Prandtl-Meyer expansion waves occur when a supersonic flow turns around a convex corner. Unlike shock waves, which are abrupt and increase pressure, expansion waves are gradual and decrease pressure, density, and temperature while increasing the Mach number.

The Prandtl-Meyer function, ν(M), relates the Mach number to the angle through which the flow has turned from an initial state of M=1. It is a fundamental concept in supersonic aerodynamics, used to design nozzles and airfoils.