[m2montazari]

hi,
if you neglect pressure variation in plane of boundary, writing bernouli+headloss equation you see that for these two streamlines all variables are the same:
1-streamline starting from inlet and ending at outlet. start point and end point are both far from box location and both are far from the wake.
2-streamline starting from inlet and ending at outlet. start point and end point are both near the centre of duct. end point is in the wake of box.
for both, pressure diff. is the same. headloss is the same. so velocities should be the the same. as equations dont have anything to set at inlet for velocity but a simple fully developed flow profile, the this type of boundary forces outlet velocity to be the same as inlet, afully developed flow with no effect of wake. so by setting pressure-pressure bc you force the wake to end before the outlet. and this is wrong!
but if you specify periodic, these two streamlines have all variables the same and similarly the velocities should be the same. but now the software places velocity profile of outlet at the inlet and continue the solution. so instead of changing outlet velocity according to inlet velocity(in pressure-pressure case), the periodic case would change the inlet velocity according to outlet velocity. which is the thing we want and THIS is physical.
yours,