River depth and speed of flow 2

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What type of experiment is this?

Experimental procedure and explanation:

• How does the river depth change the speed of the flow? There are functions that are different from the ones in the experiment, “River depth and speed of flow 1”.
• A shallow area was created in the middle of the straight section of the river flow experimental device. The gray part in the video is the shallow section (approximately 5 mm deep), and the black part is the deep section (approximately 15 mm deep). Please note that the water depth is deeper than the shallow section in the experiment “River depth and speed of flow 1” (approximately 2 mm deep).
• Floating a small piece of paper in the current shows that the flow speed increases in the shallow section. This is because there is a shallow water area partway, and the flow channel became smaller.
• The continuity equation was described in “Flow of narrowing river”. The value obtained by multiplying the cross-sectional area of the flow channel by the average flow speed (value of flow speed averaged within cross-section) is the flow rate, and the flow rate is same on both upstream and downstream sides.
  (flow rate) = (flow channel cross-sectional area) × (average flow speed) = upstream = downstream.
  This is called the continuity equation. It can be observed that a smaller flow channel area results in a faster flow.
• In this experiment, we observe that the flow speeds up in the shallow section due to a reduction in flow channel area. This is because the water has to pass through a narrow passage, causing the flow to accelerate at the upper and side parts of the shallow section. In an actual river, there may be variations in flow speed due to the shape and unevenness of the river bed.
• However, if the shallow section is too shallow (i.e., approximately 2 mm deep), like in “River depth and speed of flow 1”, then the flow that avoids the shallow section will flow to the side instead of over the shallow section and will not advance well. The depth of the shallow section must be sufficient.

[Keywords]	Relationship between water depth and speed, contraction flow
[Related items]	Flow of narrowing river、River depth and speed of flow 1
[Reference]	“Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, 190–191. “The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, p. 151–152.

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