Hello,

Is it possible to set a constant nullspace for a NonlinearVariationalSolver?

I tried the following:

order = 2
V3 = FunctionSpace(mesh, "CG", order)
V1 = FunctionSpace(mesh, "BDM", order)
V2 = FunctionSpace(mesh, "DG", order-1)
W = MixedFunctionSpace([V3, V1, V2])
v_basis = VectorSpaceBasis(constant=True)
null_space = MixedVectorSpaceBasis(W, [W.sub(0), W.sub(1), v_basis])

# Build Nonlinear Solver
uprob = NonlinearVariationalProblem(L, w1)
usolver = NonlinearVariationalSolver(uprob, nullspace=null_space, solver_parameters = {
    'snes_max_it': '50',
    'snes_rtol': '1e-12',
    'mat_type': 'aij',
    'ksp_type': 'preonly',
    'pc_type': 'lu',
    'snes_monitor': True,
    'snes_view': False,
    'snes_converged_reason': True,
    'ksp_converged_reason': True})

but the linear solver did not converge:

0 SNES Function norm 7.964422825545e-02
Linear firedrake_1_ solve did not converge due to DIVERGED_PCSETUP_FAILED iterations 0
PCSETUP_FAILED due to FACTOR_NUMERIC_ZEROPIVOT
Nonlinear firedrake_1_ solve did not converge due to DIVERGED_LINEAR_SOLVE iterations 0

I can send the full code if necessary. I'm trying to solve Navier-Stokes in velocity-vorticity-pressure formulation (https://arxiv.org/pdf/1604.00257.pdf) but the solver doesn't converge. If I add a small parameter times a pressure mass-matrix, the solver converges, but I was hoping to avoid doing this.

Thanks,

Sander