Prandtl–Meyer function

Summary

The Prandtl–Meyer function $\nu(M)$ is the angle through which a sonic flow of an ideal gas must be turned to reach the Mach number $M > 1$. The turning is a smooth isentropic expansion. It turns the computation of supersonic expansions around corners into a simple rule: the change of $\nu$ equals the turning angle. It increases from $0$ at $M = 1$ to a finite limit as $M \to \infty$, so a supersonic flow can turn only through a bounded angle. It is a basic tool of the method of characteristics for plane supersonic flow.