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Looking at flow with coffee and milk 3 (behind a straw)

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 straw, a circulating flow (vortex) is created behind it, and this phenomenon is called "separation". The same movement will occur with a round rod (cylinder) rather than a straw.
  • The vortices generated by separation (separation vortices) move away from the straw sequentially. At this time, clockwise and counterclockwise vortices are generated alternately. Such regular alternating vortices at the same time interval are called "Karman vortex streets".
  • Karman vortex streets can often generate sound and shaking of the object (vibration). If strong vibration continues, the object may break, and this can lead to an accident.
  • The technology to visualize this flow 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 "Karman vortex street" is a turbulent flow that is constantly fluctuating, so many people think that this is a "turbulent flow". However, "seemingly turbulent flow" and "turbulent flow" are different. In the "Karman vortex street", vortices are generated regularly, and the structure of each vortex is uniform. Therefore, the normal "Karman vortex street" occurs in the case of "laminar flow". In the case of "turbulent flow", vortex structures may be formed alternately behind the object; however, the typical "Karman vortex", which has highly regular and uniform vortex structures at the same time interval, occurs in the case of "laminar flow"..

The flow in this experimental video is referred to as "laminar flow", which can be confirmed by the fact that the milk line remains clearly. "Turbulence" refers to a flow with microscopic fluctuations that cannot be seen with the naked eye.
[Keywords] Separation, Karman vortex street, flow visualization
[Related items]

Looking at flow with coffee and milk 1, Looking at flow with coffee and milk 2,  Sinking a Japanese Penny Underwater


“Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, P68-69, P198-199.
“The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, P174-179.

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Last Update:4.13.2021