ENH: Surface film - added new deltaMass function

src/regionModels/surfaceFilmModels/kinematicSingleLayer/kinematicSingleLayer.H

@@ -472,6 +472,9 @@ public:
             //- Return the net film mass available over the next integration
             inline tmp<volScalarField> netMass() const;
 
+            //- Return the change in film mass due to sources/sinks
+            inline tmp<volScalarField> deltaMass() const;
+
             //- Return the gravity normal-to-patch component contribution
             inline tmp<volScalarField> gNorm() const;
 

src/regionModels/surfaceFilmModels/kinematicSingleLayer/kinematicSingleLayerI.H

@@ -198,6 +198,12 @@ inline tmp<volScalarField> kinematicSingleLayer::netMass() const
 }
 
 
+inline tmp<volScalarField> kinematicSingleLayer::deltaMass() const
+{
+    return rhoSp_*magSf()*time().deltaT();
+}
+
+
 inline tmp<volScalarField> kinematicSingleLayer::gNorm() const
 {
     tmp<volScalarField> tgNorm