How to set 'boundary condition' inside the boundary?

General
11

The link

I suggested didnt suggest adding a physical tag, but using the embed command, as follows:

import gmsh
from mpi4py import MPI
from dolfinx.io.gmshio import model_to_mesh
from dolfinx.io import XDMFFile
# https://github.com/FEniCS/dolfinx/blob/main/python/demo/demo_gmsh.py
rank = MPI.COMM_WORLD.rank

gmsh.initialize()
R = 1
gdim = 2  # Geometric dimension of the mesh
model_rank = 0
mesh_comm = MPI.COMM_WORLD
if mesh_comm.rank == model_rank:

    # Define geometry for background
    background = gmsh.model.occ.addDisk(0, 0, 0, R, R)
    
    gmsh.model.occ.synchronize()
    
    # Define geometry for center
    center = gmsh.model.occ.addPoint(0, 0, 0)

    gmsh.model.occ.synchronize()   
    gmsh.model.mesh.embed(0, [center], 2, background)
    background_domain = gmsh.model.addPhysicalGroup(gdim, [background], 1)
    
    # Generate mesh
    gmsh.model.mesh.field.add("Distance", 1)
    
    gmsh.model.mesh.field.add("Threshold", 2)
    gmsh.model.mesh.field.setNumber(2, "IField", 1)
    gmsh.model.mesh.field.setNumber(2, "LcMin", 0.01)
    gmsh.model.mesh.field.setNumber(2, "LcMax", 0.1)
    gmsh.model.mesh.field.setNumber(2, "DistMin", 0.2)
    gmsh.model.mesh.field.setNumber(2, "DistMax", 0.7)
    
    gmsh.model.mesh.field.add("Min", 5)
    gmsh.model.mesh.field.setNumbers(5, "FieldsList", [2])
    gmsh.model.mesh.field.setAsBackgroundMesh(5)
    
    gmsh.model.mesh.generate(gdim) 
    gmsh.model.mesh.optimize("Netgen")

domain, ct, _ = model_to_mesh(gmsh.model, mesh_comm, model_rank, gdim)
gmsh.finalize()

with XDMFFile(MPI.COMM_WORLD, "mt.xdmf", "w") as xdmf:
    xdmf.write_mesh(domain)
    xdmf.write_meshtags(ct)
import numpy as np
from dolfinx.mesh import locate_entities
print(locate_entities(domain, 0, lambda x: np.isclose(x[0],0) & np.isclose(x[1], 0)))

Please read through the links I post before asking a question.