I noticed a strange effect while using this common type of public bathroom hand dryer. For anyone not familiar, the device works by blowing warm air over your hands, with quite some force, trough two oppositely oriented horizontal slits.

When putting one's hands halfway between the slits, the force from each side is approximately equal. However, the force from a slit decreases noticeably as you move your hands closer to that slit. In other words, if your hands deviate from the middle ground they will be pushed further in that direction by the other slit.

Does anyone have a good explanation of this effect?


This is probably due to the effect of the side streams of air between your hand and the structures where the streams emanate. When you get nearer to one slit with your hand, the velocity of the side-stream there increases and the velocity of the side stream on the other side decreases. Thus, according to Bernoulli's equation, the pressure on one side of the hand decreases and the pressure on the other side of the hand increases. This produces a net force on your hand pulling your hand to the nearer slit.

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