Whats the exact procedure in total pressure BC in subsonic inflow?
 

I'm confused.I need to know how does pressure inlet boundary condition works in FLUENT.I've read user guide and help.but I'm not sure whats the algorithm behind pressure inlet BC.
my case is subsonic but compressible internal flow in a tube like shock tube.
does static pressure is read from internal field of domain (adjacent cells) or its calculated from isentropic relation?if latter how when Mach is un known?
if former then Mach is obtained from isentropic p relation?if so the pressure on patch and in adjacent will be the same then whats forces flow to be inflow?
whats the procedure?
thanks.

is there any help?

CFD Multi flow phase in pipe analysis, can somebudy help me about it?
 

CFD Multi flow phase in pipe analysis, can somebudy help me about it?

could anyone guide me to understand total pressue in inlet better?:(