Why Do Golf Balls Have Dimples? The Answer is in the Aerodynamics
A conversation in the early 1900s:
Tom: Jerry, I’m entering the golf course. Feel free to join me…
I should select the smoothest possible ball from the basket. Oops! There is only one unused left. Let me use the last one.
Jerry: That’s so mean! I don’t have a choice.
Everybody prefers to play with a brand new ball. The notion is that smoother balls have less resistance to travel through air, hence travels a longer distance. In the end of round 1:
Tom: What just happened? In principle, the ball I hit should have traveled a longer distance, which isn’t the case.
This was the idea behind having irregularities in a golf ball, called “dimples“. Since then, a lot of research has gone into this to find out the perfect combination of the irregularities.
Yes, you read it right: perfect combination (symmetry) for the irregularity.
Dimples and Fluid Dynamics
Dimples and their impact on the golf ball traveling through the air can be described using fluid dynamics.
Basically, there are two types of fluid dynamic flows — laminar and turbulent. In general, many real life applications are turbulent in nature. This can be generally obtained from a factor called Reynolds number (fluid mechanics newbies please do a quick Google search to understand this term). In the current scenario, however, when the ball is smooth it gives rise to something close to a laminar flow. In this case, the fluid flow downstream detaches from the surface of the ball, in the form of vortices. This phenomenon is called flow separation, which gives rise to a viscous wake behind the ball that slows it down.
What do dimples do? They act as artificial turbulators, creating turbulence next to the ball surface giving rise to two layers of air going around the ball. The top layer is going faster than the bottom layer, i.e. air clings to the ball’s surface, which creates turbulence. This reduces the drag and helps the ball to travel farther than a smooth ball. Yet another new term — drag. Drag is a force component which arises as a result of a difference in velocities of a solid and fluid body, and it opposes the solid motion through the air, in this case, the golf ball. A dimpled ball probably has about only half the drag of a smooth ball. Now back to our story — the reduction of drag allows the ball to travel faster due to less resistance.
Similar to drag there is another component called Lift. Lift occurs when the fluid is turned by the solid, which creates an opposing force. If the ball spins such that it pushes the air downward then an upward lift force is experienced by the ball. One important thing to note is that this factor comes into play only when the ball is spinning. What happens if it is spinning? The spinning action makes the air pressure on the bottom of the ball higher than the air pressure on the top and this imbalance creates an upward force on the ball. Ball spin contributes to one-half of a golf ball’s lift. The other half of lift is provided by the dimples, which allow for optimization of the lift force.
A small round of discussion — when the ball spins around an axis perpendicular to the plane in which it travels, we experience lift. What if the spin is around any of the other 2 axes? That’s the world famous Roberto Carlos or Beckham rocket kick in football. The ball gets to travel in a projectile and this effect is called the MAGNUS EFFECT. A similar effect can be seen in the game of cricket — when a fast bowler tries to swing the ball.
The number of dimples on the ball is usually between 330 to 500. Golf balls are usually covered with dimples in a highly symmetrical way. The ball will wobble if it’s not symmetrical, or its flight will depend on which part of the ball is forwards or sideways as the ball spins.
Assume dimples on only one side of the ball. The ball tends to curve towards the side on which there are dimples since the wake is generated towards the side which is smooth. For example, if the ball shown in the below figure is being hit along the same viewing direction, it’ll travel towards the left.
Hope this article was useful in understanding the impact of fluid dynamics in golf and other sports. If you’d like to try a simulation by yourself, remember that you can create a free SimScale account and start an analysis in just a few minutes. It works 100% in the cloud, so no hardware on installation needed.
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Originally published at www.simscale.com on July 19, 2016.
