Issue when apply MPC and dirichletbc together

dolfinx
1

was trying to use dolfinx_MPC to apply periodic boundary conditions together with Dirichlet bcs, something like
u3 = u2 + u4, with u4=10.
However, the results show that only part of the Dirichlet BCS are applied. Can create_general_constraint be used with fem.dirichletbc together? Same codes are as following

Thanks in advance


bc1x = fem.dirichletbc(0.0, dofs=n1x, V=V.sub(0))
bc1y = fem.dirichletbc(0.0, dofs=n1y, V=V.sub(1))

bc2x = fem.dirichletbc(0.0, dofs=n2x, V=V.sub(0))

bc4x = fem.dirichletbc(10.0, dofs=n4x, V=V.sub(0))
bc4y = fem.dirichletbc(0.0, dofs=n4y, V=V.sub(1))

bcs = [bc1x, bc2x, bc1y, bc4y, bc4x]

#MPC
mpc = MultiPointConstraint(V)

mpcx = {np.array([ a/2, b/2], dtype=np.float64).tobytes():
       {np.array([ -a/2, b/2], dtype=np.float64).tobytes(): 1,
        np.array([  a/2, -b/2], dtype=np.float64).tobytes(): -1}}

mpcy = {np.array([a/2, b/2], dtype=np.float64).tobytes():
       {np.array([ -a/2, b/2], dtype=np.float64).tobytes(): 1}}

mpc.create_general_constraint(mpcx, subspace_master=0, subspace_slave=0)
mpc.create_general_constraint(mpcy, subspace_master=1, subspace_slave=1)

mpc.finalize()

problem = LinearProblem(a, L, mpc, bcs=bcs,
                        petsc_options={"ksp_type": "preonly", "pc_type": "lu"})
2

That kind of behavior is currently not supported.

I started some work with this a while back, But I didn’t have time to finish the user interface for it.

As a workaround, I would apply the Dirichlet condition weakly with Nitsches method, see
https://jsdokken.com/dolfinx-tutorial/chapter1/nitsche.html

1 Like
3

Thanks. Can I use dolfinx_MPC to apply MPC and assemble matrices? Then, once the Jacobian matrices and residual are obtained, then manually apply the Dirichlet conditions. Is it feasible?

4

No. As you can see in the readme of dolfinx MPC, it computes K^TAK to eliminate dofs from the system, thus applying it afterwards won’t enforce it in a constraint.

1 Like
5

I see. Thanks. Can both Dirichlet conditions and MPC both exist in to model when using create_general_constraint, if Dirichlet conditions are not applied to nodes involving MPC?

6

Yes, then it can be used together