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Possible Duplicate:
Why is exhaling more forceful than inhaling?

If I put my hand behind my computer's system block, I can feel a very strong flux of air coming out, as it is being pushed by the fans. However, if I put my hand in front of the box, where the air is coming in, I can barely feel the air flow. Why is that?

I understand that this is probably because the air is coming into the box from a greater solid angle than the air coming out, so the incoming air has a lower velocity. But what is the reason for such asymmetry? Can it be explained in simple terms, without solving the Navier-Stokes equations?

Just to clarify: it is definitely not due to the difference in the area through which the air can come in and out: my computer has holes all over the back side, so no nozzles there :) In addition, all regular floor fans has the same property: the air flow coming out feels stronger than the air flow coming in.

Community
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drlemon
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1 Answers1

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Simply put, it's because a fan imparts momentum on the air (i.e. accelerates it), so in front of the fan you get a roughly conical jet of high speed air. At the back side of the fan there is a low-pressure region which makes the surrounding air move towards the fan (following the pressure gradient) from a large solid angle, as you already stated.

This is similar to the observation that you can easily blow out a candle, but not extinguish it by sucking in air near it.

Christoph
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