Blowing Away a Japanese Penny

Let's take a look!

Watch the video on YouTube.

What kind of experiment is this?

Experimental procedure and explanation:

• When you blow air across a Japanese penny on a table, the penny will be blown away.
• When the flow initially hits the penny, “flow separation” takes place at the corner, forming a recirculation region.
• Streamlines will curve around this region. In a curved flow, the outside will have a higher pressure than the inside (streamline curvature theorem). Thus, the pressure at the separation zone will become smaller than atmospheric pressure, thereby sucking the penny up. Once the penny rises from the table, the bottom side will receive air, and the penny is blown away.

[Note]	An incorrect explanation in accordance with Bernoulli's theorem exists; that is, the pressure at an area where flow exists is lower (than the ambient pressure) because velocity there is higher. Even if you blow air with a dryer or with your mouth into an empty space, the pressure will remain almost at the atmospheric level. Bernoulli's theorem describes the energy conservation law in a fluid. Forcing air to flow increases its energy relative to that of the surrounding air. Therefore, Bernoulli's theorem cannot be used to compare the moving air with the surrounding, stationary air. Please be careful about this as several books have used this erroneous explanation.
[Keywords]	flow separation, streamline curvature theorem
[Reference]	“The Wonders of Flow” Japan Society of Mechanical Engineering, Kodansha Blue Backs pp. 136-139, pp. 120-127