When no water passes, the board hangs downward; when water passes, the board rises. Why is that? Someone told me this is because water has viscosity or tension. Is this explanation correct? I found that even if the experiment was done under the water, the plate would still rise, so this phenomenon should not be related to the tension of the water. So, is this phenomenon related to the viscosity of water? No water flowing through There is water flowing through


It does not depend on viscosity, rather it is related both to the surface tension and to the momentum of the fluid: the board goes up because there is a decrease in pressure due to the flow (Coandă effect) and because there is a tension, a cohesion between the fluid and the board.

I would say that however the descrease in pressure is the main responsible, since you can obtain the same lift of the board with a fast enough flow of air (having enough momentum not to be pulled down onto the board surface due to the lack of pressure).

The tension should be responsible as we see that cohesion forces at the interface induce a deviation of the flow: the classic example is that of water flowing on a spoon.

I would say that the underwater experiment (as you present it at least) does not disprove the importance of the tension for two reason: it is not evident that the tension at the interface cannot play a role underwater, and the underwater phenomenon is not simply the exact same phenomenon we have in the setting we discussed (out of water), in the sense that the tension could even contribute only out of the water and you would still observe the lifting of the board underwater.

However the tension is still there, the only difference is that now you have it on the other side of the board too. This surely makes the board give some more resistance to being lifted, but this will not exactly cancel the tension on the flow side (this I think depends on viscosity as the thickness of the layers of liquid which are effectively being lifted with the board depends on the viscosity).

In conclusion these two effects should be causing the board to lift.

Viscosity could change details of the Coandă effect, but it should happen also with frictionless fluids. At least this is my understanding from https://en.m.wikipedia.org/wiki/Coandă_effect

  • $\begingroup$ The tension of water is not the cause, because even under water there is the same phenomenon: the board will rise. $\endgroup$
    – enbin
    Apr 2 '19 at 6:20
  • $\begingroup$ Some people think that viscosity is the cause of Coanda effect, if this is correct, then viscosity is also the cause of the rise of the board. $\endgroup$
    – enbin
    Apr 2 '19 at 6:35
  • $\begingroup$ You don't explain why momentum changes. $\endgroup$
    – enbin
    Apr 2 '19 at 6:48
  • $\begingroup$ the tension of the water is not the only cause, as I said. First of all it is not a trivial thing that just because you are under water the flow is not conditioned by the contact with the interface. Plus I have no evidence that the underwater phenomenon works exactly as the one we are discussing. About the Coandă effect we should try to find an accurate discussion maybe this one en.m.wikipedia.org/wiki/Coandă_effect . The momentum changes because the flow makes the board rise, so the board will equally try to pull down the flow. If the flow is strong enough it does not bend. $\endgroup$
    – AoZora
    Apr 2 '19 at 7:06
  • $\begingroup$ This is all the result of conservation of momentum. The board is diverting the flow of water downward, which requires the board to exert a downward force on the water. The reaction force of the water on the board is upward, which results in the rise of the board. $\endgroup$ Apr 2 '19 at 11:56

I think the main reasons are adhesive forces of water and board, pressure force due to flowing water has less pressure than atmospheric pressure. This force is even responsible for airplane's take off, peeling away of rooftops at the time of hurricanes.


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