Regarding Ourlet Boundary Condition
 

Hi,
Is it ok to use 'exit static pressure with target mass flow rate' boundary condition for a high subsonic flows (Mn ~ 0.7-0.9)?

Absolutely. The exit static pressure is a very robust boundary condition and often makes the most physical sense and should be used whenever appropriate.

You may want to guess an exit pressure and run the solver for awhile without enabling the target mass flow rate option. Once the main flow features have settled and the solution has stabilized, then the target mass flow rate option can be turned on. You may have convergence trouble if the target mass flow rate option is turned on from the 1st iteration.