★ In-compressible flows are the flows that have a constant density (?). Whereas, the compressible flows are those that consists of variable densities.

★ The flows that exist are compressible in nature. Whereas, in-compressible flows, doesn't exist in nature or are very rare.

★ In-compressible flows are used to model aerodynamic problems without loosing any detrimental accuracy i.e. most problems that exist in hydrodynamics considers the density (?) = constant. Whereas, compressible flow is hardly used as a mathematical model to, represent the hydrodynamics.

★ High speed flows are and must be treated as compressible, whereas in-compressible flows are not considered for high speed flows.