In aerodynamics, the Prandtl–Meyer function describes the angle through which a flow turns isentropically from sonic velocity (M=1) to a Mach (M) number greater than 1. The maximum angle through which a sonic (M = 1) flow can be turned around a convex corner is calculated for M = . For an ideal gas, it is expressed as follows,
By convention, the constant of integration is selected such that
As Mach number varies from 1 to , takes values from 0 to , where
For isentropic expansion,
For isentropic compression,
where, is the absolute value of the angle through which the flow turns, is the flow Mach number and the suffixes "1" and "2" denote the initial and final conditions respectively.