Monday, November 25, 2013

Bernoulli Versus Coanda or What Keeps the Ball Flying

There was an insurance commercial where a woman proclaimed that if you read it on the Internet it had to be true. The more I read on the Internet the more I question the validity of anything I read but that is true for all sources of information.  The path that lead me to the research on air flow started with purchasing a simple kit at Radio Shack known as the RadioShack Turbo Air Kit, cost under $10.

Air Stream Pointed Straight Up

This project assembles very quickly and no soldering is required, what it demonstrates is flying a foam ball in a stream of air. Many people have done this same experiment with either a vacuum cleaner reversed or a hair dryer and a ping pong ball.  Once you get the ball positioned correctly in the air stream it tends to stay in the air stream even if the column of air is tilted but at a certain point the ball will fall out of the stream if tilted too far.

Air Stream Titled at an Angle

What started the confusion and the controversy is when I tried to research physics behind this. Air pressure is pushing the ball upwards with more force than the force of gravity pulling it down, I think everyone would agree on that. The physics that keeps the ball centered in the column of air is where opinions differ.

 Many people cited the "Coanda Effect" for the explanation but other people said it was because of Bernoulli's Principle.  Really trying to simplify the Coanda effect is that it deals with fluid moving around a curved surface. Bernoulli's principle correlate an increase in air velocity decreasing pressure. 

Back of Spoon in Water Stream

The classic example of demonstrating the Coanda Effect is holding the back of a spoon in a stream of water. There is an article on Wikipedia that disputes this experiment as a valid demonstration but this article is labeled as questionable. When doing this research this leads into the discussion of how an airplane wing actually works which appears to be an even bigger can of worms.

Bill Kuhl


No comments:

Post a Comment