[DrZoidberg]

Dear CFD community,


I want to simulate the loss coefficient in a zigzag duct. My basic case setup is:

-strucutred mesh ~1e6 elements (see image of inlet section enclosed)
-steady simulation, PBS coupled solver, kwSST, air ideal-gas
-spatial discr. all 2nd order, HOTR engaged, momentum URF 0.3, Pseudo Timestep 0.01

-adibatic walls, operating pressure 1bar, pressure inlet is varied and pressure outlet with 0Pa


Inlet pressure is varied from 100Pa to 3MPa. The loss coeff profile of this duct is used for an outlet vent in a more complex simulation.


Problem: As soon as the inlet pressure is increased beyond the choking point (somewhere around 230kPa, here Ma at the outlet becomes 0.99) the Ma at outlet becomes >1, p_outlet=1bar is no longer kept but raises, net mass flow becomes > 1% of inlet mass flow and the residuals keep oscillating.


According to Fanno, the velo may be max Ma=1 at the outlet. But there is no way to set an appropiate pressure there, right?

Is the only way to fix this problem by a more complex outlet domain, i.e. setting p_out=1bar it in the "far-field"? Do you maybe have a tip how this could be modeled or what dimensions are needed? I already tried to add a square of 40mm edge length there (and used similar mesh size like the duct), but the solver diverged right from the start with this setting.



Br