Where is the gold dust?

Let's take a look!

What type of experiment is this?

Experimental procedure and explanation:

• Once upon a time, an old man lived near a curved river. The old man was collecting gold dust by scooping it up.
• Where should the old man look for the gold dust? “1. On the inner side of the curve,” “2. Around the middle of the river,” or “3. On the outer side of the curve”?
• Instead of gold dust, we prepared pieces of colored copy paper that were shredded into small pieces (you may also cut the paper into small pieces using scissors) and soaked them in water. These pieces will sink in water, so they flow near the bottom of the river.
• Upon releasing these pieces upstream, they flow toward the inner side at the curve and accumulate there. Therefore, the correct answer is “1,” i.e., the old man should look for the gold dust on the inner side of the curve.
• As explained in “Secondary flow and sediment in curved river”, secondary flow at curves causes water to flow outward near the surface and inward near the bottom. Therefore, if something sinks to the bottom, it flows on the inner side. Furthermore, near the end of the curve, the flow speed decreases, and sedimentation becomes much more likely.
• Additionally, near the beginning of the curve, the flow slows down on the outer side, possibly leading to sedimentation. However, the amount collected on the inner side by the secondary flow is less than in the second half of the curve.
• This video of the experiment was produced with the support of JSPS KAKENHI 18K03956.

[Keywords]	Curved river, secondary flow
[Related items]	Secondary flow and sediment in curved river
[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, p. 60–61.

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