A supersonic expansion fan, technically known as
Prandtl–Meyer expansion fan, is a centred expansion process that occurs when a supersonic flow turns around a
convex corner. The fan consists of an infinite number of
Mach waves, diverging from a sharp corner. In case of a smooth, circular corner, these waves can be extended backwards to meet at a point. Each wave in the expansion fan turns the flow gradually (in small steps). It is physically impossible for the flow to turn through a single "shock" wave because this would violate the
second law of thermodynamics. Across the expansion fan, the flow accelerates (velocity increases) and the
Mach number increases, while the
static pressure,
temperature and
density decrease. Since the process is
isentropic, the
stagnation properties (e.g. total pressure and total temperature) remain constant across the fan.