Welcome back to the Demo Highlight of the Week! This week, we’re exploring the motion of air with physics student Kathleen Hamilton-Campos and demonstration F5-09, the Coănda Effect with a hair dryer and a ping-pong ball.

Named for Romanian scientist Henri Coandă, the Coandă Effect describes the phenomenon where a stream of moving fluid will tend to stay in contact with a curved surface, and conversely an object in a stream of moving fluid will tend to remain within that stream. While superficially similar to the Bernoulli Effect, which describes the changes in speed and pressure in a constrained fluid, the differences between the two can be important when analyzing things like the movement of aircraft, and in this demonstration! 

 

A ping-pong ball is placed in the stream of air coming out of a hair dryer. The moving stream of high-speed air entrains the slower air around it, pulling it along. Around the surface of the ball, though, this becomes asymmetric, creating a low-pressure region in the center of the stream with a high pressure region around it. The ball is effectively trapped in this low-pressure core.