Secondary Flow inside a Basin

Let's take a look!

Watch the video on YouTube.

What kind of experiment is this?

Experimental procedure and explanation:

• When a basin of water is rotated, a forced vortex forms.
• When the rotation of the basin is stopped, the bottom and side walls stop. However, the water has momentum and continues rotating for a while. This is basically the same situation as the rotating water inside the mug of “Collecting Tea Leaves.”
• Close to the stationary bottom, the flow slows down because of viscosity and smaller centrifugal force. This causes an inward flow, because of the push by the higher pressure near the walls. This flow forms a rising current at the center, an outward flow at the top, a downward flow along the walls, and thereby causing circulation. (In fluid dynamics, we often observe this phenomenon along with rotating fluid particles (rotating coordinate system). With this in mind, and with the help of apparent and centrifugal forces, we will explain the current phenomenon.)
• Though the overall flow (primary flow) is a rotation, a secondary flow also forms perpendicular to the rotation. The tea leaves accumulate at the center because of this secondary flow.

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

[Keywords]	vortex, secondary flow
[Related items]	Collecting Tea Leaves, Centrifuge, Centrifugal Force
[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs pp. 60-61