When reading the codes in Fenicsx tutorial about heat equation, I have a question about the following codes: boundary_facets=mesh.locate_entities_boundary( domain,fdim,lambda x:np.full(x.shape[1],True,dtype=bool)) What does ‘x.shape[1]’ mean? When implementing the function ‘locate_entities_bo…

The third input argument to locate_entities_boundary is a python function, that takes in coordinates x, of shape (3, num_points), such that x[0] are all points in the x-coordinate, x[1] all points in y-coordinate etc. locate_entities_boundary works on the computational domain (domain) in your case…

Thank you very much.I still have some questions :‘locate_entities_boundary’ will only look for facets which are already on boundary ,or it will look for all facets including ones inside domain?And ‘x’ is stored in (3,num_points) form, where num_points for all mesh points or dof points?Who supplies ‘…

There is another function for looking at all facets (including interior ones, namely dolfinx.mesh.locate_entities. locate_entities_boundary uses exterior_facet_indices to get the facets. Then for each of the facets; it finds the vertices, and checks if all vertices satisfies the condition in the Py…

Sorry for another question: this np.full() function return an narray full of true but locate_entities_boundary need a marker function which returns a True or False? Do I misunderstand anything?

In general it needs a function that returns an array of booleans, whose shape is the same as the second axis of the input. See for instance: Hyperelasticity — FEniCSx tutorial on how to mark parts of a boundary using numpy.isclose

Oh,I just look for the document which says that in locate_entities_boundary, the marker function returns an array of boolean value of size num_points, which is True if it is on the boundary.So np.full() returns what it wants.

Hello dokken, I have another question about it.Since x[0] and x[1] are just different in x or y coordinates but with the same number of points, I replace x.shape[1] with x.shape[0] but this time the code throws an error : Length of array of markers is wrong.So what is the reason here?

The shape of x is (3, number of points). Thus x.shape[0] will return 3. not len(x[0]),. len(x[0])=len(x[1])=number of points.

Dear dokken, much later I’m coming back to this. I’m also a newbie and am trying to identify the boundary entities of an imported .msh - file. I browsed some of the older forum posts, where I often stumbled into the exterior_facet_indices() - method (and that it only works on certain dolfinx versio…

[image] berzan_abi: facets_ids = dolfinx_mesh.exterior_facet_indices(dolfinx_mesh.topology) This should be facets_ids = dolfinx.mesh.exterior_facet_indices(dolfinx_mesh.topology) from the dolfinx.mesh module.

Question about locate_entities_boundary

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dokken March 4, 2025, 9:17pm 19

See for instance: How to collect the global mesh without writing a file - #4 by dokken for some inspiration for this.

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