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50 votes

Why does my tea periodically alternate its rotational speed after stirring? (Link to video below)

Just looking at the video, it appears that the shape of the surface is varying quasi-periodically, as if the liquid is moving outward (and upward) toward the cup walls, then moving inward and rising ...
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38 votes

Why does my tea periodically alternate its rotational speed after stirring? (Link to video below)

You have created a Lissajous spiral from the tea: the raising of the mixing utensil creates a partial secondary flow which deviates some of the concentric flow up and down. The continuous rotational ...
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29 votes

Emptying a bottle faster by swirl

Assuming you start with a full bottle of water, when you tip the bottle upside down, a 'partial vacuum' (ie below atmospheric pressure) is created at top of the bottle as the water pours out the ...
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25 votes
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Emptying a bottle faster by swirl

When water leaves the bottle, the pressure above it drops. This reduces the net force pushing the water out of the opening, until it stops and a bubble can rise up. When the bubble has left the mouth ...
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24 votes
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Can a water vortex (whirlpool) be used to efficiently store energy?

Yes, this would be horribly inefficient compared to pumped hydro or even a regular flywheel. With a rotating fluid, there's a lot of viscosity. This viscosity generates heat and slows the fluid down....
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14 votes
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What is a Physically Accurate Explanation for the Kutta Condition?

The Kutta condition is completely artificial. The potential equations are completely artificial. The potential equations are a mathematical construct we use because it's much simpler than the full ...
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14 votes

Why does my tea periodically alternate its rotational speed after stirring? (Link to video below)

To corroborate on the observation of @Vladimir Kalitvianski, this hypothesis might be understood by simple Newtonian physics. There are two timescales in the system - $\tau_J$ timescale of the ...
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12 votes
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Why do co-rotating vortices coalesce, but not counter-rotating ones?

Intuitive start of an answer: If you have counter rotating vortices they have zero net angular momentum (to first order). If they merged they would have to have no motion -> where did the energy go. ...
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11 votes

Why do co-rotating vortices coalesce, but not counter-rotating ones?

Because where they come close together the air in between circulates in such a way as to join them in a single path. Floris is right, but maybe this picture helps.
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11 votes
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What is the mechanism between whirlpools repelling and attracting each other?

For liquids that are moving Bernoulli's principle states that $$P+h\rho g+\frac{1}{2}\rho v^2$$ is constant. This means that the pressure exerted by a faster flowing liquid is reduced (for the same ...
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10 votes

Why do "quantum vortices" form in superfluids?

The superfluid can be in a state with zero vortices. But if one imparts angular momentum to the superfluid, then the angular momentum is physically expressed either by the superfluid rotating as a ...
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8 votes

How many independent properties can a photon have?

Short answer: The orbital angular momentum (or vorticity) of photons is part of the spatial degrees of freedom. It would therefore replace the momentum degree of freedom and should not be consider as ...
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8 votes

Why does my tea periodically alternate its rotational speed after stirring? (Link to video below)

When a liquid spins, there is often a vortex that spins quickly. Velocity drops away from the center. Usually the vortex spins quietly in the center of the vessel. In this case, there may be a ...
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7 votes
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Smoke ring in space?

The smoke ring is an instance of vortex, as you have mentioned. So, to begin with - How does the vortex work? The vortex effect is possible due to the viscous friction between the substance you ...
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7 votes

Can a water vortex (whirlpool) be used to efficiently store energy?

As mentioned by other writers, water viscosity will rapidly convert any kinetic energy into heat. However, efficient energy storage could be attainable with superfluids. Superfluids are related to ...
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7 votes

Why does a vortex last so long?

The smaller the fluid's viscosity, the longer the vortex will last. Viscosity is a measure of friction effects within a fluid. As with other forms of friction, viscosity transfers energy from ...
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6 votes
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Physical intuition for Lamb vector

Consider the Navier-Stokes equations $$\frac{\partial \bf{u}}{\partial t}+\bf{u}\cdot\nabla \bf{u}=-\nabla p+\nu \nabla^2\bf{u}.$$ We can rewrite the inertial term as $$\bf{u}\cdot\nabla \bf{u}=\...
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6 votes
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What is the physical meaning of irrotational flow?

The simplest physical interpretation is to imagine a little paddle wheel moving with the flow. If it spins, the flow is rotational. If it maintains its orientation (even if it's orbiting a core of ...
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5 votes

Whirlpools and Tornados

I think this is a good question, for two reasons. First, I believe that whirlpools in the bath and tornadoes have much in common. Second, while whirlpools can be easily observed in detail, tornadoes ...
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5 votes
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Prevention of fluid vortex formation

The vortex forms due to a process called vortex stretching. Essentially as soon as the plug is pulled and the tank begins draining, a vortex forms at the center of the drain where the length of the ...
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5 votes
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Why is the eye of a cyclone a forced vortex?

Empirical evidence: First of all, to fix our ideas, I assume that you are talking about an tangential wind profile that looks something like this: $\phantom{texttexttexttexttexttexttexttexttextte}$ ...
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5 votes
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Is there an explicit connection between rolling-shutter images of rotating propellers and interference patterns with optical vortices?

Yes indeed, there is a connection. And, as one can imagine, it is a geometrical one. To make the connection, one needs to define three-dimensional spaces for the two scenarios. For the optical ...
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5 votes
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$\nabla \times \bf{u} \neq 0$ but $\oint_{c} \bf{u} \cdot \textit{d}r \textit{=0}$?

The path you have chosen is special in a way, for this vector field. The path is cylindrically symmetric. The fact that $z$ is constant in your path means that $u_z$ becomes irrelevant to the integral....
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4 votes

What is the physical basis of the $\rho \bf{u} \times \bf{\omega}$ force on a fluid vortex line?

Bernoulli's equation does not require that the flow be irrotational, just inviscid. Let's consider a vortex filament, and denote its surface by $S$ and volume by $V$. Using the identity you ...
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4 votes
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Why does the tea in the middle of the cup spin faster when I stir it?

Ultimately, the Navier-Stokes equations explain this :) OK, that's not a useful answer: here's how they explain the phenomenon in some cases. Under steady state conditions for a fluid (inviscid, ...
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4 votes

Emptying a bottle faster by swirl

I think you've understood it all, air gets into the bottle faster. Without the vortex, the air is able to pull on the liquid, preventing it from escaping. This is why you can pour orange juice faster ...
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  • 1,375
4 votes
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Navier Stokes: what about angular momentum?

The basic answer was given here: In a fluid, why are the shear stresses $\tau_{xy}$ and $\tau_{yx}$ equal?. Angular momentum conservation follows from linear momentum conservation (expressed by the ...
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4 votes

Why does bathwater make a vortex in the plughole?

Do this at the equator, so you can forget about the angular momentum of things on the earth. Is the container circular, with no obstructions on the bottom? Is the drain in the center? Is the water in ...
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