Why do golf balls have dimples? – Oct 2011 Historical Background Before golf balls started being produced with dimples, golf players originally played with smooth balls. However, eventually players started noticing that the older balls that had more dents and scratch marks on them would fly further. Golfers were drawn towards this advantage, and so more golf balls were intentionally grooved by manufacturing companies. More research went into shaping balls to fly further and more efficiently up until today where modern day golf balls have dimples on them to have an average of 330 dimples .
One would think that the extra scratch marks would give golfer’s a disadvantage during their gameplay, but it was discovered that this is not always the case. So if dimpled golf balls do really give the edge to make them travel further distances what is the physics behind this phenomenon? So how does having dimples on the golf ball reduce drag? Firstly we have to look at the aerodynamics around golf balls, since the only force that can slow down a golf ball while in mid-air is the air flowing around the surface of it.
We call these resistive forces drag forces, and there are two kinds which inevitably slow down spherical golf balls; pressure drag and skin friction drag. Skin friction drag is the shear forces on the sphere from the air molecules. Pressure drag is a resistive force caused by turbulent airflow behind the sphere in the wake area (explained later), which is only experienced once golf ball reaches a certain speed as to create turbulence in the system. When a ball moves slowly it experiences laminar airflow and no pressure drag since the pressure distribution is symmetric – the pressure in front and behind the ball is the same.
As the ball speeds up the ball eventually experiences turbulent airflow which breaks the symmetry of pressure where the front of the speeding ball experiences higher pressure than behind, creating a drag force on the sphere. We can tell whether a sphere experiencing air flow is going through laminar airflow or turbulent airflow by looking at Reynold’s number. Reynold’s number is a dimensionless number that characterises fluid flow through a system, in this case the sphere. Reynold’s number can be calculated using the following equation.
Re= (d? V?? atm)/? Where Re is displayed as Reynold’s number, d the diameter of the sphere, V the velocity, ? atm the atmospheric density and ? the viscosity. As the speed of the ball increases so does Reynold’s number. At high Reynold’s number of around 100000 is where the dimpled ball experiences less overall drag compared with the smooth ball. However, as not shown in the figure, at low Reynold’s number (relatable with lower speeds) the dimpled ball experiences a greater overall drag than the smooth ball.
This, along with the Magnus effect,explainswhy long golf shots land straight from the sky almost perpendicular to the ground. The idea behind the dimpled ball having a less overall drag force than the smooth ball during high Reynold’s number is the turbulent airflow produced near the surface of the dimpled ball, also known as the boundary layer. The turbulent boundary layer allows for rapid mixing of air within itself and also the nearby laminar flow. This in a sense drags air toward the ball and allows for the separation point to be further behind the ball, creating a smaller wake.
The wake is a trail of air behind the ball, and the smaller wake results in a smaller pressure drag as shown in Figure 4. Even though the dimpled ball has more skin friction drag due to the separation point being further down the ball, the pressure drag more than makes up for it resulting in less overall drag on the ball. The effect of replacing a laminar boundary layer with a turbulent one is huge; it can mean the difference between a golf drive of 70 yards and one of 250 yards!  Conclusion Golf balls have dimples to reduce the overall drag on them.
The resultant turbulent boundary layers and turbulent airflow produces a small enough wake to significantly reduce the pressure drag. It is relevant to have dimples on golf balls since they travel at higher speeds with high Reynold’s number. This allows dimpled golf ball to go further than smooth balls. References  Number of dimples on a golf ball, http://www. golfblogger. com/index. php/golf/comments/purpose_and_number_of_golf_ball_dimples/  Separation of boundary layers, http://www. see. ed. ac. k/~johnc/teaching/fluidmechanics4/2003-04/fluids14/separation. html  Fluid Dynamics, http://electron9. phys. utk. edu/phys136d/modules/m1/bernoulli. htm  Aerospaceweb. org – Ask Us Golf Ball Dimples and Drag http://www. aerospaceweb. org/question/aerodynamics/q0215. shtml  Why are golf balls dimpled? – http://math. ucr. edu/home/baez/physics/General/golf. html  Classic Moth Boat Association (Figure 1) – http://www. mothboat. com/Building/waxon. html  Louis A. Bloomfield, How Things Work: the physics of everyday life, Wiley. 4th Ed. , 2010.
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