solver modules: Moved constructing the face velocity/momentum into preSolve()

The velocity boundary conditions are corrected before the construction of the
face velocity or momentum but for multi-region cases with interacting velocity
boundary conditions this is only possible after all the region solver modules
have been constructed so it is better to delay the optional construction of the
face velocity/momentum until preSolve().

applications/modules/VoFSolver/VoFSolver.C

@@ -146,27 +146,6 @@ Foam::solvers::VoFSolver::VoFSolver
 {
     mesh.schemes().setFluxRequired(p_rgh.name());
 
-    if (mesh.dynamic() || MRF.size())
-    {
-        Info<< "Constructing face momentum Uf" << endl;
-
-        // Ensure the U BCs are up-to-date before constructing Uf
-        U_.correctBoundaryConditions();
-
-        Uf = new surfaceVectorField
-        (
-            IOobject
-            (
-                "Uf",
-                runTime.name(),
-                mesh,
-                IOobject::READ_IF_PRESENT,
-                IOobject::AUTO_WRITE
-            ),
-            fvc::interpolate(U_)
-        );
-    }
-
     if (LTS)
     {
         Info<< "Using LTS" << endl;
@@ -221,6 +200,27 @@ void Foam::solvers::VoFSolver::preSolve()
     // Read the controls
     readControls();
 
+    if ((mesh.dynamic() || MRF.size()) && !Uf.valid())
+    {
+        Info<< "Constructing face momentum Uf" << endl;
+
+        // Ensure the U BCs are up-to-date before constructing Uf
+        U_.correctBoundaryConditions();
+
+        Uf = new surfaceVectorField
+        (
+            IOobject
+            (
+                "Uf",
+                runTime.name(),
+                mesh,
+                IOobject::READ_IF_PRESENT,
+                IOobject::AUTO_WRITE
+            ),
+            fvc::interpolate(U_)
+        );
+    }
+
     if (transient())
     {
         correctCoNum();