Hi Joshua, yes D is fixed, and the value on the polygon is always positive; Indeed, here is what I do;
I have an old concentration field at one time step "OLDC" (all concentration is between 0 and 1). then I update the geometry of the polygon (it increases its size a little bit) I create a new mesh, and a new variable "NEWC" . I put the value 0. at the creation of NEWC, and then I copy the values of OLDC in NEWC. so, so far no negative values. and then I iterate one timestep, the diffusion equation on NEWC. and sometimes, I get negative values !!! or values > 1 On Tue, Feb 15, 2011 at 3:33 PM, Joshua Bush <[email protected]> wrote: > Julien, > > Would you mind giving us more information? > Is D a fixed value? What is the center polygon's initial concentration > value? > On Feb 15, 2011 6:29 AM, "Julien Derr" <[email protected]> wrote: > > Hi all, > > > > I am still bothering you with my simple diffusion problem between an > outer > > circle (Boundary condition value=1) > > and an inner polygon (Boundary flux=0) > > > > I am getting negative values on the concentration field !! > > a simple diffusion process ( eq = TransientTerm() == > > DiffusionTerm(coeff=D)) should not allow that , no ? > > > > even with a fixed flux boundary ? >
