Revert "Revert "DPMFoam: Multiply the gravitational force by the gas phase fraction""

This reverts commit 0a6395b6.

applications/solvers/lagrangian/DPMFoam/UcEqn.H

@@ -15,7 +15,7 @@ surfaceScalarField rAUcf("Dp", fvc::interpolate(rAUc));
 surfaceScalarField phicForces
 (
    (fvc::interpolate(rAUc*cloudVolSUSu/rhoc) & mesh.Sf())
- + rAUcf*(g & mesh.Sf())
+ + alphacf*rAUcf*(g & mesh.Sf())
 );
 
 if (pimple.momentumPredictor())