MomentumTransferPhaseSystem::partialElimination: Removed the phase velocity mean adjustment

The phase velocity mean adjustment was introduced for consistency with phase
flux mean adjustment which is necessary to ensure the mean flux divergence is
preserved.  However for systems with very high drag it has proved preferable to
not adjust the velocity of the phases to conserve momentum rather than ensure
consistency with the fluxes.

applications/modules/multiphaseEuler/phaseSystems/PhaseSystems/MomentumTransferPhaseSystem/MomentumTransferPhaseSystem.C

@@ -1344,22 +1344,6 @@ void Foam::MomentumTransferPhaseSystem<BasePhaseSystem>::partialElimination
 
     phaseSystem::phaseModelList& phases = this->phaseModels_;
 
-    // Calculate the mean velocity from the current velocity
-    // of the moving phases
-    volVectorField Um(this->movingPhases()[0]*this->movingPhases()[0].U());
-
-    for
-    (
-        label movingPhasei=1;
-        movingPhasei<this->movingPhases().size();
-        movingPhasei++
-    )
-    {
-        Um +=
-            this->movingPhases()[movingPhasei]
-           *this->movingPhases()[movingPhasei].U();
-    }
-
     // Remove the drag contributions from the velocity and flux of the phases
     // in preparation for the partial elimination of these terms
     forAll(phases, i)
@@ -1509,7 +1493,8 @@ void Foam::MomentumTransferPhaseSystem<BasePhaseSystem>::partialElimination
         }
     }
 
-    this->setMixtureU(Um);
+    // Adjust the phase-fluxes such that the mean flux
+    // obtained from the pressure solution is retained
     this->setMixturePhi(alphafs, this->phi());
 }
 
@@ -1643,6 +1628,8 @@ void Foam::MomentumTransferPhaseSystem<BasePhaseSystem>::partialEliminationf
         }
     }
 
+    // Adjust the phase-fluxes such that the mean flux
+    // obtained from the pressure solution is retained
     this->setMixturePhi(alphafs, this->phi());
 }