How to compute the number of dof from a generated mesh?

mesh
1

Hi,

Please could you tell me a simple way to compute the total number of dof ?

Here is my minimal code

#Geometry 
L, H = 1, 1
approx_cell_size = 0.05
#Mesh 1 : triangular elements, linear interpolation
approx_number_of_elements_on_edge = L/approx_cell_size
geom = Rectangle(Point(0., 0.), Point(L, H))
mesh = generate_mesh(geom, approx_number_of_elements_on_edge)

#Mesh 2 : triangular elements (crossed), linear interpolation
#mesh = RectangleMesh(Point(0., 0.), Point(L, H), L/approx_cell_size, H/approx_cell_size, "crossed")

minimal_cell_size = mesh.hmin()
ndim = mesh.topology().dim()
plt.figure(figsize=(5, 5))
plot(mesh)
number_of_cells = mesh.num_cells()
print("Total number of cells %.0f, Minimal cell size %.2f, %.0f dof "%(number_of_cells, minimal_cell_size, ? ))
2

If you are thinking of the “dofs” as the vertices of the mesh, you can use mesh.num_vertices(), as shown in the next snippet:


number_of_cells = mesh.num_cells()
number_of_vertices = mesh.num_vertices()
print("Total number of cells %.0f, Minimal cell size %.2f, %.0f dof "%(number_of_cells, minimal_cell_size,  number_of_vertices))

However, if you are actually interested in the degrees of freedom (dofs), you need to create the function space you are interested in, i.e.:

V = FunctionSpace(mesh, "CG", 1)
V2 = FunctionSpace(mesh, "CG", 2)
print(V.dim(), V2.dim())
3

Thank you very much.

By the way, do you have any information about the official website of FEniCS ? I face problems when i try to open fenicsproject.org… It is not working

4

The link you have posted above works fine for me, and the documentation can be found at: https://fenicsproject.org/olddocs/dolfin/latest/python/

5

Hi there,

It seems that with the 0.9.0 update to FEniCSx the solutions presented here no longer work. In order to find the number of dofs, I’m currently doing

len(domain.geometry.x[:,0])

where domain is my mesh object.

Is there a better way of doing this?

6 
from mpi4py import MPI
import dolfinx

mesh = dolfinx.mesh.create_unit_square(MPI.COMM_WORLD, 32, 32)
V = dolfinx.fem.functionspace(mesh, ("CG", 3))
print(f"{V.dofmap.index_map.size_global:,}")

Yields

9,409
1 Like
7

That was just the kind of thing I was looking for, thank you!