The Mach number, Ma is a dimensionless parameter that is commonly used in the discussion of compressible flows. The Mach number is defined as, Ma := Vc , where V is the flow speed and c is the speed of sound in the flow. Mach number: Ma < 1 subsonic Ma = 1 sonic Ma > 1 supersonic Ma < 0.3 incompressible Ma ⇡ 1 transonic Ma > 5 hypersonic The square of the Mach number, Ma2, is a measure of a flow’s macroscopic kinetic energy to its microscopic kinetic energy.