Dear all, I am working with multi-D DMPlex's coupled with PetscFV and a TS to solve hyperbolic systems of equations. To add boundaries, I use the PetscDSAddBoundary as shown in some examples throughout the documentation. I do CFD, meaning I often have an object around which I want to simulate the flow. And I would like to know how some variables (pressure for instance) behave at the wall (literally _at_ the wall, and not only in the first fluid cell above the wall). After going through the documentation and the PETSc source code, I think the way to go is to use something like what ex23.c (of plex/tests) does : mark boundary faces that match the _wall_ boundary condition, create a submesh with them and somehow project the PetscFV/TS solution onto those facets based on the order of the spatial discretization (like if it's order 1 then just use the value in the first fluid cell above the boundary condition, if it's order 2 use the gradient and the value to get something at the facet, etc...). But I cannot figure out two things : 1/ how to mark the right boundary faces ? I tried using the DMPlexMarkBoundaryFaces with (what I thought was) the ID of the wall boundary condition (I mean, I DMView'd the DMPlex and I picked the right id in the Face Sets group), but then when I use the label thus created with DMPlexCreateSubmesh, it just returns ... nothing. The "submesh" is empty when I DMView it. 2/ how to project on the facets in a PetscFV paradigm ? Thank you very much in advance for any advice/pointers you could give me ! Take care and have a good day, Thibault