Mach Number and Compressibility

The Mach number is the most important parameter when analyzing high-speed, compressible flows. Unlike low-speed (incompressible) flow where air density is assumed constant, at high speeds, this assumption breaks down. The Mach number categorizes flow into distinct regimes: subsonic (M < 1), transonic (0.8 < M 1), and hypersonic (M > 5). Each regime has unique physical characteristics.

In the subsonic regime, air behaves much like an incompressible fluid, and pressure disturbances propagate away from the aircraft in all directions. As an aircraft approaches Mach 1 (the transonic regime), air ahead of it has less ‘warning’ of its approach. Airflow begins to reach sonic speed in some areas, like the curved top of the wing, even if the aircraft itself is subsonic. This creates localized shock waves, which are abrupt discontinuities in pressure, density, and temperature. These shocks can cause a dramatic increase in drag (wave drag) and a loss of lift, a phenomenon known as the sound barrier.

Once an aircraft exceeds Mach 1 (supersonic flight), it outruns its own pressure waves. These waves coalesce to form a powerful shock wave, typically a cone-shaped one at the nose and tail, which is heard on the ground as a sonic boom. In supersonic and hypersonic flight, the physics is dominated by these shock waves. Aerodynamic design shifts from smooth, rounded shapes to sharp leading edges to manage the intense heating and forces associated with strong shocks. The study of compressibility is therefore essential for any vehicle designed to travel near or faster than the speed of sound.