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.

  • 2
    $\begingroup$ Well, right. When the air goes out of the fan, it's directed, so the air molecules are hitting your hand. On the other hand, when the air goes into a fan, it may go from any direction to fill the pressure gap and the solid angle is therefore much greater, reducing the velocity. I think that your intuition about a "natural symmetry" is a good one and if there were no turbulence, I believe that the velocities would be symmetric and had the same absolute values. So the symmetry violation is a consequence of complicated turbulent phenomena. $\endgroup$ Oct 21, 2011 at 18:57
  • $\begingroup$ All dissipating processes such as friction and viscosity break the time-reversal symmetry of the effective laws of physics. $\endgroup$ Oct 21, 2011 at 18:58
  • $\begingroup$ @Lubos: except here there is no real viscosity in the relevant region, except touching the fan blades. You can also have irreversibility in the initial conditions, like the irreversibility of radiation going out from charges and not inward, is caused by zero radiative field initial conditions. Here also, the room air conditions can lead to irreversibility without obvious dissipation. $\endgroup$
    – Ron Maimon
    Oct 21, 2011 at 21:36
  • $\begingroup$ It's essentially d'Alembert's paradox. Potential flow theory would predict no lift, no drag, symmetric flow into and out of the fan, helicopters would not fly, etc. The standard explanation is Prandtl boundary layer theory, the boundary condition at the surface of the blades creates a thin layer where viscous forces are important, vorticity is generated etc.. $\endgroup$
    – user1631
    Oct 22, 2011 at 0:00
  • 1
    $\begingroup$ this is not really a duplicate... $\endgroup$
    – bharal
    Jan 11, 2013 at 11:28

1 Answer 1


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.


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