Reply To: High velocity flow of air in tube

[Mark Owens]

For an ideal gas

density = W*Pabs /(R*T)

where

 

W = molecualr weight

Pabs = absolute Pressure = solver pressure + reference pressure

R = ideal gas constant

T = temperature

 

You should check what reference pressure you have set for the domain.

 

For a velocity inlet both the velocity and temperature is fixed  and only the inlet density (and hence mass flow) will be a function of the solution pressure which will be changing over time to satisfy the boundary conditions. For subsonic flow pressure propagates upstream from the outlet. For a mass flow inlet both the density and velocity  will vary as the solver tries to find a solution. If you cannot get a velocity  inlet to work then it is unlikely that you will get a mass flow inlet to work.

 

In either case you should be able to use the ideal gas formula to check the density. If it is too high it could be because the reference pressure is too high. That might also explain why the velocity inlet is giving wranings about a high mach number since the speed of sound varies inversley with the square root of the density.