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[Phys-L] Re: tea



HI,

In a stirred "Tea Cup" ( a classic problem)
problem, there is a secondary circulation. In a
vertical section which includes the axis of the
primary circulation, the pattern of this
circulation is a pair of loops which decend along
the either edge and rise in the middle, thus
dragging the tea leaves to the middle.

<- <- -> ->
| | | |
| ^ ^ |
| | | |
|-> ->| |<- <-|


The secondary circulation is associated with the
tea near the cup moving slower than the tea a
little way toward the center, due to the drag of
the cup on the tea. The air-tea surface is higher
at the outside than the center and at the tea-cup
interface the hydrulic pressure is not enough to
support this extra height and so it sinks, thus
driving the circulation.

Thanks
Roger

Anthony Lapinski wrote:

When tea is stirred, why do the leaves end up in the CENTER of the cup?
After all, they are more dense than the surrounding water, and I would
expect them to be "thrown to the outside" (like a centrifuge) due to their
larger inertia. I am thinking of the cork/water demo, where a cork is
suspended (upside down) by a string in water. If you hold the bottle and
spin around, the cork (with less inertia) moves toward the center of the
circle. Why do the tea leaves behave differently? Does it have to do with
some internal forces within the spinning tea?
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