Boundary condition for not fully-developed flow
 

I have a situation where I have flow through a pipe system.

The flow is not fully-developed at one pipe outlet. I believe that the flow must be fully developed for an outflow boundary condition. Therefore I may create a pressure outlet condition.

However, I only have data on the downstream pressure and flow velocity for that outlet.

Also, my aim is to determine the mass flow rate through this outlet. Therefore I do not want to use outflow BC where the volume flow rate is specified by the user.

Does anyone have any solutions for my boundary condition problem?

Thank you for your help.