surfaceScalarField & phi2
surfaceScalarField & phi1
surfaceScalarField ddtPhi2((phi2 - phi2.oldTime())/runTime.deltaT())
surfaceScalarField ddtPhi1((phi1 - phi1.oldTime())/runTime.deltaT())
GeometricField< scalar, fvsPatchField, surfaceMesh > surfaceScalarField