To find the pressure downstream of the expansion wave, follow these steps:
Find the downstream Prandtl Meyer function using the upstream Mach number and the deflection angle.
Using the result from step 1, use the Prandtl Meyer function – Mach number table or solve the Prandtl Meyer equation for downstream Mach number to obtain the downstream Mach number.
Calculate the total pressure using the upstream pressure and isentropic pressure ratio – Mach number table.
Obtain the downstream pressure by using the total pressure and the isentropic pressure ratio and the Mach number table.
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