mass source
 

I have a question about the " Mass source " in flow 3d.
I want to simulation a tube with a steady flow. the stream entered the vertical pipe & fill the whole pipe section at top . I created the mass source top of the vertical pipe, that the center of mass is on the center of pipe, & the direction is -z .
the mass source is circular and its radius is equal to pipe radius.
but the problem is: after simulation , i saw the mass source filled the half pipe section.
I don't know why? please advise me .Thank you very much.