Several features make two-phase flow an interesting and challenging branch of fluid mechanics:
*Surface tension makes all dynamical problems nonlinear (see Weber number)
*In the case of air and water at standard temperature and pressure, the density of the two phases differs by a factor of about 1000. Similar differences are typical of water liquid/water vapor densities
*The sound speed changes dramatically for materials undergoing phase change, and can be orders of magnitude different. This introduces compressible effects into the problem
*The phase changes are not instantaneous, and the liquid vapor system will not necessarily be in phase equilibrium
*The change of phase means flow-induced pressure drops can cause further phase-change (e.g. water can evaporate through a valve) increasing the relative volume of the gaseous, compressible medium and increasing exit velocities, unlike single-phase incompressible flow where closing a valve would decrease exit velocities
*Can give rise to other counter-intuitive, negative resistance-type instabilities, like Ledinegg instability, geysering, chugging, relaxation instability, and flow maldistribution instabilities as examples of static instabilities, and other dynamic instabilities