Fully Expanded Jet Mach Number.
 

Hi everyone,
I am running an Euler simulation for an underexpanded jet. The design Mach number is Md = 2.2, and this jet operates at a pressure ratio (p at jet exit to ambient p) = 1.2.
The fully expanded jet Mach number is
Mj = sqrt ((2/gamma-1)((P0/Pe)^((gamma-1)/gamma) -1)),
where P0 is the Chamber Pressure, gamma=1.4, Pe =14.7 psi (freestream).

Mj = 2.3166 for PR=1.2 by the above formula, but in the simulations the flow Mach number goes up to 3.2 in the first shock.
Could anyone please give some insight. Why the Mach number in the simulation is same as the Mj value by the formula.
Thank you.