Actual source code: ex22.c
petsc-3.6.4 2016-04-12
1: static const char help[] = "Time-dependent advection-reaction PDE in 1d, demonstrates IMEX methods.\n";
2: /*
3: u_t + a1*u_x = -k1*u + k2*v + s1
4: v_t + a2*v_x = k1*u - k2*v + s2
5: 0 < x < 1;
6: a1 = 1, k1 = 10^6, s1 = 0,
7: a2 = 0, k2 = 2*k1, s2 = 1
9: Initial conditions:
10: u(x,0) = 1 + s2*x
11: v(x,0) = k0/k1*u(x,0) + s1/k1
13: Upstream boundary conditions:
14: u(0,t) = 1-sin(12*t)^4
16: Useful command-line parameters:
17: -ts_arkimex_fully_implicit - treats advection implicitly, only relevant with -ts_type arkimex (default)
18: -ts_type rosw - use Rosenbrock-W method (linearly implicit IMEX, amortizes assembly and preconditioner setup)
19: */
21: #include <petscdm.h>
22: #include <petscdmda.h>
23: #include <petscts.h>
25: typedef PetscScalar Field[2];
27: typedef struct _User *User;
28: struct _User {
29: PetscReal a[2]; /* Advection speeds */
30: PetscReal k[2]; /* Reaction coefficients */
31: PetscReal s[2]; /* Source terms */
32: };
34: static PetscErrorCode FormRHSFunction(TS,PetscReal,Vec,Vec,void*);
35: static PetscErrorCode FormIFunction(TS,PetscReal,Vec,Vec,Vec,void*);
36: static PetscErrorCode FormIJacobian(TS,PetscReal,Vec,Vec,PetscReal,Mat,Mat,void*);
37: static PetscErrorCode FormInitialSolution(TS,Vec,void*);
41: int main(int argc,char **argv)
42: {
43: TS ts; /* time integrator */
44: SNES snes; /* nonlinear solver */
45: SNESLineSearch linesearch; /* line search */
46: Vec X; /* solution, residual vectors */
47: Mat J; /* Jacobian matrix */
48: PetscInt steps,maxsteps,mx;
49: PetscErrorCode ierr;
50: DM da;
51: PetscReal ftime,dt;
52: struct _User user; /* user-defined work context */
53: TSConvergedReason reason;
55: PetscInitialize(&argc,&argv,(char*)0,help);
57: /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
58: Create distributed array (DMDA) to manage parallel grid and vectors
59: - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
60: DMDACreate1d(PETSC_COMM_WORLD,DM_BOUNDARY_NONE,-11,2,2,NULL,&da);
62: /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
63: Extract global vectors from DMDA;
64: - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
65: DMCreateGlobalVector(da,&X);
67: /* Initialize user application context */
68: PetscOptionsBegin(PETSC_COMM_WORLD,NULL,"Advection-reaction options","");
69: {
70: user.a[0] = 1; PetscOptionsReal("-a0","Advection rate 0","",user.a[0],&user.a[0],NULL);
71: user.a[1] = 0; PetscOptionsReal("-a1","Advection rate 1","",user.a[1],&user.a[1],NULL);
72: user.k[0] = 1e6; PetscOptionsReal("-k0","Reaction rate 0","",user.k[0],&user.k[0],NULL);
73: user.k[1] = 2*user.k[0]; PetscOptionsReal("-k1","Reaction rate 1","",user.k[1],&user.k[1],NULL);
74: user.s[0] = 0; PetscOptionsReal("-s0","Source 0","",user.s[0],&user.s[0],NULL);
75: user.s[1] = 1; PetscOptionsReal("-s1","Source 1","",user.s[1],&user.s[1],NULL);
76: }
77: PetscOptionsEnd();
79: /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
80: Create timestepping solver context
81: - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
82: TSCreate(PETSC_COMM_WORLD,&ts);
83: TSSetDM(ts,da);
84: TSSetType(ts,TSARKIMEX);
85: TSSetRHSFunction(ts,NULL,FormRHSFunction,&user);
86: TSSetIFunction(ts,NULL,FormIFunction,&user);
87: DMSetMatType(da,MATAIJ);
88: DMCreateMatrix(da,&J);
89: TSSetIJacobian(ts,J,J,FormIJacobian,&user);
91: /* A line search in the nonlinear solve can fail due to ill-conditioning unless an absolute tolerance is set. Since
92: * this problem is linear, we deactivate the line search. For a linear problem, it is usually recommended to also use
93: * SNESSetType(snes,SNESKSPONLY). */
94: TSGetSNES(ts,&snes);
95: SNESGetLineSearch(snes,&linesearch);
96: SNESLineSearchSetType(linesearch,SNESLINESEARCHBASIC);
98: ftime = 1.0;
99: maxsteps = 10000;
100: TSSetDuration(ts,maxsteps,ftime);
102: /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
103: Set initial conditions
104: - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
105: FormInitialSolution(ts,X,&user);
106: TSSetSolution(ts,X);
107: VecGetSize(X,&mx);
108: dt = .1 * PetscMax(user.a[0],user.a[1]) / mx; /* Advective CFL, I don't know why it needs so much safety factor. */
109: TSSetInitialTimeStep(ts,0.0,dt);
111: /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
112: Set runtime options
113: - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
114: TSSetFromOptions(ts);
116: /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
117: Solve nonlinear system
118: - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
119: TSSolve(ts,X);
120: TSGetSolveTime(ts,&ftime);
121: TSGetTimeStepNumber(ts,&steps);
122: TSGetConvergedReason(ts,&reason);
123: PetscPrintf(PETSC_COMM_WORLD,"%s at time %g after %D steps\n",TSConvergedReasons[reason],(double)ftime,steps);
125: /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
126: Free work space.
127: - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
128: MatDestroy(&J);
129: VecDestroy(&X);
130: TSDestroy(&ts);
131: DMDestroy(&da);
132: PetscFinalize();
133: return 0;
134: }
138: static PetscErrorCode FormIFunction(TS ts,PetscReal t,Vec X,Vec Xdot,Vec F,void *ptr)
139: {
140: User user = (User)ptr;
141: DM da;
142: DMDALocalInfo info;
143: PetscInt i;
144: Field *f;
145: const Field *x,*xdot;
149: TSGetDM(ts,&da);
150: DMDAGetLocalInfo(da,&info);
152: /* Get pointers to vector data */
153: DMDAVecGetArrayRead(da,X,(void*)&x);
154: DMDAVecGetArrayRead(da,Xdot,(void*)&xdot);
155: DMDAVecGetArray(da,F,&f);
157: /* Compute function over the locally owned part of the grid */
158: for (i=info.xs; i<info.xs+info.xm; i++) {
159: f[i][0] = xdot[i][0] + user->k[0]*x[i][0] - user->k[1]*x[i][1] - user->s[0];
160: f[i][1] = xdot[i][1] - user->k[0]*x[i][0] + user->k[1]*x[i][1] - user->s[1];
161: }
163: /* Restore vectors */
164: DMDAVecRestoreArrayRead(da,X,(void*)&x);
165: DMDAVecRestoreArrayRead(da,Xdot,(void*)&xdot);
166: DMDAVecRestoreArray(da,F,&f);
167: return(0);
168: }
172: static PetscErrorCode FormRHSFunction(TS ts,PetscReal t,Vec X,Vec F,void *ptr)
173: {
174: User user = (User)ptr;
175: DM da;
176: Vec Xloc;
177: DMDALocalInfo info;
178: PetscInt i,j;
179: PetscReal hx;
180: Field *f;
181: const Field *x;
185: TSGetDM(ts,&da);
186: DMDAGetLocalInfo(da,&info);
187: hx = 1.0/(PetscReal)info.mx;
189: /*
190: Scatter ghost points to local vector,using the 2-step process
191: DMGlobalToLocalBegin(),DMGlobalToLocalEnd().
192: By placing code between these two statements, computations can be
193: done while messages are in transition.
194: */
195: DMGetLocalVector(da,&Xloc);
196: DMGlobalToLocalBegin(da,X,INSERT_VALUES,Xloc);
197: DMGlobalToLocalEnd(da,X,INSERT_VALUES,Xloc);
199: /* Get pointers to vector data */
200: DMDAVecGetArrayRead(da,Xloc,(void*)&x);
201: DMDAVecGetArray(da,F,&f);
203: /* Compute function over the locally owned part of the grid */
204: for (i=info.xs; i<info.xs+info.xm; i++) {
205: const PetscReal *a = user->a;
206: PetscReal u0t[2] = {1. - PetscPowRealInt(PetscSinReal(12*t),4),0};
207: for (j=0; j<2; j++) {
208: if (i == 0) f[i][j] = a[j]/hx*(1./3*u0t[j] + 0.5*x[i][j] - x[i+1][j] + 1./6*x[i+2][j]);
209: else if (i == 1) f[i][j] = a[j]/hx*(-1./12*u0t[j] + 2./3*x[i-1][j] - 2./3*x[i+1][j] + 1./12*x[i+2][j]);
210: else if (i == info.mx-2) f[i][j] = a[j]/hx*(-1./6*x[i-2][j] + x[i-1][j] - 0.5*x[i][j] - 1./3*x[i+1][j]);
211: else if (i == info.mx-1) f[i][j] = a[j]/hx*(-x[i][j] + x[i-1][j]);
212: else f[i][j] = a[j]/hx*(-1./12*x[i-2][j] + 2./3*x[i-1][j] - 2./3*x[i+1][j] + 1./12*x[i+2][j]);
213: }
214: }
216: /* Restore vectors */
217: DMDAVecRestoreArrayRead(da,Xloc,(void*)&x);
218: DMDAVecRestoreArray(da,F,&f);
219: DMRestoreLocalVector(da,&Xloc);
220: return(0);
221: }
223: /* --------------------------------------------------------------------- */
224: /*
225: IJacobian - Compute IJacobian = dF/dU + a dF/dUdot
226: */
229: PetscErrorCode FormIJacobian(TS ts,PetscReal t,Vec X,Vec Xdot,PetscReal a,Mat J,Mat Jpre,void *ptr)
230: {
231: User user = (User)ptr;
233: DMDALocalInfo info;
234: PetscInt i;
235: DM da;
236: const Field *x,*xdot;
239: TSGetDM(ts,&da);
240: DMDAGetLocalInfo(da,&info);
242: /* Get pointers to vector data */
243: DMDAVecGetArrayRead(da,X,(void*)&x);
244: DMDAVecGetArrayRead(da,Xdot,(void*)&xdot);
246: /* Compute function over the locally owned part of the grid */
247: for (i=info.xs; i<info.xs+info.xm; i++) {
248: const PetscReal *k = user->k;
249: PetscScalar v[2][2];
251: v[0][0] = a + k[0]; v[0][1] = -k[1];
252: v[1][0] = -k[0]; v[1][1] = a+k[1];
253: MatSetValuesBlocked(Jpre,1,&i,1,&i,&v[0][0],INSERT_VALUES);
254: }
256: /* Restore vectors */
257: DMDAVecRestoreArrayRead(da,X,(void*)&x);
258: DMDAVecRestoreArrayRead(da,Xdot,(void*)&xdot);
260: MatAssemblyBegin(Jpre,MAT_FINAL_ASSEMBLY);
261: MatAssemblyEnd(Jpre,MAT_FINAL_ASSEMBLY);
262: if (J != Jpre) {
263: MatAssemblyBegin(J,MAT_FINAL_ASSEMBLY);
264: MatAssemblyEnd(J,MAT_FINAL_ASSEMBLY);
265: }
266: return(0);
267: }
271: PetscErrorCode FormInitialSolution(TS ts,Vec X,void *ctx)
272: {
273: User user = (User)ctx;
274: DM da;
275: PetscInt i;
276: DMDALocalInfo info;
277: Field *x;
278: PetscReal hx;
282: TSGetDM(ts,&da);
283: DMDAGetLocalInfo(da,&info);
284: hx = 1.0/(PetscReal)info.mx;
286: /* Get pointers to vector data */
287: DMDAVecGetArray(da,X,&x);
289: /* Compute function over the locally owned part of the grid */
290: for (i=info.xs; i<info.xs+info.xm; i++) {
291: PetscReal r = (i+1)*hx,ik = user->k[1] != 0.0 ? 1.0/user->k[1] : 1.0;
292: x[i][0] = 1 + user->s[1]*r;
293: x[i][1] = user->k[0]*ik*x[i][0] + user->s[1]*ik;
294: }
295: DMDAVecRestoreArray(da,X,&x);
296: return(0);
297: }