GENERAL

Multiphase flow is a common observation such as occurs in evaporation or condensation in which a liquid particle is transformed into a gas or vice versa. Other examples include phase changes involved in the boiling of liquids, tracking of free surfaces between gas and liquid, or rocket solid propellants which, upon ignition, change into a liquid phase and subsequently into a gas phase. Furthermore, rigid body motions of solids in the presence of gases or liquids such as in sedimentation and fluidized beds, and reactive laminar and turbulent flows in fluid-particle mixtures, are the complicated physical phenomena in multiphase flows.

Interfaces are present as a specified initial condition or as a result of phase changes through evaporation, condensation, melting, solidification, merging (coalescence), or breakup. Surface tension plays an important role in these interfaces. Interface kinematics dealing with interface tracking in the free surface flows may be described and solved in many different ways. Among them are: volume tracking methods, front tracking methods, level set methods, phase field formulations, continuum advection schemes, boundary integral methods, particle-based methods, and moving mesh methods. A brief review of these methods is given below.

Volume tracking methods, often known as the volume of fluid (VOF) method, were originated by Nichols and Hirt [1975] and Noh and Woodward [1976], and further extended by Hirt and Nichols [1981]. Since then, the VOF method has been improved significantly over the years [Rudman, 1997; Rider and Kothe, 1998].