Secondary flow and sediment in curved river

Let's take a look!

What type of experiment is this?

Experimental procedure and explanation:

(The explanation below is almost identical to that in “Sediment in a curved river” that was previously published)

• Secondary flows were explained in the previous experiment, “Sediment in a curved river,” but let’s observe its behavior.
• If shredded paper is floated on the water surface, it can be observed that the paper moves to the outer side of the water surface.
• Meanwhile, if the water-soaked paper is released into the water, it sinks and moves to the inner side.
• When it rains heavily, this becomes a muddy stream that is mixed with sediment, but where does this sediment accumulate?
• As explained in “Speed of flow in a curved river 2” and “Flow of curved river”, in a curve, the pressure increases on the outer side.
  Additionally, near the river bed, the flow is decelerated by viscous forces. The centrifugal force then becomes proportionally smaller (in fluid dynamics, when dealing with a flow in a curve or a rotating flow, there are cases where phenomena are observed from the viewpoint of the fluid particles (rotating system). In this case as well, the apparent force and centrifugal force can explanation the phenomenon).
• Meanwhile, in the upper layers away from the river bed, the viscous action is weak, and the flow is fast. Therefore, the centrifugal force is greater than near the river bed.
• The centrifugal force is small near the river bed, so the sediments get pushed by the high pressure on the outer side and flow to the inner side. Conversely, in the upper layer, the centrifugal force is large, so an outward flow is generated, which gives rise to a circulating flow, as shown by the red line in the figure.
• The main flow of the river flows downstream, but there is also a component of the flow perpendicular to this direction called the secondary flow. This flow causes small pieces of paper submerged in water to move towards the inner side and get deposited. This phenomenon is observed in actual rivers as well. In tight curves, a gentle reverse flow (separation) may occur in the downstream area on the inner side, making it easier for sediment to deposit.
• If the curve continues, the flow on the inner side may slow down due to the influence of the secondary flow; a supplementary explanation of this is given in  [Advanced].
• This video of the experiment was produced with the support of JSPS KAKENHI 18K03956.

[Advanced]	The basic concept for flow in a curve is “slower on the outer side and faster in the inner side”. However, in an actual river, various factors, such as the water depth, topography, river bed undulations, secondary flow, and separation, may form an area where “flows are fast on the outer side and slow on the inner side”. The effect of water depth was explained in the [Advanced] section of “Speed of flow in a curved river 2”, so the effects of secondary flow and separation are explained here. Secondary flows occur in curved rivers, and they are characterized by flow near the river bed moving towards the inner side. Viscous friction slows down the flow near the river bed, but if the curve is long enough, the slow flow moves to the inner side and the inner section slows down. However, the effect of the inner side slowing down due to the secondary flow is more pronounced when the curving angle is large, such as when the direction is changed by 180° (as in this experimental video) or by 90°. It is also more likely to occur in tight, sharp turns, but such extreme curving is relatively rare in actual rivers. Additionally, it is important to note that at the end of a curve, the low-pressure flow on the inner side rises to its original pressure. As a result, the flow on the inner side decelerates, and the overall flow moves towards the outer side by the same amount, resulting in slower flow on the inner side and faster flow on the outer side. In the video of the experiment, the paper scraps accumulate in the low-speed area around the end of the curve and in the downstream area. Furthermore, in the case of a tight (i.e., sharp) turn, reverse flows can occur in this low-speed area, known as separation.
[Keywords]	Secondary flow
[Related items]	Speed of flow in a curved river 2, Flow in a River at a Corner, Collecting tea leaves, Centrifugal
[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, p. 60–61. p. 151–152.

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