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Looking at flow with coffee and milk 1 (behind a card)

Let's take a look!

What type of experiment is this?

Experimental procedure and explanation:

  • Float milk on coffee and observe the flow.
  • When you move the card, a circulating flow (vortex) is created behind it, and this phenomenon is called "separation". If you move the flat plate rather than the card, the flow will be the same. Separation is one of the major causes of fluid resistance (acting as a brake by the force pushing an object in the downstream direction).
  • The flow cannot turn sharply. In the case of an object such as a flat plate, the flow bends at a right angle at the end and cannot wrap around behind, creating a vortex.
  • The vortices generated by separation (separation vortices) move away from the card sequentially. At this time, clockwise and counterclockwise vortices are generated alternately. In the image, the vortex generated on the upper side rotates clockwise, and the vortex generated on the lower side rotates counterclockwise.
  • Water and air are transparent and invisible, but as in this experiment, you can make the flow visible by placing visible things (ink, smoke, fine particles, etc.) in the flow. The technology to do this is called "flow visualization". It is often used in fluid dynamics research and development.
  • After the experiment was over, the staff enjoyed the coffee used in this experiment.

The phenomenon in which vortices are formed one after another behind an object because of separation is a seemingly turbulent flow, so many books describe this as "turbulent flow", but this is not correct. "Turbulence" is not a term that refers to "seemingly turbulent flow" or "separation." "Turbulence" is not a disturbance that can be seen with the naked eye, but is rather a microscale fluctuation of the flow. Such microscale fluctuations often lead to large visible fluctuations. However, not all "seemingly turbulent flows" are "turbulent flows". The flow in this experimental video is constantly fluctuating and turbulent, but it is a typical "laminar flow". This can be seen from the clear lines of milk that remain. In the case of turbulence, the milk is immediately agitated by microscale fluctuations, and the milk lines are blurred.
Also, many people may think that a "Karman vortex street" that forms a regular vortex street behind an object is a "turbulent flow", but a normal "Karman vortex" occurs in cases of "laminar flow" (reference Looking at flow with coffee and milk 3).

[Keywords] Separation, flow visualization
[Related items]

Air Flow behind a sheet, Sinking a Japanese Penny Underwater, Looking at flow with coffee and milk 2, Looking at flow with coffee and milk 3

[Reference] “Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, P68-69.
“The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, P174-179.

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Last Update:4.13.2021