The Wind Tunnel (Page 1)

We wanted to show you the aerodynamic flow around a tennis ball and compare that to the flow around a smooth ball. How does the fuzz on the ball affect its flight?

If you like, you can bypass the next few pages of details and explanations and go right to viewing the wind tunnel movies or read the background on the test before proceeding onto the results. We've also posted the calculations and materials that we submitted to NASA to prepare for the wind tunnel test. You can follow the link to them from here, if you like.

We have a small sample of the calculations we did to prepare for the wind tunnel test. We'll refer to it as we explain how the test was done and the results.

We chose an 11 inch diameter Wilson novelty tennis ball and a child's 9 inch diameter beach ball. Some of you might ask "Isn't comparing two different size balls like comparing apples to oranges? Isn't varying the size and surface cover of the two balls too many changes at once".

The answer is "no". The reason deals with one of the most important concepts in aerodynamics. In 1882, Osborne Reynolds, a British engineer conducted experiments on pipe flow. He determined a mathematical relationship between density, viscosity, velocity and length. This relationship is a parameter now called the Reynolds number. What Reynolds discovered was that two similarly shaped but different sized objects (like a Boeing 747 and a scale model of a Boeing 747) would have the same aerodynamics as long as the Reynolds numbers for the two objects matched.

Let's start by calculating the Reynolds number (Re) of a standard size tennis ball.