gas or another dissolved liquid, or with homogeneous suspension of very fine particles of solids, can be considered and treated as a single-phase flow, although in reality two phases are involved. A flow is said to be incompressible if the density of any particle in the flow, be it a fluid or a solid particle, remains constant as the particle travels with the flow. A flow is said to be homogeneous if the density is constant throughout the flow. A
single-phase incompressible flow is a homogeneous flow, whereas a multiphase incompressible flow is not homogeneous. For instance, for a pipe flow of water carrying gravel, the density of the flow is not the same everywhere at a given time, depending on whether water or gravel exists at the location at a given time. Normally, both liquid and gas are treated as incompressible flow. However, when the speed of a gas approaches, equals to, or exceeds the velocity of sound, large density changes occur in the flow within short distances and the flow can no longer be treated as incompressible. Also, when any gas is flowing through a long pipeline, there can be substantial change of the density of the gas over a long distance due to pressure change along the pipe even when the speed involved is low. Therefore, not all gas pipelines can be treated as incompressible, even when the velocity is low.
