Issues Solving Pure Neumann Problems in dolfinx

dokken · February 17, 2023, 11:09pm

You can remove the constant nullspace with PETSc. See for instance

ComputationalPhysiology/oasisx/blob/main/src/oasisx/fracstep.py#L269


      
          self._M.assemble()
          _fem.petsc.assemble_matrix(self._K, self._stiffness_Vi)
          self._K.assemble()
          
          
# Assemble stiffness matrix with boundary conditions
          _fem.petsc.assemble_matrix(self._Ap, self._stiffness_Q, bcs=[
              bc._bc for bc in self._bcs_p])
          self._Ap.assemble()
          
          
if len(self._bcs_p) == 0:
              nullspace = _PETSc.NullSpace().create(constant=True, comm=self._mesh.comm)
              self._Ap.setNullSpace(nullspace)
          
          
if not self._low_memory:
              for i in range(self._mesh.geometry.dim):
                  # Preassemble pressure matrix for tentantive velocity
                  _fem.petsc.assemble_matrix(self._grad_p_M[i], self._grad_p[i])
                  self._grad_p_M[i].assemble()
          
          
        # Assemble pressure RHS matrices
                  _fem.petsc.assemble_matrix(self._p_M[i], self._p_rhs[i])

github.com

ComputationalPhysiology/oasisx/blob/main/src/oasisx/fracstep.py#L449


      
          def pressure_solve(self) -> np.int32:
              """
              Solve pressure correction problem
              """
          
          
    # Set difference vector to previous time step
              self._dp.x.array[:] = -self._ps.x.array[:]
          
          
    if len(self._bcs_p) == 0:
                  nullspace = self._Ap.getNullSpace()
                  nullspace.remove(self._b_c.vector)
              converged = self._solver_p.solve(self._b_c.vector, self._p)
          
          
    # Compute phi  = p^(n-1/2) - p*
              self._dp.vector.axpy(1, self._p.vector)
              return converged
          
          
def velocity_update(self, dt) -> npt.NDArray[np.int32]:
              """
              Compute Velocity update
              """

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Issues Solving Pure Neumann Problems in dolfinx

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