[petsc-users] Bug in boundary conditions (ex25.c) ?

[Christophe Ortiz]

Hi guys,

I was having some troubles with my code (dof >> 1) and I decided to go back
to simple things. Therefore, I took ex25.c, which is a nice case for me:
diffusion of 2 species and 1 reaction.

First, to have a better mesh I set 256 points (instead of the 11) in

  ierr = DMDACreate1d(PETSC_COMM_WORLD,DMDA_BOUNDARY_NONE,256,2,2,NULL,&da);

Then, I did something simple. I removed reaction terms by commenting

ierr = TSSetRHSFunction(ts,NULL,FormRHSFunction,&user);CHKERRQ(ierr);

So we are left with 2 diffusion equations:

u_t -alpha u_xx = 0
v_t - alpha v_xx = 0

Now, do something simple:
In FormInitialSolution, for variable v, change it and simply set it to 5
(for instance), ie, v will have uniform values from i=0 to i=mx-1.

Now, for boundary conditions (Dirichlet here), set vleft and vright to 10,
for instance.

Since values at the boundaries (10.0) are higher than values of initial
solution (5.0), what you expect is some diffusion towards the center from
the boundaries. Since we use Dirichlet boundary conditions here, you expect
the values of v at i=0 and i=mx-1 to be 10.0. It can't be something else.
Well, that's not the case. I see that the value of v at i=0 and at i=mx-1
is always the initial value (t=0). Here is a sample of the final values
after diffusion:

0    5.000000
1    8.951153
2    8.414078
3    7.964193
4    7.581196
...
...
77    5.000235
78    5.000207
79    5.000182
80    5.000161
...
...
252    7.964193
253    8.414078
254    8.951153
255    5.000000

Then I checked the value of v inside IFunction at each timestep. I found
out that each time IFunction is called, the first value of v that is used
to evaluate F is the initial value (5.0), instead of of the value set by
Dirichlet (10). For next evaluations within the same timestep, it is 10.0,
the value imposed by Dirichlet boundary conditions.
Some output:

      TSAdapt 'basic': step 324 accepted t=9.73302    + 9.496e-02
wlte=0.798 family='arkimex' scheme=0:'1bee' dt=9.569e-02
5.000000
10.000000
10.000000
10.000000
10.000000

      TSAdapt 'basic': step 325 accepted t=9.82798    + 9.569e-02
wlte=0.798 family='arkimex' scheme=0:'1bee' dt=9.642e-02
5.000000
10.000000
10.000000
10.000000
10.000000

Is it a bug or did I miss something ? I would expect that to evaluate F
within IFunction, it would always use the value set by Dirichlet.


I use petsc 3.4.1and configured it with
 --with-mpi=0 --with-fc=ifort --with-cc=icc --with-debugging=1
--with-scalar-type=real --with-precision=double
--with-blas-lapack-dir=/opt/intel/mkl.

The options I used to run the example are
-ts_adapt_monitor -ts_adapt_basic_clip 0.1,1.1 -draw_pause -2
-ts_arkimex_type 1bee -ts_max_snes_failures -1 -snes_type newtonls
-snes_linesearch_type bt.


Christophe
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