read unstructured vtk grid into dolfinx

read_unstructured_vtk.py

import pyvista as pv
import numpy as np
import ufl
import basix
from dolfinx.mesh import create_mesh
from mpi4py import MPI
import dolfinx

# Define the points
points = [
    [1.0, 1.0, 1.0],
    [1.0, -1.0, -1.0],
    [-1.0, 1.0, -1.0],
    [-1.0, -1.0, 1.0],
    [2.0, 2.0, -1.0],  # Additional point for the second tetrahedron
]

# Define the cells (two tetrahedra sharing a face)
cells = [4, 0, 1, 2, 3, 4, 0, 1, 2, 4]  # First tetrahedron  # Second tetrahedron

# Define the cell types
celltypes = [pv.CellType.TETRA, pv.CellType.TETRA]

grid = pv.UnstructuredGrid(cells, celltypes, points)
# grid.plot(show_edges=True, show_axes=True)

grid.cell_data["mean"] = np.arange(grid.n_cells)

# save to vtk file
grid.save("original.vtk")


# Extract connectivity information
cell_connectivity = grid.cells_dict[10]

degree = 1  # Set polynomial degree
cell = ufl.Cell("tetrahedron")
mesh_element = basix.ufl.element("Lagrange", cell.cellname(), degree, shape=(3,))

# Create dolfinx Mesh
mesh_ufl = ufl.Mesh(mesh_element)
dolfinx_mesh = create_mesh(MPI.COMM_WORLD, cell_connectivity, grid.points, mesh_ufl)
function_space = dolfinx.fem.functionspace(dolfinx_mesh, ("DG", 0))
u = dolfinx.fem.Function(function_space)

u.x.array[:] = grid.cell_data["mean"][dolfinx_mesh.topology.original_cell_index]

writer = dolfinx.io.VTXWriter(MPI.COMM_WORLD, "u_as_dg0.bp", u, "BP5")
writer.write(t=0)