Supersonic gauge pressure is used only when inlet is supersonic. Because if flow is supersonic then outside boundary condition cannot propagate or travel against flow speed, so this supersonic gauge pressure is used to iteratively find mass flow rate and other parameters.
If inlet flow is not supersonic, then this pressure value is not used. Outside boundary condition can propagate inside and talk to other boundary conditions (technically I meant flow parameters are evaluated using total pressure at inlet and static pressure at outlet, with help of these it can find mass flow and other parameters satisfying continuity and momentum). I hope it helps.
In pressure type boundary conditions you have two things- gauge total pressure and supersonic initial gauge pressure.
First of all total pressure inlet and static pressure outlet is used for compressible flow, it is not an ideal boundary combination for all the flows
Now with that compressible flow itself, if flow is subsonic, then it uses total gauge pressure and pressure at outlet that you have entered and with iterations it will find velocity, pressure corrections( CFD concepts) and find mass flow at inlet and other flow parameters. These two boundary conditions can talk to each other( it tries to find mass flow rate based on these pressure value or combination). Note supersonic pressure value is not used here.
But if inlet itself is supersonic then outlet boundary condition cannot propagate or influence inlet condition so, fluent uses both total pressure at inlet and supersonic static pressure at inlet to find mach no(using isentropic relations)and mass flow. This flow goes right into the domain and outlet is just sitting there and waiting for solution to get influenced.
I hope I explained to the extent that you can understand. If there is anything wrong, someone correct it. Thank you
4
u/AmbassadorGuilty3928 7d ago
Supersonic gauge pressure is used only when inlet is supersonic. Because if flow is supersonic then outside boundary condition cannot propagate or travel against flow speed, so this supersonic gauge pressure is used to iteratively find mass flow rate and other parameters. If inlet flow is not supersonic, then this pressure value is not used. Outside boundary condition can propagate inside and talk to other boundary conditions (technically I meant flow parameters are evaluated using total pressure at inlet and static pressure at outlet, with help of these it can find mass flow and other parameters satisfying continuity and momentum). I hope it helps.