Please note that If you specify mass flow rate, you still need to input initial velocity (these are independent inputs). the coupling mechanism refers to the interaction between the phases. It is my understanding that one-way coupling accounts for the interaction of the fluid over the solid phase only and two-way coupling accounts for both interactions, e.g. fluid over solid phase & solid over fluid phase. Depending on you turbulence model, you may find these options in the DPM setup. I suggest you to read the theory guide. n