In two-phase fluid flow, the gravitational force plays a very important role because the fluid with lower density (e.g.,, gas) is always above the fluid with higher density. Stratified flows are very common in nature, for example, in the ocean and the atmosphere. In the internal flows, the stratified flow occurs at low liquid and gas velocities. As the velocity of the gas increases, the horizontal interface becomes more disturbed, and waves may be formed. This flow regime is usually known as the stratified-wavy flow.