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A couple of weeks ago, a friend and I were brewing a new batch of beer. We boil our wort in a turkey fryer. Pretty standard setup for beginners. Anyway, our wort was getting pretty close to boiling (not quite bubbling on the top yet.) I started stirring the wort and then stopped. However, the wort continued spinning and the spinning was accelerating. The "pit" of the whirlpool was continually getting deeper until it was a few inches deep. Then, while the whirlpool continued spinning at a seemingly constant rate, the wort started shooting up from the middle of the whirlpool in spurts like a tiny geyser. This went on for about 30 seconds or so as we gawked and thought it was cool. But it started making a mess, so we stuck our stirring spoon in there to stop the wort from stirring, which successfully stopped the whole process. A few minutes later when I stirred it again, the same thing happened.

I tried using Google to find what this effect might be, but I was unsuccessful in turning up any info.

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  • $\begingroup$ Since you did not find any help in the last weeks, this might be a better fit for Physics. (You can flag it for migration if you would like to try.) $\endgroup$ – Martin Jul 3 '14 at 2:45
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    $\begingroup$ Are you brewing your beer in a witch's cauldron? Is your garage built over a cemetery? Does your recipe include eye of newt, virgin's blood or frog toes? If the answer to any of these is yes, you may have opened a Hellmouth. This will affect the color and bitterness of your beer. To compensate, cut back on the hops, use a lighter malt, and add a dash of holy water (any denomination will do). $\endgroup$ – Schwern Jul 3 '14 at 6:36
  • $\begingroup$ @Schwern Haha. Best comment ever. $\endgroup$ – Tom Jul 3 '14 at 16:31
  • $\begingroup$ Aside from middle English "wort" and OE "wyrt' for plant, this is the first time I've heard this archaic word in this apparently (for brewing) correct usage (clearly linked to the ME). Kind of sets the scene for @Schwern 's comment. $\endgroup$ – Selene Routley Jun 5 '15 at 6:36
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Not sure about this - best guess is that you have observed the formation of an irrotational vortex. When you stir, the liquid tends to rotate as a rigid body.
enter image description here

When you stop stirring, then

In the absence of external forces, a vortex usually evolves fairly quickly toward the irrotational flow pattern, where the flow velocity u is inversely proportional to the distance r. For that reason, irrotational vortices are also called free vortices.

irrotational vortex

The pressure drops as you approach the core - this can lead to the empty core you observed. I can't really explain the wort

shooting up from the middle of the whirlpool in spurts like a tiny geyser,

but the very low/zero pressure in the center must be relevant.

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The liquid shooting upward from the pit of your vortex may be explained by at least two effects.

The first effect was indirectly addressed by Albert Einstein in a 1926 paper he wrote on the erosion of river banks (http://people.ucalgary.ca/~kmuldrew/river.html). This effect is also called the tea leaves paradox, from an experiment Einstein performed in a tea cup. He found when he stirred the cup that tea leaves gathered in a little pile in the center rather than toward the sides, where they might be expected to migrate due to the centrifugal force.

Einstein reasoned that angular velocity of the vortex against the sides of the cup was slowed by friction, diverging from the general increase of velocity with increasing distance from the center. He also reasoned that angular velocity at the bottom of the cup is less that at the top, due to friction against the bottom. The velocity gradient at the sides from top to bottom causes the vortex contents to migrate downward at the sides and into the center, forming a circular flow perpendicular to the plane of the vortex and depositing the contents of the vortex in the center.

Thus, there's a tendency for the bottom of the vortex to leap upward in the center as it tries to complete its circular flow from top to bottom to center and back to top. See figure 1 in Einstein's paper, linked above.

The second effect is caused by the tendency of gases to become less soluble as temperature increases, and by the fact that the center of an irrotational vortex tends toward singularity - it rotates slowly or not at all.

You said that your wort was near the boiling point. At this temperature, gas starts to vaporize out of the liquid when it finds points of nucleation. The rapidly swirling outer reaches of the vortex do not allow gas to accumulate at nucleation sites. But as the center point is either stationary or moving very slowly, it provides a nucleation site for gas bubbles to form.

Coupled with the tendency of the vortex contents to leap upward at the center due to Einstein's circulation, these bubbles may propel spurts of liquid straight up, which explains the phenomenon you observed.

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This is my guess: heated water from the bottom is flowing up the warm sides of the pot, while the cooler water from the top is being forced down the middle of the whirlpool to the bottom where it can be heated. These opposing flows of water have caused a whirlpool that acts in effect as a heat pump.

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    $\begingroup$ But this would drive a flow with a torus shape and not a rotating flow. $\endgroup$ – fibonatic Sep 10 '14 at 18:12

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