Friday, February 06, 2009
Back to our regularly scheduled programming......
with apologies to this, we will now determine why balls have dimples.
GOLF balls people. jeez. get your mind out of the gutter! Although this does get us back to the issue of drag......and also beer pong?
I'm talking aerodynamics here. Of a sphere.
The picture below is from aerospaceweb.org. I will try to translate their wonderful explanation.
Imagine if you will, a smooth round object (like a ping pong ball for instance) and the gust of air that it encounters as it tries to make it's way into a plastic cup at the other end of the table. Hey. Focus! Ok, so there are two types of flow: laminar flow and turbulent flow.
When a fluid (in this case air) flows in parallel layers without disruption then the flow is smooth or laminar. Here's a video.
When the fluid is disrupted in any way then turbulent flow around the object results.
The resistance that the ball experiences as it traverses the atmosphere to the cup (be it the pin at the 9th hole or the dixie cup at the end of the table) is not only dependent on: 1) the force exerted by the air on the front of the ball, 2) the friction of the air against the ball as the flow splits, as well as, 3) the pressure disruption/drag BEHIND the ball. By placing dimples on the ball, you disrupt the laminar flow around the ball thereby creating turbulent flow behind it. This actually decreases the drag on the ball (see the smaller area behind the ball as illustrated above). click the picture for a cool video example!
Turns out that you can actually calculate how much drag is created behind an object!!! It's called the Reynolds number (no units) that is a ratio between the inertia force and the viscous force affecting an object. Fun with fractions reminds us that if the Reynolds number is large than viscous forces around the ball are negligible (ie small denominator = big fraction). If the Reynolds number is small, then the viscosity of the fluid around the object has a more significant effect. Dimpling makes the Reynolds number DECREASE. (would this make a great exam question or what!!??)
So, we have learned that the dimpling makes the flow around the ball turbulent, thereby decreasing the drag. But--get this--changing the shape and configuration of the dimple also affects the Reynolds number. That's why the "best" golf ball has hexagonal or pentagonal dimples rather than circular ones. No matter the size or shape of the dimples on your balls, you will still need special skills to rescue you from this:
Now, I would be remiss if I didn't mention the OTHER force on a golf ball---LIFT.
But we actually already covered the importance of lift in a previous SSF. (You're welcome).
So, with that dear reader, I will leave it to you to field test the lift and drag on your balls......