# Unknown outlet BC for internal transient compressible supersonic flow

Question asked by Jimmy Welch on Sep 11, 2014
Latest reply on Sep 11, 2014 by Jared Conway

I am having difficulty understanding how to set boundary conditions for a model. The first goal is to find the pressure with respect to time of the outlet, for which boundary conditions are unknown. Transient inlet boundary conditions are known for pressure and temperature. Velocity is also known but this is a bit of a wildcard because there is a projectile that is being forced in front of the expanding gas (air).  The projectile velocity dictates the Z-velocity (Vz) of the expanding gas, however the X & Y velocities (Vx, Vy), which denote the velocity of the expanding gas normal to the projectile flight path, are Vx = Vy = 4Vz, at the inlet.

Initial conditions are as follows:

Variable
InletOutlet

Pressure

3000 psia14.7 psia
Temperature2500 F70 F
Z-Velocity850 ft/s0 ft/s
X-Velocity3400 ft/s0 ft/s
Y-Velocity3400 ft/s0 ft/s

As stated initially, transient boundary conditions (dependency) of each of these parameters at the inlet are known, and unknown at the outlet.

Does anyone have any suggestions/experience with this sort of situation?