// Dispersed phase velocity
// const volVectorField Ud(mixture_.U() + Udm_);
// Use the mixture rather than the dispersed-phase velocity to approximate
// the dispersed-phase acceleration to improve stability as only the mixture
// momentum equation is coupled to continuity and pressure
//
// This approximation is valid only in the limit of small drift-velocity.
// For large drift-velocity an Euler-Euler approach should be used in
// which both the continuous and dispersed-phase momentum equations are
// solved and coupled to the pressure.
const volVectorField& Ud = mixture_.U();
return g_ - (Ud & fvc::grad(Ud));
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