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I read about the Tea Leaf Paradox on Wikipedia, but didn't understand enough, so I wanted to see the secondary flow with an experiment. I set it up and filmed this. It's water and some tea from different tea bags. I stir it for a minute and then see what happens when the stirring stops.

I am not able to see the secondary flow as shown in this figure from the Wikipedia article:

Secondary flow

Am I doing something wrong?

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  • $\begingroup$ What exactly were you expecting to see? $\endgroup$
    – probably_someone
    Commented Nov 25, 2019 at 14:29
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    $\begingroup$ From the moment i turn off the stirring, I expect to see some tea leaves spiraling downwards close to the glass, and spiraling upwards close to the center. Instead I see everything slowly spiraling down, and tea leaves in the center rotating faster than tea leaves near the edge. $\endgroup$
    – Jostein Trondal
    Commented Nov 25, 2019 at 14:31
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    $\begingroup$ The motion of objects in a fluid and the motion of the fluid itself aren't necessarily the same. This is especially true if the objects are large, which your tea fragments definitely are, and not neutrally-buoyant, which is also true (you can see that they sink in water). You wouldn't expect a feather to just suspend itself in the air if there's no wind, after all. But you see the primary effect, which is exactly the same one that's shown in the Wikipedia article - the tea ends up piling up in the center. $\endgroup$
    – probably_someone
    Commented Nov 25, 2019 at 14:34
  • $\begingroup$ Yes, I'm looking for something that's neutrally buoyant (and easily available), to maybe see the secondary flow better. I can definitely see the primary effect. I exploit it in beer making. $\endgroup$
    – Jostein Trondal
    Commented Nov 25, 2019 at 14:38
  • $\begingroup$ Neutral buoyancy will only help you so much - if the particles are too large, their inertia will make their motion very different than the motion of the fluid. You really need a really fine, neutrally-buoyant powder to have much of a chance, probably - industrial solutions for this almost certainly exist. $\endgroup$
    – probably_someone
    Commented Nov 25, 2019 at 14:45

1 Answer 1

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It is necessary to take a photo from above, then you can see the toroidal vortex as in the first frames in Fig. 1

Figure 1

In the numerical model, it is possible to determine the velocity field in a liquid and the particle trajectory as in Fig. 2 Figure 2

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  • $\begingroup$ Interesting! About how strong is the velocity field of the primary flow compared to the secondary flow? $\endgroup$
    – Jostein Trondal
    Commented Nov 25, 2019 at 15:03
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    $\begingroup$ It depends on the time. Figure 2 shows a scale of 0.2, and the main stream is 1. $\endgroup$
    – Alex Trounev
    Commented Nov 25, 2019 at 15:15

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