pressure far field boundary condition

Hello!

forgive my curiosity, but for a far-field BC you still need the static pressure. What would be the advantage of a far-field in this case?

In Omnis you should be able to set the external BC by changing the type:

Hello, thx for reply. Ah so the typ selection to " external" was right, as I thought :)

so i would like to use a fixed bc for the outlet. If all flow fluctuations up to the outlet are damped, then I can also use such a boundary condition. To do this, I run several simulations and change my domain length for this, if the flow becomes independent of the domain length, it fits, otherwise I try to find the domain length until all fluctuations are damped. And for this I use a far-field boundary condition at the exit. Ah okay, well maybe I can estimate the static pressure also with rule of thumb ?

I am investigating a compressible flow, have defined the total pressure, total temperature and velocity angle ("Arctg(Vt/Vz)" at "Angle from axial direction") at the inlet, then the mentioned far field boundary condition with a static pressure at the outlet. Unfortunately, my solution diverges with a 100% mass error. The exit mass flow is 0 kg/s

( hmm, I see now for the external setup I didn't use the same Values of k and epsilon like in the inlet maybe this is the mistake ? )

For EXT BC, the mass flow rate is zero. The solver doesn't output how much mass is flowing in or out of the domain.

 Generally, we avoid EXT conditions when we can use a simple static pressure: the far-field requires the knowledge of the velocity and it must be placed really far from any gradient. Static pressure is a much more robust condition. 

ah thank you very much for this information, my supervisor also rather advised me against this bc. Do you know how I could estimate the static pressure at the outlet with a rule of thumb ? I have the total pressure at the inlet, I could possibly find out the total loss from the literature. 

maybe with these formula ? So its the total pressure to static pressure ratio but for isentropic flow. Therefore I I would subtract the total pressure drop from the total pressure at the inlet and with a Mach number at the outlet I could estimate the static pressure ? Or maybe an easier way ? :)