In fluid mechanics, two phase flow refers to a system containing gas and liquid with a meniscus separating the two phases. The most commonly considered fluids are air and water.

Everyday examples include

• bubbles
• rain
• waves on the sea
• foam
• fountains
• mousse
• oil slicks
• clouds

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
• In many problems, the air is compressible and the water almost perfectly incompressible