Ball Dimples

Tiger Woods knows about drag during a drive. Once off the tee, a golf ball immediately starts to slow down. It's aerodynamic. We deal with it by making golf balls that have dimples.

Please consider the following: Well hit golf balls go about 200 kilometers per hour (120 miles per hour). That's fast. And, at that airspeed, sticky air slows a ball down a lot. That's right, air is sticking to the ball, and that can create a bumpy ride. Think about this: when a ball falls in a water hazard, it gets wet, right? So, when a ball is streaming through the air, it gets wet with air. We say it has a "wetted surface."

 Because air is sticking to the surface it streams over, it makes sense that the less area you have on a ball, the less sticking you'd get and the less drag you'd have to overcome. So, you might think that a very smooth, air-hardly-sticks-to-it-at-all coating on a polished sphere would travel farther down range than a ball with a roughed up lemon or lime skin surface. But, at the airspeeds that golf balls go, it doesn't quite work that way.

Imagine yourself as a tiny entity, smaller than a gnat, perhaps the size of a micro crustacean -- so small you could fit between grains of sand on a beach. Now imagine riding on the surface of a golf ball in flight, or in the wind tunnel at the lab. You realize that right at the surface of the ball, the air is still and it sticks to the plastic as the air molecules are dragged along like syrup running from the rim on a little pitcher at a popular pancake breakfast place.

But as you swim up away from the surface, say as far as the thickness of three sheets of paper, you notice that the air is going full speed. You're in, what we in golf science call, the "free stream" which moves at 200 kph. (You may have to ride a few flights to collect all the data we're covering here. That's of course after you get that micro entity thing worked out.) Anyway, the air streaming over a golf ball forms a "boundary layer" of relatively slow moving air. It's distinct. Right at the surface, the air is stuck. A millimeter away from the surface, the air is going full blast. In between -- in the boundary layer -- it's just slurping along.

So here's the deal. That slow moving air in the boundary layer is a drag. Uh, I mean, it's a source of drag. It lets the air stick to the surface and tumble behind the ball in wild whipping whirlpools.

The energy in the boundary layer is lost energy. The tumbling air behind the ball allows a large (relatively large, it's just a golf ball) region of low pressure to form, creating a partial vacuum that would suck the ball back toward the tee. So, the thinner we can make the boundary layer, the less slurpy drag we'll have, and the sooner the air behind the ball can get back up to the "free stream" speed (at least from your micro entity point of view). The less drag, the farther the ball will be driven.

Here's where the dimples do their job. Dimples make the molecules in the layer tumble. They start roiling against one another. We say the boundary layer becomes "turbulent." The molecules in the layer are no longer just sliding across the surface gently jostling. Now, they're rolling and bouncing and bumping each other along. When the molecules are in a turbulent boundary layer, they're moving closer to the free-stream speed. There is less of a difference between the speed of the tumbling molecules and the speed of the ball.

It turns out that the air flow in a turbulent boundary layer on a dimpled golf ball is thinner than a smooth or "laminar" flow on, say, a ping-pong ball. Boundary layers are laminar or turbulent, or somewhere in between. We say they're in "transition." Dimples make the transition quick-- not a smooth transition, a turbulent one, ha! (A little fluid dynamics gag there...) When the layer is turbulent and thin, the ball loses less energy to the free stream air. And, drag is lower. Isn't that weird? The dimples make the ball develop less drag. Weird, huh?

By the way, boundary-layer control is a big deal for birds and airplanes, too. Bird feathers have all manner of turbulent tumbler tines. And, airplanes have splitter plates, strakes, chines, vanes and turbulators to make the boundary layer do what we want it to. By the time the boundary layer reaches the aft end of a 777, it can be a meter thick. It's sticky stream that we have to manage.

When it comes to drag on a drive, dimples smooth things out. Golf is a game, but down deep in the dimples, it's science!