atmomospheric backflow intensity
 

hi, i have a pressure outlet boundary condition the flow will go to atmospheric air from a pipe (hydraulic diameter =3mm) what is the value of turbulance intensity of backflow?

Are you expecting any backflow? If there is no backflow or reversed flow then it will not matter what you set as the turbulence intensity of the backflow.

If outlet is to ambient, then ambient really should have 0 turbulence intensity. Another estimate is to use the turbulence intensity at the outlet as the backflow intensity, or something in between. The real question is how the heck you are going to have backflow at an outlet.