Wikipedia:Reference desk/Archives/Science/2021 September 24

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September 24

Why can't I feel sucking when standing behind table fan?

If front side of the fan feeling like blowing, why cant it gives me the same sucking feel when standing behind? Is it fan blade design is the factor here?Rizosome (talk) 02:25, 24 September 2021 (UTC)

The fan imparts linear momentum in a specific direction to the air that is blown out. It does not do that to the air at the other side in anticipation of it going to be sucked in. The air is equally being sucked in at the backside of the fan from all directions that lead to the spinning blades, which means that most of it comes in sideways.  --Lambiam 09:23, 24 September 2021 (UTC)

There will still be a low vacuum behind the fan. Try dangling a piece of paper near there, for example. 2601:648:8202:350:0:0:0:1598 (talk) 18:46, 24 September 2021 (UTC)

Deflection of a dangling piece of paper will merely show the presence and direction of the horizontal component of a local airflow. There is definitely no vacuum behind the fan, unless it is operating in a vacuum – in which case there is no airflow at all. To the extent the pressure is locally lower, this is almost entirely due to Bernoulli's principle. With an upright table fan operating in normal conditions, one can easily demonstrate the airflow behind the fan. This will show it as converging in from all directions. If a source of smoke is available, it may give the best demonstration.  --Lambiam 11:12, 25 September 2021 (UTC)

Is the airflow turbulent, thus rendering Bernoulli inapplicable? Imagine Reason (talk) 12:15, 25 September 2021 (UTC)

It is true that the velocity of the air approaching the fan is slower than the air departing from the fan. Wikipedia has an article on the matter. See Momentum theory which also addresses the difference in pressure upstream and downstream from the fan. Dolphin (t) 14:09, 25 September 2021 (UTC)

@User:Lambian, are you implying that the air flow becomes symmetrical between back and front as Re tends to zero? catslash (talk) 17:04, 25 September 2021 (UTC)

I'm not trying to imply that. A table fan is essentially a propeller, which generates thrust (a force in one specific direction) by exerting force on the air in the other direction. If you mount the fan on a wheeled platform, it may act as a propeller and push the system backwards. But under normal operating conditions, friction keeps the fan from sliding off the table. The force on the air is only imparted near the spinning blades and gives it momentum in the opposite direction of the (reactive) thrust. The momentum carries the air stream forward over possibly a considerable distance. There is no operating principle behind the fan to give the air momentum in a single direction. The air flow cannot be expected to tend to become symmetrical until the blades stop spinning.  --Lambiam 22:01, 25 September 2021 (UTC)

@User:Lambian, if the flow is not symmetric when inertia can be neglected, then momentum is not the cause of the asymmetry. catslash (talk) 20:32, 27 September 2021 (UTC)

If you put a box fan in a window, facing outward, and open some other windows, the fan will pull some air through the house. Just not with the kind of force imparted outward by the blades. ←Baseball Bugs ^{What's up, Doc?} carrots→ 18:53, 25 September 2021 (UTC)

If you stand behind a fan, or place your hand behind it, you will block the airflow, so it will pull air from elsewhere. But if you stand back and blow smoke at it you'll see that it does indeed pull air predominantly from the rear. nagualdesign 19:23, 26 September 2021 (UTC)

From the rear, but not preferentially in a direction parallel to the axle of the fan. If the source of the smoke is to the side, it will move in sideways. But it is blown out in a jet of air with a preferred direction before it disperses.  --Lambiam 10:04, 27 September 2021 (UTC)