     and solved using Newton's method alone.

     27:   ----------------------------------------------------------------------------- */
     28: /*
     29:     Include "petscts.h" to use the PETSc timestepping routines. Note that
     30:     this file automatically includes "petscsys.h" and other lower-level
     31:     PETSc include files.
     32: */
     33: #include <petscts.h>

     35: /*
     36:   Create an application context to contain data needed by the
     37:   application-provided call-back routines, FormJacobian() and
     38:   FormFunction().
     39: */
     40: typedef struct {
     41:   PetscReal param;          /* test problem parameter */
     42:   PetscInt  mx;             /* Discretization in x-direction */
     43:   PetscInt  my;             /* Discretization in y-direction */
     44: } AppCtx;

     46: /*
     47:    User-defined routines
     48: */
     49: extern PetscErrorCode  FormJacobian(TS,PetscReal,Vec,Mat,Mat,void*), FormFunction(TS,PetscReal,Vec,Vec,void*), FormInitialGuess(Vec,AppCtx*);

     51: int main(int argc,char **argv)
     52: {
     53:   TS             ts;                 /* timestepping context */
     54:   Vec            x,r;               /* solution, residual vectors */
     55:   Mat            J;                  /* Jacobian matrix */
     56:   AppCtx         user;               /* user-defined work context */
     57:   PetscInt       its,N;                /* iterations for convergence */
     59:   PetscReal      param_max = 6.81,param_min = 0.,dt;
     60:   PetscReal      ftime;
     61:   PetscMPIInt    size;

     63:   PetscInitialize(&argc,&argv,NULL,help);if (ierr) return ierr;
     64:   MPI_Comm_size(PETSC_COMM_WORLD,&size);
     65:   if (size != 1) SETERRQ(PETSC_COMM_WORLD,PETSC_ERR_SUP,"This is a uniprocessor example only");

     67:   user.mx    = 4;
     68:   user.my    = 4;
     69:   user.param = 6.0;

     71:   /*
     72:      Allow user to set the grid dimensions and nonlinearity parameter at run-time
     73:   */
     74:   PetscOptionsGetInt(NULL,NULL,"-mx",&user.mx,NULL);
     75:   PetscOptionsGetInt(NULL,NULL,"-my",&user.my,NULL);
     76:   N  = user.mx*user.my;
     77:   dt = .5/PetscMax(user.mx,user.my);
     78:   PetscOptionsGetReal(NULL,NULL,"-param",&user.param,NULL);
     79:   if (user.param >= param_max || user.param <= param_min) SETERRQ(PETSC_COMM_WORLD,PETSC_ERR_ARG_OUTOFRANGE,"Parameter is out of range");

     81:   /*
     82:       Create vectors to hold the solution and function value
     83:   */
     84:   VecCreateSeq(PETSC_COMM_SELF,N,&x);
     85:   VecDuplicate(x,&r);

     87:   /*
     88:     Create matrix to hold Jacobian. Preallocate 5 nonzeros per row
     89:     in the sparse matrix. Note that this is not the optimal strategy; see
     90:     the Performance chapter of the users manual for information on
     91:     preallocating memory in sparse matrices.
     92:   */
     93:   MatCreateSeqAIJ(PETSC_COMM_SELF,N,N,5,0,&J);

     95:   /*
     96:      Create timestepper context
     97:   */
     98:   TSCreate(PETSC_COMM_WORLD,&ts);
     99:   TSSetProblemType(ts,TS_NONLINEAR);

    101:   /*
    102:      Tell the timestepper context where to compute solutions
    103:   */
    104:   TSSetSolution(ts,x);

    106:   /*
    107:      Provide the call-back for the nonlinear function we are
    108:      evaluating. Thus whenever the timestepping routines need the
    109:      function they will call this routine. Note the final argument
    110:      is the application context used by the call-back functions.
    111:   */
    112:   TSSetRHSFunction(ts,NULL,FormFunction,&user);

    114:   /*
    115:      Set the Jacobian matrix and the function used to compute
    116:      Jacobians.
    117:   */
    118:   TSSetRHSJacobian(ts,J,J,FormJacobian,&user);

    120:   /*
    121:        Form the initial guess for the problem
    122:   */
    123:   FormInitialGuess(x,&user);

    125:   /*
    126:        This indicates that we are using pseudo timestepping to
    127:      find a steady state solution to the nonlinear problem.
    128:   */
    129:   TSSetType(ts,TSPSEUDO);

    131:   /*
    132:        Set the initial time to start at (this is arbitrary for
    133:      steady state problems); and the initial timestep given above
    134:   */
    135:   TSSetTimeStep(ts,dt);

    137:   /*
    138:       Set a large number of timesteps and final duration time
    139:      to insure convergence to steady state.
    140:   */
    141:   TSSetMaxSteps(ts,10000);
    142:   TSSetMaxTime(ts,1e12);
    143:   TSSetExactFinalTime(ts,TS_EXACTFINALTIME_STEPOVER);

    145:   /*
    146:       Use the default strategy for increasing the timestep
    147:   */
    148:   TSPseudoSetTimeStep(ts,TSPseudoTimeStepDefault,0);

    150:   /*
    151:       Set any additional options from the options database. This
    152:      includes all options for the nonlinear and linear solvers used
    153:      internally the timestepping routines.
    154:   */
    155:   TSSetFromOptions(ts);

    157:   TSSetUp(ts);

    159:   /*
    160:       Perform the solve. This is where the timestepping takes place.
    161:   */
    162:   TSSolve(ts,x);
    163:   TSGetSolveTime(ts,&ftime);

    165:   /*
    166:       Get the number of steps
    167:   */
    168:   TSGetStepNumber(ts,&its);

    170:   PetscPrintf(PETSC_COMM_WORLD,"Number of pseudo timesteps = %D final time %4.2e\n",its,(double)ftime);

    172:   /*
    173:      Free the data structures constructed above
    174:   */
    175:   VecDestroy(&x);
    176:   VecDestroy(&r);
    177:   MatDestroy(&J);
    178:   TSDestroy(&ts);
    179:   PetscFinalize();
    180:   return ierr;
    181: }
    182: /* ------------------------------------------------------------------ */
    183: /*           Bratu (Solid Fuel Ignition) Test Problem                 */
    184: /* ------------------------------------------------------------------ */

    186: /* --------------------  Form initial approximation ----------------- */

    188: PetscErrorCode FormInitialGuess(Vec X,AppCtx *user)
    189: {
    190:   PetscInt       i,j,row,mx,my;
    192:   PetscReal      one = 1.0,lambda;
    193:   PetscReal      temp1,temp,hx,hy;
    194:   PetscScalar    *x;

    196:   mx     = user->mx;
    197:   my     = user->my;
    198:   lambda = user->param;

    200:   hx = one / (PetscReal)(mx-1);
    201:   hy = one / (PetscReal)(my-1);

    203:   VecGetArray(X,&x);
    204:   temp1 = lambda/(lambda + one);
    205:   for (j=0; j<my; j++) {
    206:     temp = (PetscReal)(PetscMin(j,my-j-1))*hy;
    207:     for (i=0; i<mx; i++) {
    208:       row = i + j*mx;
    209:       if (i == 0 || j == 0 || i == mx-1 || j == my-1) {
    210:         x[row] = 0.0;
    211:         continue;
    212:       }
    213:       x[row] = temp1*PetscSqrtReal(PetscMin((PetscReal)(PetscMin(i,mx-i-1))*hx,temp));
    214:     }
    215:   }
    216:   VecRestoreArray(X,&x);
    217:   return 0;
    218: }
    219: /* --------------------  Evaluate Function F(x) --------------------- */

    221: PetscErrorCode FormFunction(TS ts,PetscReal t,Vec X,Vec F,void *ptr)
    222: {
    223:   AppCtx            *user = (AppCtx*)ptr;
    224:   PetscErrorCode    ierr;
    225:   PetscInt          i,j,row,mx,my;
    226:   PetscReal         two = 2.0,one = 1.0,lambda;
    227:   PetscReal         hx,hy,hxdhy,hydhx;
    228:   PetscScalar       ut,ub,ul,ur,u,uxx,uyy,sc,*f;
    229:   const PetscScalar *x;

    231:   mx     = user->mx;
    232:   my     = user->my;
    233:   lambda = user->param;

    235:   hx    = one / (PetscReal)(mx-1);
    236:   hy    = one / (PetscReal)(my-1);
    237:   sc    = hx*hy;
    238:   hxdhy = hx/hy;
    239:   hydhx = hy/hx;

    241:   VecGetArrayRead(X,&x);
    242:   VecGetArray(F,&f);
    243:   for (j=0; j<my; j++) {
    244:     for (i=0; i<mx; i++) {
    245:       row = i + j*mx;
    246:       if (i == 0 || j == 0 || i == mx-1 || j == my-1) {
    247:         f[row] = x[row];
    248:         continue;
    249:       }
    250:       u      = x[row];
    251:       ub     = x[row - mx];
    252:       ul     = x[row - 1];
    253:       ut     = x[row + mx];
    254:       ur     = x[row + 1];
    255:       uxx    = (-ur + two*u - ul)*hydhx;
    256:       uyy    = (-ut + two*u - ub)*hxdhy;
    257:       f[row] = -uxx + -uyy + sc*lambda*PetscExpScalar(u);
    258:     }
    259:   }
    260:   VecRestoreArrayRead(X,&x);
    261:   VecRestoreArray(F,&f);
    262:   return 0;
    263: }
    264: /* --------------------  Evaluate Jacobian F'(x) -------------------- */

    266: /*
    267:    Calculate the Jacobian matrix J(X,t).

    269:    Note: We put the Jacobian in the preconditioner storage B instead of J. This
    270:    way we can give the -snes_mf_operator flag to check our work. This replaces
    271:    J with a finite difference approximation, using our analytic Jacobian B for
    272:    the preconditioner.
    273: */
    274: PetscErrorCode FormJacobian(TS ts,PetscReal t,Vec X,Mat J,Mat B,void *ptr)
    275: {
    276:   AppCtx            *user = (AppCtx*)ptr;
    277:   PetscInt          i,j,row,mx,my,col[5];
    278:   PetscErrorCode    ierr;
    279:   PetscScalar       two = 2.0,one = 1.0,lambda,v[5],sc;
    280:   const PetscScalar *x;
    281:   PetscReal         hx,hy,hxdhy,hydhx;

    284:   mx     = user->mx;
    285:   my     = user->my;
    286:   lambda = user->param;

    288:   hx    = 1.0 / (PetscReal)(mx-1);
    289:   hy    = 1.0 / (PetscReal)(my-1);
    290:   sc    = hx*hy;
    291:   hxdhy = hx/hy;
    292:   hydhx = hy/hx;

    294:   VecGetArrayRead(X,&x);
    295:   for (j=0; j<my; j++) {
    296:     for (i=0; i<mx; i++) {
    297:       row = i + j*mx;
    298:       if (i == 0 || j == 0 || i == mx-1 || j == my-1) {
    299:         MatSetValues(B,1,&row,1,&row,&one,INSERT_VALUES);
    300:         continue;
    301:       }
    302:       v[0] = hxdhy; col[0] = row - mx;
    303:       v[1] = hydhx; col[1] = row - 1;
    304:       v[2] = -two*(hydhx + hxdhy) + sc*lambda*PetscExpScalar(x[row]); col[2] = row;
    305:       v[3] = hydhx; col[3] = row + 1;
    306:       v[4] = hxdhy; col[4] = row + mx;
    307:       MatSetValues(B,1,&row,5,col,v,INSERT_VALUES);
    308:     }
    309:   }
    310:   VecRestoreArrayRead(X,&x);
    311:   MatAssemblyBegin(B,MAT_FINAL_ASSEMBLY);
    312:   MatAssemblyEnd(B,MAT_FINAL_ASSEMBLY);
    313:   if (J != B) {
    314:     MatAssemblyBegin(J,MAT_FINAL_ASSEMBLY);
    315:     MatAssemblyEnd(J,MAT_FINAL_ASSEMBLY);
    316:   }
    317:   return 0;
    318: }

    321: /*TEST

    323:     test:
    324:       args: -ksp_gmres_cgs_refinement_type refine_always -snes_type newtonls -ts_monitor_pseudo -snes_atol 1.e-7 -ts_pseudo_frtol 1.e-5 -ts_view draw:tikz:fig.tex

    326:     test:
    327:       suffix: 2
    328:       args: -ts_monitor_pseudo -ts_pseudo_frtol 1.e-5

    330: TEST*/