Speed of flow in a curved river 3

Let's take a look!

What type of experiment is this?

Experimental procedure and explanation:

• In “Speed of flow in a curved river 2”, the basic concept is that “flows are slow on the outer side and fast on the inner side.” However, it was explained that various influences such as the water depth, topography, river bed undulations, secondary flow (explained in “Secondary flow and sediment in curved river”), and separation may form an area where “flows are fast on the outer side and slow on the inner side.”
• Hence, let’s observe a bit more closely to see how the speed changes at each part of the curve. Take colored copy paper shredded into small pieces and float them in water to see how they flow.
• (1) At the beginning of the curve, the flow is slow on the outer side and fast on the inner side (which is opposite to what is usually explained in 5th-grade science textbooks). (2) Indeed, around 45° in the first half, the flow is slow on the outer side and fast on the inner side. (3) At around 90°, there is an area on the inner side where the speed decreases, with the flow fastest near the center, due to the effect of secondary flow, where the flow near the bottom decelerates by viscous friction with the bottom and moves to the inner side. This flow rises to the water surface on the inner side and gives rise to a low-speed area. (4) At around 45° in the second half, the speed difference between the outer and inner sides is not clear, with the flow slightly faster around the center. (5) Around the end of the curve, the flow is fast on the outer side and slow on the inner side (identical to what is written in 5th-grade science textbooks). Around the end of the curve, the flow on the inner side increases in pressure as it travels downstream and decelerates, while the flow on the outer side accelerates by the same amount (see the “Advanced” section in “Secondary flow and sediment in curved river”). As a result, the outer side is fast, and the inner side is slow.
• In some cases, a phenomenon called “separation” may occur near the end of a curve. In our previous experiment, we reduced the flow rate and observed backflow on the inside near the end of the curve, flowing downstream. This phenomenon is known as “flow separation,” which is more likely to occur when the pressure increases as the flow moves further downstream.

[Keywords]	Curved river, speed distribution, separation
[Related items]	Speed of flow in a curved river 2、Secondary flow and sediment in curved river
[Note]	This experiment shows the basic tendencies of a curved river, and the tendencies of actual rivers may differ due to various factors such as water depth, topography (whether curve is sharp, angle of the curve, etc.), river bed undulations, and changes in flow rate.
[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, p. 52–59, p. 60–61, p.136–139.

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