Prandtl–Meyer function

Varition in the Prandtl–Meyer function ($\nu$) with Mach number ($M$) and ratio of specific heat capacity ($\gamma$). The dashed lines show the limiting value ${\displaystyle \nu _{\text{max}}}$ as Mach number tends to infinity.

Prandtl–Meyer function describes the angle through which a flow can turn isentropically for the given initial and final Mach number. It is the maximum angle through which a sonic (M = 1) flow can be turned around a convex corner. For an ideal gas, it is expressed as follows,

${\displaystyle {\begin{aligned}\nu (M)&=\int {\frac {\sqrt {M^{2}-1}}{1+{\frac {\gamma -1}{2}}M^{2}}}{\frac {\,dM}{M}}\\&={\sqrt {\frac {\gamma +1}{\gamma -1}}}\cdot \arctan {\sqrt {{\frac {\gamma -1}{\gamma +1}}(M^{2}-1)}}-\arctan {\sqrt {M^{2}-1}}\\\end{aligned}}}$

where $\nu \,$ is the Prandtl–Meyer function, $M$ is the Mach number of the flow and $\gamma$ is the ratio of the specific heat capacities.

By convention, the constant of integration is selected such that $\nu (1)=0.\,$

As Mach number varies from 1 to $\infty$, $\nu \,$ takes values from 0 to $\nu _{\text{max}}\,$, where

${\displaystyle \nu _{\text{max}}={\frac {\pi }{2}}{\bigg (}{\sqrt {\frac {\gamma +1}{\gamma -1}}}-1{\bigg )}}$

For isentropic expansion,	${\displaystyle \nu (M_{2})=\nu (M_{1})+\theta \,}$
For isentropic compression,	${\displaystyle \nu (M_{2})=\nu (M_{1})-\theta \,}$

where, $\theta$ is the absolute value of the angle through which the flow turns, $M$ is the flow Mach number and the suffixes "1" and "2" denote the initial and final conditions respectively.

See also

• Gas dynamics
• Prandtl–Meyer expansion fan

References

• Liepmann, Hans W.; Roshko, A. (2001) [1957]. Elements of Gasdynamics. Dover Publications. ISBN 0-486-41963-0.