Flow in a River at a Corner

Let's take a look!

Watch the video on YouTube.

What kind of experiment is this?

Experimental procedure and explanation:

• We install a cylinder at the center of the basin and conduct an experiment similar to “Secondary Flow Inside a Basin.”
• When the basin is rotated, a forced vortex forms.
• When the rotation of the basin is stopped, the bottom and side walls become stationary. However, the water keeps rotating for a while.
• Close to the stationary bottom, the flow slows due to viscosity and smaller centrifugal force. This causes a secondary, inward flow, because of the push by the higher pressure at the walls. (In fluid dynamics, we often observe this phenomenon with rotating fluid particles (rotating coordinate system). On the basis of this phenomenon and with the help of apparent and centrifugal forces, we will explain the flow.)
• Because of the secondary flow, tea leaves accumulate near the center.
• As the pressure is higher near the walls, the water level is higher near the walls.
• n nature, when a high flow is caused by heavy rain, the flow becomes violent, eroding soil from river bottoms and riverbanks. This soil is carried downstream. But when the river turns a corner, the carried soil will accumulate at the inside of the corner because of the occurrence of the secondary flow there.

(An explanation was added as Mr. Takuya Matsuda (honorary professor, Kobe University) pointed out that it was misleading.)

[Keywords]	secondary flow
[Related items]	Secondary Flow inside a Basin, Centrifugal Force
[Reference]	“The Wonders of Flow” Japan Society of Mechanical Engineering, Kodansha Blue Backs pp. 60-61.