Blowing Air between Two Sheets of Paper

Let's take a look!

Watch the video on YouTube.

What kind of experiment is this?

Experimental procedure and explanation:

• When you hold two sheets of paper in parallel and blow air between them, the two papers pull together.
• A region with slower flow, called a “boundary layer,” forms on the surface of the sheets of paper due to viscosity of the fluid. Because of this, the flowing air is narrower in the middle section and the flow at the center becomes faster. The flow is faster compared to the upstream side (atmospheric pressure), so the pressure in the middle section becomes less than atmospheric pressure. The sheets get sucked closer to each other.
• Once the sheets are pulled together, the gap gets smaller, and the flow further increases. Therefore, the pressure drops further, and the papers are pulled together in a consistent manner.

[Note]	The explanation that pressure at an area where flow exists is lower (than the ambient pressure) because velocity there is higher, in accordance with Bernoulli's theorem, is incorrect. 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, boundary layer, Bernoulli’s theorem
[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Koudansha Blue Backs, pp. 144-147 “Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, pp. 206-211