Allow setting a non-zero initial guess for nonlinear solver in tutori…

…al 17

tutorials/17_navier_stokes.ipynb

@@ -246,6 +246,20 @@
     "        \"\"\"Return the boundary conditions for the problem.\"\"\"\n",
     "        return self._bcs\n",
     "\n",
+    "    def create_snes_solution(self) -> petsc4py.PETSc.Vec:  # type: ignore[no-any-unimported]\n",
+    "        \"\"\"\n",
+    "        Create a petsc4py.PETSc.Vec to be passed to petsc4py.PETSc.SNES.solve.\n",
+    "\n",
+    "        The returned vector will be initialized with the initial guesses provided in `self._solutions`,\n",
+    "        properly stacked together in a single block vector.\n",
+    "        \"\"\"\n",
+    "        x = dolfinx.fem.petsc.create_vector_block(self._residual_cpp)\n",
+    "        with multiphenicsx.fem.petsc.BlockVecSubVectorWrapper(x, [self._VQ[0].dofmap, self._VQ[1].dofmap]) as x_wrapper:\n",
+    "            for x_wrapper_local, sub_solution in zip(x_wrapper, self._solutions):\n",
+    "                with sub_solution.vector.localForm() as sub_solution_local:\n",
+    "                    x_wrapper_local[:] = sub_solution_local\n",
+    "        return x\n",
+    "\n",
     "    def update_solutions(self, x: petsc4py.PETSc.Vec) -> None:  # type: ignore[no-any-unimported]\n",
     "        \"\"\"Update `self._solutions` with data in `x`.\"\"\"\n",
     "        x.ghostUpdate(addv=petsc4py.PETSc.InsertMode.INSERT, mode=petsc4py.PETSc.ScatterMode.FORWARD)\n",
@@ -292,7 +306,7 @@
     "        jacobian_mat = dolfinx.fem.petsc.create_matrix_block(self._jacobian_cpp)\n",
     "        snes.setJacobian(self._assemble_jacobian, J=jacobian_mat, P=None)\n",
     "        snes.setMonitor(lambda _, it, residual: print(it, residual))\n",
-    "        solution = dolfinx.fem.petsc.create_vector_block(self._residual_cpp)\n",
+    "        solution = self.create_snes_solution()\n",
     "        snes.solve(None, solution)\n",
     "        self.update_solutions(solution)\n",
     "        residual_vec.destroy()\n",