Flow of narrowing river

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What type of experiment is this?

Experimental procedure and explanation:

• How does the speed change in a river as the flow becomes narrower downstream?
• Let’s conduct an experiment with a flow channel that narrows down partway. When finely-shredded paper is floated on the water surface and allowed to flow it speeds up as the flow narrows.
• The cross-sectional area of the flow channel is smaller on the downstream side than on the upstream side. Hence, the flow speed becomes larger.
• The speed of the flow is inversely proportional to the cross-sectional area of the flow channel. The product of the cross-sectional area of the flow channel and the average flow speed (the speed averaged within the cross-section) gives the flow rate, which is the volume of fluid flowing per second. Another way to state this is that the flow rate remains the same for both the upstream and downstream sides.
  (flow rate) = (flow channel cross-sectional area) × (average flow speed) = equivalent upstream and downstream.
  This is called the continuity equation.
• Therefore, the flow rate is higher in narrow rivers and lower in wider rivers.
• (Supplement) For this experiment, we need not narrow the flow channel width gradually; it can be narrowed all at once. This is because there is almost no separation like that seen in “Flow of narrowing river”.
• This video of the experiment was produced with the support of JSPS KAKENHI 18K03956.

[Keywords]	Contraction flow, continuity equation
[Related items]	Flow of widening river
[Reference]	“Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing,“The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, p. 151–152.

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