# Incompressible viscous flow

1. Sep 26, 2011

### Stonescar

Hello,
I'm looking at this problem wich states:

"A parallel air flow along a semi infinite flat plate has undisturbed parameters as follows: "

Then they list the speed, temprature, viscousity and pressure.

"Demonstrate that the boundary layer can treated by means of incompressible analysis. "

The solution says:

Incompressible if M$^{2}_{\infty}$>>4

Where M$^{2}_{\infty}$=$\frac{U^{2}}{\gamma R T}$

I'm just wondering what formula this is? I've never seen it before.
What is M ?

2. Sep 26, 2011

### SteamKing

Staff Emeritus
M is the Mach Number.