Added Urel for post-processing and function objects

applications/solvers/incompressible/MRFSimpleFoam/MRFSimpleFoam.C

@@ -65,6 +65,10 @@ int main(int argc, char *argv[])
 #           include "pEqn.H"
         }
 
+        // Calculate relative velocity
+        Urel == U;
+        mrfZones.relativeVelocity(Urel);
+
         turbulence->correct();
 
         runTime.write();

applications/solvers/incompressible/MRFSimpleFoam/createFields.H

@@ -44,3 +44,19 @@
 
     MRFZones mrfZones(mesh);
     mrfZones.correctBoundaryVelocity(U);
+
+    // Create Urel as a permanent field to make it available for on-the-fly
+    // post-processing operations
+    volVectorField Urel
+    (
+        IOobject
+        (
+            "Urel",
+            runTime.timeName(),
+            mesh,
+            IOobject::NO_READ,
+            IOobject::AUTO_WRITE
+        ),
+        U
+    );
+    mrfZones.relativeVelocity(Urel);