Get field value at mesh vertex

Question

Hi,

I compute a displacement field u (defined as a VectorFunction on a 2D mesh), and i need to export its value at a given vertex (= geometrical point ?) of the 2D mesh. I was thinking of using :
u.vector()[vertex_no]
But I wonder how I can get the vertex_no corresponding to a given geometry ?

Thank you in advance !

Answer 1

If you just want to get the value at certain points, you can just do:

print u(x, y)

e.g. for every vertex in the mesh, you could do:

for v in vertices(mesh):
    x = v.point().x()
    y = v.point().y()
    print x, y, u(x, y)

Alternatively, you can look at get_vertex_values() and/or vertex_to_dof_map()

Comments

Is there no other way to get the field values at each mesh coordinate? The tutorial book Solving PDEs in Minutes on p. 126 (see the box "Cheap vs expensive function evaluation") says that this is the most expensive way to access values. But, is there no other option?

Answer 2

This link also might be helpful:
Evaluation of function at mesh coordinates