Thanks very much! That works great for me. Best,
Isaac Le mardi 6 mai 2025 à 16:01:43 UTC+2, Guyer, Jonathan E. Dr. (Fed) a écrit : > Isaac - > > What you've set up is an > [internal fixed gradient]( > https://pages.nist.gov/fipy/en/latest/USAGE.html#internal-fixed-gradient) > whereas your boundary condition defines a fixed flux. > I see that we don't actually cover this case, but it's actually easier to > set up than fixed gradient IMO, > because the finite volume method naturally works with fluxes via the > divergence theorem. > > First, you'll want to turn off diffusion at your boundary: > > ```diff > D0 = 1 > D = FaceVariable(mesh=mesh, value=D0) > +D.setValue(0., where=internal_boundary_mask) > ``` > > Next , you need to specify the orientation of your flux. > `mesh.faceNormals` defines the orientation of faces *when pointing from > cell 0 to cell 1*. > This is useful for FiPy internally, but cell 0 vs cell 1 doesn't mean much > to humans. > > Instead, we can take advantage of the knowledge you have about what's > inside and outside > your domains of interest. By taking the gradient of your mask (after > converting it from Boolean) > and normalizing it by the distance between adjacent cells, we obtain a > unit vector that points in > (or out) of the domain of interest: > > ```diff > +solid_mask_var = CellVariable(mesh=mesh, value=solid_mask_flat * 1.) > +boundary_normals = solid_mask_var.faceGrad * mesh._cellDistances > > +flux = 1. > +boundary_flux = internal_boundary_mask * flux * boundary_normals > ``` > > Finally, add the divergence of this boundary flux as a source term: > > ```diff > -gradient = - 1 / D0 > - > -largeValue = 1e10 > > > eq = (TransientTerm() == DiffusionTerm(coeff=D) > - + DiffusionTerm(coeff=largeValue * internal_boundary_mask) > - - ImplicitSourceTerm((internal_boundary_mask * largeValue * gradient > - * mesh.faceNormals).divergence)) > - > + + boundary_flux.divergence) > ``` > > For anything further, please start a GitHub Discussion: > https://github.com/usnistgov/fipy/discussions > It's a much easier environment for talking about math and code than this > mailing list. > > ________________________________________ > From: fi...@list.nist.gov <fi...@list.nist.gov> on behalf of Isaac Paten < > isaac...@gmail.com> > Sent: Saturday, May 3, 2025 07:49 > To: FIPY > Subject: [fipy] Setting internal "constant flux" boundary conditions > correctly > > Hi, > > I am trying to solve a diffusion equation using FiPy, with a constant flux > condition on internal boundaries - I have attached both a pdf describing > the equation, and also a file with a MWE of my code. > > I am using a 2D geometry with two phases, labelled 0 and 1. Phase 1 is a > square immersed in phase 0. I only need to solve the equation in phase 1, > and the constant-flux BC exists only on the faces between cells labelled 0 > and 1. > > I do not get any errors when I run the code, but the solution is 0 > everywhere, so something is going wrong. The definition of my internal > boundary seems ok (I plotted it). Is there something wrong with the way I > am inputting the implicit source term? > > Thanks, > > Isaac > > -- > To unsubscribe from this group, send email to fipy+uns...@list.nist.gov > > View this message at https://list.nist.gov/fipy > To unsubscribe from this group and stop receiving emails from it, send an > email to fipy+uns...@list.nist.gov<mailto:fipy+uns...@list.nist.gov>. > -- To unsubscribe from this group, send email to fipy+unsubscr...@list.nist.gov View this message at https://list.nist.gov/fipy To unsubscribe from this group and stop receiving emails from it, send an email to fipy+unsubscr...@list.nist.gov.
