Transonic

Transonic refers to a regime of aerodynamic flow that occurs when an object moves at speeds near the speed of sound, in which both subsonic and supersonic flow patterns can exist on and around the surface. In practice, transonic speeds are typically associated with Mach numbers roughly in the range of 0.8 to 1.2, though the exact boundaries depend on geometry and flow conditions. The defining characteristic is the presence of locally sonic flow—flow that reaches Mach 1 at some points on the surface—while other regions remain subsonic.

A key consequence of transonic flow is the formation of shock waves on surfaces such as wings and fuselages, which produces abrupt changes in pressure and can cause a rapid increase in aerodynamic drag known as wave drag. The interaction between subsonic and supersonic regions also leads to complex phenomena such as boundary-layer separation, buffet, and unsteady forces, collectively referred to as transonic effects. The critical Mach number (Mcr) is the free-stream Mach number at which sonic flow first appears on the body; when Mcr is exceeded, shocks form and drag rises more rapidly with speed.

Transonic flow has significant implications for aircraft design. It is especially relevant for high-subsonic transports and some fighters, where reducing drag and delaying flow separation are priorities. Design responses include the use of supercritical airfoils, wing sweep and thickness distribution strategies, area ruling, and advanced computational fluid dynamics to predict mixed subsonic–supersonic behavior. The transonic regime remains a central area of aerodynamics research and wind-tunnel testing, reflecting the mixed-flow nature that dominates near-sound-speed flight.