Mass balance issue when solving an elliptic diffusion problem using pde toolbox

Hi

I'm trying to solve a diffusion like, elliptic equation using pde toolbox. I used 'pednonlin' function but the answer I get, gets me a bit confused regarding mass conservation issue since the solution should be steady-state.

Here are more about problem:

The equation is simply -div(K(u)*del(u)) = 0 on a rectangular domain with Dirichlet B.C. on top and bottom and no flux B.C. on side boundaries. But when I plot the solution using:

 [cux,cuz] = pdecgrad(p,t,c,u) ;
 pdeplot(p,e,t,'flowdata',[-cux;-cuz])

the flux has a pattern which induces mass is not conserved!

Does anyone have an idea where I'm going wrong?

Thanks a lot.

Answer 1

[Mohammad Monfared]

The issue was that the Jacobian has to be calculated using 'full' parameter, not 'fixed' or 'lump'.