Date: 2021-03-03T22:27:58+00:00

Actual source code: ex1f.F
petsc-3.14.5 2021-03-03
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      1: !
      2: !   Solves the time dependent Bratu problem using pseudo-timestepping
      3: !
      4: !   Concepts: TS^pseudo-timestepping
      5: !   Concepts: TS^pseudo-timestepping
      6: !   Concepts: TS^nonlinear problems
      7: !   Processors: 1
      8: !
      9: !   This code demonstrates how one may solve a nonlinear problem
     10: !   with pseudo-timestepping. In this simple example, the pseudo-timestep
     11: !   is the same for all grid points, i.e., this is equivalent to using
     12: !   the backward Euler method with a variable timestep.
     13: !
     14: !   Note: This example does not require pseudo-timestepping since it
     15: !   is an easy nonlinear problem, but it is included to demonstrate how
     16: !   the pseudo-timestepping may be done.
     17: !
     18: !   See snes/tutorials/ex4.c[ex4f.F] and
     19: !   snes/tutorials/ex5.c[ex5f.F] where the problem is described
     20: !   and solved using the method of Newton alone.
     21: !
     22: !
     23: !23456789012345678901234567890123456789012345678901234567890123456789012
     24:       program main
     25: #include <petsc/finclude/petscts.h>
     26:       use petscts
     27:       implicit none

     29: !
     30: !  Create an application context to contain data needed by the
     31: !  application-provided call-back routines, FormJacobian() and
     32: !  FormFunction(). We use a double precision array with three
     33: !  entries indexed by param, lmx, lmy.
     34: !
     35:       PetscReal user(3)
     36:       PetscInt          param,lmx,lmy,i5
     37:       parameter (param = 1,lmx = 2,lmy = 3)
     38: !
     39: !   User-defined routines
     40: !
     41:       external FormJacobian,FormFunction
     42: !
     43: !   Data for problem
     44: !
     45:       TS                ts
     46:       Vec               x,r
     47:       Mat               J
     48:       PetscInt           its,N,i1000,itmp
     49:       PetscBool  flg
     50:       PetscErrorCode      ierr
     51:       PetscReal  param_max,param_min,dt
     52:       PetscReal  tmax
     53:       PetscReal  ftime
     54:       TSConvergedReason reason

     56:       i5 = 5
     57:       param_max = 6.81
     58:       param_min = 0

     60:       call PetscInitialize(PETSC_NULL_CHARACTER,ierr)
     61:       if (ierr .ne. 0) then
     62:          print*,'Unable to initialize PETSc'
     63:          stop
     64:       endif
     65:       user(lmx)        = 4
     66:       user(lmy)        = 4
     67:       user(param)      = 6.0

     69: !
     70: !     Allow user to set the grid dimensions and nonlinearity parameter at run-time
     71: !
     72:       call PetscOptionsGetReal(PETSC_NULL_OPTIONS,PETSC_NULL_CHARACTER,                    &
     73:      &     '-mx',user(lmx),flg,ierr)
     74:       itmp = 4
     75:       call PetscOptionsGetInt(PETSC_NULL_OPTIONS,PETSC_NULL_CHARACTER,            &
     76:      &                        '-my',itmp,flg,ierr)
     77:       user(lmy) = itmp
     78:       call PetscOptionsGetReal(PETSC_NULL_OPTIONS,PETSC_NULL_CHARACTER,           &
     79:      &                        '-param',user(param),flg,ierr)
     80:       if (user(param) .ge. param_max .or.                               &
     81:      &                                user(param) .le. param_min) then
     82:         print*,'Parameter is out of range'
     83:       endif
     84:       if (user(lmx) .gt. user(lmy)) then
     85:         dt = .5/user(lmx)
     86:       else
     87:         dt = .5/user(lmy)
     88:       endif
     89:       call PetscOptionsGetReal(PETSC_NULL_OPTIONS,PETSC_NULL_CHARACTER,       &
     90:      &                         '-dt',dt,flg,ierr)
     91:       N          = int(user(lmx)*user(lmy))

     93: !
     94: !      Create vectors to hold the solution and function value
     95: !
     96:       call VecCreateSeq(PETSC_COMM_SELF,N,x,ierr)
     97:       call VecDuplicate(x,r,ierr)

     99: !
    100: !    Create matrix to hold Jacobian. Preallocate 5 nonzeros per row
    101: !    in the sparse matrix. Note that this is not the optimal strategy see
    102: !    the Performance chapter of the users manual for information on
    103: !    preallocating memory in sparse matrices.
    104: !
    105:       i5 = 5
    106:       call MatCreateSeqAIJ(PETSC_COMM_SELF,N,N,i5,PETSC_NULL_INTEGER,    &
    107:      &     J,ierr)

    109: !
    110: !     Create timestepper context
    111: !

    113:       call TSCreate(PETSC_COMM_WORLD,ts,ierr)
    114:       call TSSetProblemType(ts,TS_NONLINEAR,ierr)

    116: !
    117: !     Tell the timestepper context where to compute solutions
    118: !

    120:       call TSSetSolution(ts,x,ierr)

    122: !
    123: !    Provide the call-back for the nonlinear function we are
    124: !    evaluating. Thus whenever the timestepping routines need the
    125: !    function they will call this routine. Note the final argument
    126: !    is the application context used by the call-back functions.
    127: !

    129:       call TSSetRHSFunction(ts,PETSC_NULL_VEC,FormFunction,user,ierr)

    131: !
    132: !     Set the Jacobian matrix and the function used to compute
    133: !     Jacobians.
    134: !

    136:       call TSSetRHSJacobian(ts,J,J,FormJacobian,user,ierr)

    138: !
    139: !       For the initial guess for the problem
    140: !

    142:       call FormInitialGuess(x,user,ierr)

    144: !
    145: !       This indicates that we are using pseudo timestepping to
    146: !     find a steady state solution to the nonlinear problem.
    147: !

    149:       call TSSetType(ts,TSPSEUDO,ierr)

    151: !
    152: !       Set the initial time to start at (this is arbitrary for
    153: !     steady state problems and the initial timestep given above
    154: !

    156:       call TSSetTimeStep(ts,dt,ierr)

    158: !
    159: !      Set a large number of timesteps and final duration time
    160: !     to insure convergence to steady state.
    161: !
    162:       i1000 = 1000
    163:       tmax  = 1.e12
    164:       call TSSetMaxSteps(ts,i1000,ierr)
    165:       call TSSetMaxTime(ts,tmax,ierr)
    166:       call TSSetExactFinalTime(ts,TS_EXACTFINALTIME_STEPOVER,ierr)

    168: !
    169: !      Set any additional options from the options database. This
    170: !     includes all options for the nonlinear and linear solvers used
    171: !     internally the timestepping routines.
    172: !

    174:       call TSSetFromOptions(ts,ierr)

    176:       call TSSetUp(ts,ierr)

    178: !
    179: !      Perform the solve. This is where the timestepping takes place.
    180: !
    181:       call TSSolve(ts,x,ierr)
    182:       call TSGetSolveTime(ts,ftime,ierr)
    183:       call TSGetStepNumber(ts,its,ierr)
    184:       call TSGetConvergedReason(ts,reason,ierr)

    186:       write(6,100) its,ftime,reason
    187:  100  format('Number of pseudo time-steps ',i5,' final time ',1pe9.2    &
    188:      &     ,' reason ',i3)

    190: !
    191: !     Free the data structures constructed above
    192: !

    194:       call VecDestroy(x,ierr)
    195:       call VecDestroy(r,ierr)
    196:       call MatDestroy(J,ierr)
    197:       call TSDestroy(ts,ierr)
    198:       call PetscFinalize(ierr)
    199:       end

    201: !
    202: !  --------------------  Form initial approximation -----------------
    203: !
    204:       subroutine FormInitialGuess(X,user,ierr)
    205:       use petscts
    206:       implicit none

    208:       Vec              X
    209:       PetscReal user(3)
    210:       PetscInt  i,j,row,mx,my
    211:       PetscErrorCode ierr
    212:       PetscOffset      xidx
    213:       PetscReal one,lambda
    214:       PetscReal temp1,temp,hx,hy
    215:       PetscScalar      xx(2)
    216:       PetscInt          param,lmx,lmy
    217:       parameter (param = 1,lmx = 2,lmy = 3)

    219:       one = 1.0

    221:       mx     = int(user(lmx))
    222:       my     = int(user(lmy))
    223:       lambda = user(param)

    225:       hy    = one / (my-1)
    226:       hx    = one / (mx-1)

    228:       call VecGetArray(X,xx,xidx,ierr)
    229:       temp1 = lambda/(lambda + one)
    230:       do 10, j=1,my
    231:         temp = min(j-1,my-j)*hy
    232:         do 20 i=1,mx
    233:           row = i + (j-1)*mx
    234:           if (i .eq. 1 .or. j .eq. 1 .or.                               &
    235:      &        i .eq. mx .or. j .eq. my) then
    236:             xx(row+xidx) = 0.0
    237:           else
    238:             xx(row+xidx) =                                              &
    239:      &        temp1*sqrt(min(min(i-1,mx-i)*hx,temp))
    240:           endif
    241:  20     continue
    242:  10   continue
    243:       call VecRestoreArray(X,xx,xidx,ierr)
    244:       return
    245:       end
    246: !
    247: !  --------------------  Evaluate Function F(x) ---------------------
    248: !
    249:       subroutine FormFunction(ts,t,X,F,user,ierr)
    250:       use petscts
    251:       implicit none

    253:       TS       ts
    254:       PetscReal  t
    255:       Vec               X,F
    256:       PetscReal  user(3)
    257:       PetscErrorCode     ierr
    258:       PetscInt         i,j,row,mx,my
    259:       PetscOffset       xidx,fidx
    260:       PetscReal  two,lambda
    261:       PetscReal  hx,hy,hxdhy,hydhx
    262:       PetscScalar  ut,ub,ul,ur,u
    263:       PetscScalar  uxx,uyy,sc
    264:       PetscScalar  xx(2),ff(2)
    265:       PetscInt     param,lmx,lmy
    266:       parameter (param = 1,lmx = 2,lmy = 3)

    268:       two = 2.0

    270:       mx     = int(user(lmx))
    271:       my     = int(user(lmy))
    272:       lambda = user(param)

    274:       hx    = 1.0 / real(mx-1)
    275:       hy    = 1.0 / real(my-1)
    276:       sc    = hx*hy
    277:       hxdhy = hx/hy
    278:       hydhx = hy/hx

    280:       call VecGetArrayRead(X,xx,xidx,ierr)
    281:       call VecGetArray(F,ff,fidx,ierr)
    282:       do 10 j=1,my
    283:         do 20 i=1,mx
    284:           row = i + (j-1)*mx
    285:           if (i .eq. 1 .or. j .eq. 1 .or.                               &
    286:      &        i .eq. mx .or. j .eq. my) then
    287:             ff(row+fidx) = xx(row+xidx)
    288:           else
    289:             u            = xx(row + xidx)
    290:             ub           = xx(row - mx + xidx)
    291:             ul           = xx(row - 1 + xidx)
    292:             ut           = xx(row + mx + xidx)
    293:             ur           = xx(row + 1 + xidx)
    294:             uxx          = (-ur + two*u - ul)*hydhx
    295:             uyy          = (-ut + two*u - ub)*hxdhy
    296:             ff(row+fidx) = -uxx - uyy + sc*lambda*exp(u)
    297:             u =  -uxx - uyy + sc*lambda*exp(u)
    298:          endif
    299:  20   continue
    300:  10   continue

    302:       call VecRestoreArrayRead(X,xx,xidx,ierr)
    303:       call VecRestoreArray(F,ff,fidx,ierr)
    304:       return
    305:       end
    306: !
    307: !  --------------------  Evaluate Jacobian of F(x) --------------------
    308: !
    309:       subroutine FormJacobian(ts,ctime,X,JJ,B,user,ierr)
    310:       use petscts
    311:       implicit none

    313:       TS               ts
    314:       Vec              X
    315:       Mat              JJ,B
    316:       PetscReal user(3),ctime
    317:       PetscErrorCode   ierr
    318:       Mat              jac
    319:       PetscOffset xidx
    320:       PetscInt    i,j,row(1),mx,my
    321:       PetscInt    col(5),i1,i5
    322:       PetscScalar two,one,lambda
    323:       PetscScalar v(5),sc,xx(2)
    324:       PetscReal hx,hy,hxdhy,hydhx

    326:       PetscInt  param,lmx,lmy
    327:       parameter (param = 1,lmx = 2,lmy = 3)

    329:       i1 = 1
    330:       i5 = 5
    331:       jac = B
    332:       two = 2.0
    333:       one = 1.0

    335:       mx     = int(user(lmx))
    336:       my     = int(user(lmy))
    337:       lambda = user(param)

    339:       hx    = 1.0 / real(mx-1)
    340:       hy    = 1.0 / real(my-1)
    341:       sc    = hx*hy
    342:       hxdhy = hx/hy
    343:       hydhx = hy/hx

    345:       call VecGetArrayRead(X,xx,xidx,ierr)
    346:       do 10 j=1,my
    347:         do 20 i=1,mx
    348: !
    349: !      When inserting into PETSc matrices, indices start at 0
    350: !
    351:           row(1) = i - 1 + (j-1)*mx
    352:           if (i .eq. 1 .or. j .eq. 1 .or.                               &
    353:      &        i .eq. mx .or. j .eq. my) then
    354:             call MatSetValues(jac,i1,row,i1,row,one,INSERT_VALUES,ierr)
    355:           else
    356:             v(1)   = hxdhy
    357:             col(1) = row(1) - mx
    358:             v(2)   = hydhx
    359:             col(2) = row(1) - 1
    360:             v(3)   = -two*(hydhx+hxdhy)+sc*lambda*exp(xx(row(1)+1+xidx))
    361:             col(3) = row(1)
    362:             v(4)   = hydhx
    363:             col(4) = row(1) + 1
    364:             v(5)   = hxdhy
    365:             col(5) = row(1) + mx
    366:             call MatSetValues(jac,i1,row,i5,col,v,INSERT_VALUES,ierr)
    367:           endif
    368:  20     continue
    369:  10   continue
    370:       call MatAssemblyBegin(jac,MAT_FINAL_ASSEMBLY,ierr)
    371:       call MatAssemblyEnd(jac,MAT_FINAL_ASSEMBLY,ierr)
    372:       call VecRestoreArray(X,xx,xidx,ierr)
    373:       return
    374:       end

    376: !/*TEST
    377: !
    378: !    test:
    379: !      TODO: broken
    380: !      args: -ksp_gmres_cgs_refinement_type refine_always -snes_type newtonls -ts_monitor_pseudo -ts_max_snes_failures 3 -ts_pseudo_frtol 1.e-5  -snes_stol 1e-5
    381: !
    382: !TEST*/