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I was sitting right in front of a big window (with a mesh) with curtains open, suddenly wind swooshes in, curtains sway forth and back. While they sway backward, they stick to the window(mesh) as if they are being sucked due to a vacuum. Somewhat like the image below:enter image description here

Now my explanation for this phenomenon was using Bernoulli's Equation:

$$P_0+\frac{\rho v^2}{2}+\rho g h = \text{constant}$$

taking $h=0$ (because I'm assuming that Air is acting $\perp$ to the curtain surface,) one can observe that: when fast blowing air moves past the curtains, Pressure in the region between window(mesh) and curtains drops causing them to sway back (and stick to mesh).

Is that right?

Any corrections and further insights into the problem are appreciated.

Edit: To better understand the problem and diagram, you can consider visualizing the "Curtain swaying back" part in the diagram as if the direction of the wind reverses when curtains start getting back to there equilibrium position, i.e. wind is pushing the curtains into mesh!

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  • $\begingroup$ I am still trying to make sure I understand the scenario, but I would recommend looking at some misapplications of Bernoulli's principle just in case :) $\endgroup$ – BioPhysicist May 28 '20 at 20:57
  • $\begingroup$ Could you perhaps make a diagram of the system? I still don't fully understand the scenario. $\endgroup$ – BioPhysicist May 28 '20 at 21:03
  • $\begingroup$ Thanks for the diagram. How large is the room? Does the air coming in have anywhere to go? $\endgroup$ – BioPhysicist May 28 '20 at 21:26
  • $\begingroup$ Room is kinda big, I don't know about the actual size, and Yes air has a lot of space to go i.e. Room is not sealed if you're taking that part into consideration.But curtains cover the window completely $\endgroup$ – Samarth May 28 '20 at 21:30
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    $\begingroup$ Do the curtains always get sucked in when pushed out or is it random? If it is random I think your arrows are in the wrong direction. Turbulent wind can come in different directions and cause the curtains to be pushed towards the window if the wind is travelling parallel to the outside wall via Bernoulli's equation. $\endgroup$ – Cell May 28 '20 at 21:39
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I highly doubt Bernoulli's principle explains this. If you blow on something, it doesn't suddenly start moving against the air. Bernoulli's equation is correctly applied when you are looking at pressure differences in a fluid resulting in changes in speed / height of the fluid. Bernoulli's equation doesn't say "fast moving fluid is at lower pressure".

The most likely reason is that, for whatever reason$^*$, air rushing into your room then tries to rush out. This pushes the curtains against the mesh. Without actually being there and seeing it / doing other tests I am not sure if there is more I can speculate about though.


$^*$ My guess is that, since before the wind is blowing, the air in your room / house(?) is at equilibrium with outside air (concentration wise), and so when more air rushes in it will then eventually try to rush back out.

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  • $\begingroup$ Actually what I was trying to convey in my explanation was that Fast moving air creates a low-pressure region between the curtains and mesh rather than saying that fast-moving air is at low pressure. And, "air rushing into your room then tries to rush out." is probably not possible because this room is well ventilated and all the doors and windows are already open, thus both room and outside air should have similar concentrations. $\endgroup$ – Samarth May 28 '20 at 21:39
  • $\begingroup$ @Samarth "thus both room and outside air should have similar concentrations." exactly. That is the case before the wind comes into your room. Then there is more air in the room when wind blows it in. It doesn't matter how well the ventilation is, there is still more air inside than it would have at equilibrium. Therefore to get back to equilibrium some air needs to leave. I suppose the time between influx and efflux would be twice the time it takes for sound to propagate across the room once the wind stops blowing? Or is it more random like Cell asked you on the main post? $\endgroup$ – BioPhysicist May 28 '20 at 21:41
  • $\begingroup$ Sounds reasonable but if so, why doesn't this happens when air is not moving as fast but still fast enough that you can feel it? $\endgroup$ – Samarth May 28 '20 at 22:00
  • $\begingroup$ @Samarth Maybe there isn't enough of a concentration difference to produce a large enough force to push the curtain against the mesh. $\endgroup$ – BioPhysicist May 28 '20 at 22:03
  • $\begingroup$ Why the ventilation doesn't matter even if air has a lot of room to move forward? $\endgroup$ – Samarth May 28 '20 at 22:07

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