This seems to be like a very basic question, so you will have to forgive me if I miss some basic concepts in flow simulation. I am trying to use the flow simulation to demonstrate the flow rate, velocity, and flow direction of a suction fan installed in the middle of a cylinder vertically. The flow direction is pointing downward.

I have created a dummy impeller in order to apply rotating region in the flow simulation. My first question is that does it have to be a proper impeller for SW to recognize its geometry in order to have the proper flow direction? I have assigned this simulation internal. Since the flow direction is moving downwards, I have assigned the rotating region along Y-axis. After this, the only boundary continue i have applied is to the environmental pressure around the pipe and the rotating region which is basically the RPM of the motor. My second question is whether I should put a negative sign in the RPM value so SW can recognize that the flow is moving vertically downward? 

I have tried both negative and positive signs and run the simulation. However, the flow direction is still upwards. Maybe I am missing something important and fundamental here. Would like to hear all the suggestions. Thanks