For multiphase flows, you can optionally include the "virtual mass effect" that occurs when a secondary phase accelerates relative to the primary phase . The inertia of the primary-phase mass encountered by the accelerating particles (or droplets or bubbles) exerts a "virtual mass force" on the particles [152]. The virtual mass force is defined as:

(14–343)

where is the virtual mass coefficient which typically has a value of 0.5. The term denotes the phase material time derivative of the form

(14–344)

The virtual mass force will be added to the right-hand side of the momentum equation for both phases ().

The virtual mass effect is significant when the secondary phase density is much smaller than the primary phase density (for example, for a transient bubble column).

By default, the virtual mass is applied as an explicit source term. You can also choose to use an implicit approach which is recommended in steady-state coupled simulations where the explicit approach may fail to converge. Refer to Including the Virtual Mass Force for details on how to include virtual mass in your simulation.