Traditional golf as known today started being played in the 15th century by fancy Scots in their country houses. Mary Queen of Scots, among many others, was one of the early devotees to this recreation, which started out as simple wood balls aimed at holes and kicked with branches.
Early on, sharp golfers started noticing how golf balls which were beaten up with bumps and slices seemed to fly further than regular, new ones. Suddenly, playing with worn out balls on the golf course became the norm.
Eventually, with the growing number of smart people who devoted themselves to this fascinating thing called aerodynamics, some insight into the mystery began to arise.
The outcome is today known worldwide: golf balls with dimples have been the state of the art design for quite some years.
Yet…
Why do rotten spheres fly longer in the air than perfectly-shaped ones?
To understand the science behind this, let’s learn some theory about the interaction between flows and solid objects.
Whenever a flow like air finds a surface, it has a tendency to stick to it due to its viscosity. Moving air will slow down as it becomes near a wall, whilst the closest particles to it can be considered to have zero velocity.
This velocity gradient creates a boundary layer.
There are two types of flow around an object: laminar and turbulent - and their characterization is a function of flow speed and the object’s characteristic length. For example, if we’re talking spheres, the important aspect would be its diameter.
Physically, what decides if a flow is laminar or turbulent is the ratio between the inertial forces and the viscous forces acting on the flow; if the flow energy is way higher than the viscous shear near the solid object, we usually have a turbulent flow — if not, it’s probably laminar.
Meaning: the stronger the flow momentum, the stronger the turbulence and most likely it is to have a turbulent boundary layer.
The numerical quantity which measures this ratio is called Reynolds number.
And how does this relate to golf balls?
The Smooth Ball
Take a look at normal airflow around a sphere:
Near the sphere, a laminar boundary layer starts to form, and the flow momentum will dissipate into viscous shear, resulting in a less energized flow.
The result of this energy loss is that air won’t be able to stay attached to the sphere much longer.
And guess what? Vortex shedding starts to happen on the dark side of the moon, meaning your trailing flow will be a giant, turbulent mess.
Looking at the forces which act upon a golf ball:
The messy, great wake region will create a big low pressure area behind the ball; and guess what happens due to the increased low pressure region?
Exactly, drag happens. And drag is the one force you do not need to win a championship.
Now, get in those dimples.
As the moving ball finds air, a laminar boundary layer starts to form, just like in the smooth ball.
But due to multiple dimples, as the airflow follows the solid surface, its turbulence will start to increase thanks to the consecutive obstacles and ups and downs. Slowly, the boundary layer becomes more energized and transitions to a turbulent regime.
The positive effect of this turbulent surface flow is the delayed wake separation when compared to the smooth ball, meaning less area of low pressure and therefore less drag.
It is said that separation happens around 110º from the tangent flight direction and that drag is around half as much as with a ball without dimples!
Extra facts
- Golf balls have around 350–500 dimples which vary in size, pattern, depth and shape;
- You can’t carve dimples in every moving object because it will only help if the drag force is mainly generated due to wake separation;
- Also, production costs and design play a great role. No one would buy a car full of delicately shaped dimples;
- It is extremely difficult to correctly simulate and predict a golf ball trajectory using Computational Fluid Dynamics. The direction, velocity and spin of a golf ball are determined by the split interaction the ball has with the club when it’s struck — this interaction usually lasts less than a millisecond;
- Until today, there is no perfect dimple pattern nor shape due to the computational complexity of the model in hands.
Now you know why your golf balls have dimples!
Keep curious!
