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A ceiling fan creates a vertical circular airflow. Does this airflow along a window or a door in the room reduce the airflow through them? I would guess air trying to flow into the room would face more resistance as the ceiling fan's speed increases.

As an example imagine there's a window along the right-hand side I contour:

Velocity contours in a room with a ceiling fan

Will having the ceiling fan spinning, reduce the rate of exchange of air between the room and the outside world or the rest of the house?

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The question reduces to: "Can a parallel airstream to an open window in a room prevent wind from outside the room from entering the room?"

In wind, the air molecules have a specified direction (on the average). The average kinetic energy of this extra directed velocity (the wind velocity) is pretty insignificant (assuming typical air conditions) compared to the average kinetic energy of the air molecules when there's no wind:

One can look up:

For typical air at room conditions, the average molecule is moving at about 500 m/s (close to 1000 miles per hour)

Just for information, because these (relative to wind velocities) high velocities (due to the heat content of the air), have nothing to do with this problem.

Let's assume typical air conditions. If there is no airstream parallel to the window (the fan is off) and a wind is blowing towards the open window than the wind can blow freely into the room, making you feel a wind blowing in the room It's the averaged wind velocity that's involved.

Now if we apply an airstream parallel to the window (fan switched on), you are creating a "wind" with a momentum parallel to the window. The difference with the case where this wind is not there (fan switched off), is that there is a net moment, contrary to non-moving air (which contains kinetic energy though de to the heat contained). This wind (fan switched on) parallel to the window can deflect a bigger part of the incoming wind momentum.
But why? Well, if the momentum of the wind is directed toward the window, the wind (fan switched on) parallel to the window gives the wind a change in momentum upward. The wind deflects from the window (of course the upward momentum of the fan wind can't change the wind momentum to Because the layer of wind parallel to the window has a certain thickness, a part of the air molecules will not reach the room because of this, which they would have if the fan was switched off.
Imagine a very strong fan. The wind caused by this fan can surely prevent (as you guessed) a part of the incoming wind from reaching your room.

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  • $\begingroup$ So the vertical air flow does not have an "air curtain" effect as sometimes found in shopping mall or airport entrances? $\endgroup$
    – Yigal
    Jul 27, 2020 at 17:09
  • $\begingroup$ @Yigal I was thorough editing when you commented. As you see I changed the answer as I first misunderstood it. And as you can read, I think your guess is right. $\endgroup$ Jul 27, 2020 at 17:34
  • $\begingroup$ Thanks for the clarification. Though your conclusion is the opposite of Adam Herbst's if I understand correctly. Does it mean having the fan on can induce airflow when the outside wind is below some threshold and reduce it when the wind is above the threshold, compared to having the fan off? $\endgroup$
    – Yigal
    Jul 27, 2020 at 20:24
  • $\begingroup$ @Yigal Sorry for reacting a bit late. I was watching a movie. When the fan is off, air (wind) can easily enter the entrance of the shopping mall's entrances. The "air curtains" are used to keep the cold or hot wind out of the mall. In the first case the temperature of the "air curtain" is relatively high and in the second case relatively low wrt to the outside temperature, so the air curtain absorbs heat or cold while at the same time blocking the wind from entering. No matter how high the outside wind, some of it won't enter the room when the fan is on. Fan out: wind will enter maximally. $\endgroup$ Jul 27, 2020 at 23:13
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    $\begingroup$ @Yigal Even when there's very little wind and the fan wind blows parallel over the window, there will not occur a Bernouilli effect which reduces the pressure in the window plane (this effect occurs only when the wind blows over a curved surface like an airplane wing). This is because you can just as well say that the outside air is moving wrt to the parallel fan wind, which would imply the air is pushed outward because of the lowered pressure outside. So in any case letting the wind produced by a fan flow along your window will stop some part of the outside wind from flowing in. $\endgroup$ Jul 27, 2020 at 23:16
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Here's my best guess:

If air blown by the fan flows parallel to an open window, then by Bernoulli's equation it should have lower pressure than the stationary air just outside the window. A parcel of that outside air feels normal atmospheric pressure from all sides, except for the window side. So it gets pushed inward, and so air flows into the room.

That's one possibility. But since air is nearly incompressible in typical conditions, the total amount (mass or number of particles) of air in the room should not change. So if air is flowing in through the window, it must be flowing out somewhere else. For example if you have a ventilation duct, it could passively flow out through the duct, while the pressure differential is actively bringing it in through the window.

Basically, if we're assuming an equilibrium laminar flow, then there will be some streamlines moving faster and others moving slower. Wherever there's a gradient of flow speed among adjacent streamlines, the lower speed (higher pressure) ones should gradually be migrating toward the higher speed (lower pressure) ones as you move along the streamline. So the flow turns. But that doesn't have to happen at the window, it could happen anywhere in the room, depending on the boundary conditions (the shape of the room and any openings it has).

So the fan may induce air exchange if your room is set up well. But I don't see any reason it would inhibit exchange. If air is not actively flowing through the window, it's because the air just inside the window is stationary anyway. And in that case, you should get the same rate of exchange that you would without the fan.

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  • $\begingroup$ Could you also please elaborate how this explanation interacts with the explanation given by @descheleschilder? $\endgroup$
    – Yigal
    Jul 27, 2020 at 20:25
  • $\begingroup$ I guess I didn't realize you're referring to wind blowing in. That would certainly be a non-equilibrium effect, probably with turbulence involved, so it does become hazy. Yet in the end, I don't see how downward flowing air provides resistance to laterally flowing air. Think of a collision between particle A moving down with speed $u$, and particle B moving left with speed $v$. If they have the same mass, B can transfer all its momentum to A, and kinetic energy is conserved because $K = \frac12m(u^2 + v^2)$ in both cases, since $u$ and $v$ are perpendicular. $\endgroup$ Jul 29, 2020 at 1:00

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