Big peach coming down the river

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What type of experiment is this?

Experimental procedure and explanation:

• Once upon a time, an old woman was washing her clothes in the river. Then, a big peach came floating down the river.
• Where should the old woman, who is downstream of the curved river, wait for the peach? “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 a peach, we will use a thin styrene foam board (or any thin board that floats on the water) as a substitute for the peach. We then release this styrene foam board in the middle of the river upstream.
• Upon approaching the curve, the peach will flow to the outer side. Therefore, the correct answer is “3,” i.e., she should wait on the outer 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, the peach floating on the water surface flows on the outer side.
• 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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