Separate convection and diffusion terms in the MULEScorr predictor

to remove interaction of the diffusion term with the flux-correction.

applications/solvers/multiphase/driftFluxFoam/alphaEqn.H

@@ -17,6 +17,11 @@
                 phi,
                 upwind<scalar>(mesh, phi)
             ).fvmDiv(phi, alpha1)
+        );
+
+        solve
+        (
+            alpha1Eqn
           - fv::gaussLaplacianScheme<scalar, scalar>
             (
                 mesh,
@@ -25,8 +30,6 @@
             ).fvmLaplacian(fvc::interpolate(mut/rho), alpha1)
         );
 
-        alpha1Eqn.solve();
-
         Info<< "Phase-1 volume fraction = "
             << alpha1.weightedAverage(mesh.Vsc()).value()
             << "  Min(alpha1) = " << min(alpha1).value()