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air will move downward r upward it depends upon bending of fans arm and its rotation.
if fan is in horizontal position and arms of fan r slightly bend dawn in forward direction and fan is moving in clockwise direction then air will move downward because bent arms of fan will push the air in the downward direction . but if you will change anyone of these either direction of rotation or bending of arm then air will move in upward direction.
u can check it practically by changing the direction of rotation of a fan.
the blades create a high pressure thrust by its rotation on one side and low pressure on the other. In case of a ceiling fan, the high pressure is towards the room while the low pressure is from the ceiling to the fan.
Well the blades are continuously striking the gas molecules and imparting momentum to them( Imagine a cricket bat hitting a ball) since the plane of the blade is making an angle with the rotational plane(which is horizontal). As the blades rotate they sweep a volume. The gas molecules being hit at a very high speed by the blades start moving perpendicular to the surface of the blade (not vertically down) towards the floor and hence the air you feel is the momentum being transferred to you by air molecules.
Now since the air in the swept volume is pushed down there is a low pressure(less air molecules compared to surroundings per unit volume) created in the volume where your fan is rotating. So the surrounding air moving randomly all over the place enters(or is sucked in from above and sides) in to the collision area and in the mean time the next blade comes down striking the new air molecules which have just entered the area and the flow of air continues.
Imagine your fan is rotating in reverse direction...imagine the collisions, also then think in which direction the air flows....yeah thats right it hits the ceiling (well if it's a ceiling fan)...else just the opposite direction to the original.
I hope I have explained giving the physical reason :)