Problem to solve a Navier-Stokes problem with a Custom Newton solver in FEniCSx

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@Karlosjusus

Creating a DirichletBC using a Constant is not supported when the Constant size is not equal to the block size of the constrained (sub-)space. Use a fem::Function to create the fem::DirichletBC.

As stated you should then use a function to define the bc values, as for instance done in

Boundary conditions about Mixed space

Please make the example reproducible. Currently you have only posted snippets of code, which doesn’t make it easy for people to reproduce the problem. An example of how to make your code reproducible is: import dolfinx from mpi4py import MPI import ufl from petsc4py import PETSc import numpy as np domain = dolfinx.mesh.create_unit_square(MPI.COMM_WORLD, 10, 10) Ve = ufl.VectorElement("CG", domain.ufl_cell(), 2) Pe = ufl.FiniteElement("CG", domain.ufl_cell(), 1) We = ufl.MixedElement([Ve, Pe]) …

How to impose a Dirichlet bc on a vector sub-space?

As the error message says, the first input has to be a function. In your code you invoke it with a Constant value: V1, _ = ME.sub(1).collapse() u_D = fem.Function(V1) u_D.x.array[:] =1 boundary_dofs_b = fem.locate_dofs_topological((ME.sub(1), V1), fdim, boundary_facets) bc_b = fem.dirichletbc(u_D, boundary_dofs_b, ME.sub(1))

you can interpolate any expression into the function.