What are expansion waves and how do they differ from shock waves?

Expansion waves (Prandtl-Meyer expansion) occur when supersonic flow turns away from itself around a convex corner, causing continuous, isentropic acceleration, pressure decrease, temperature decrease, and Mach number increase. Unlike shock waves, expansion waves are gradual (fan of Mach waves), involve no entropy increase, and are reversible. The maximum turning angle depends on upstream Mach number. Expansion waves are used in supersonic nozzle design, vehicle aerodynamics, and understanding wave patterns around supersonic bodies. They often follow oblique shocks as flow navigates around objects.