Why are Golf Balls Dimpled

Mitch BlancMatt Smith

ME 41212/10/15

Definition of the Problem

How does adding dimples to a golf ball affect its flight? What percentage does the drag decrease by adding dimples?

Types of Drag

Friction Drag Arises from the friction of the fluid against the "skin" of the object that is

moving through it. Part of drag that is due to shear stress on the object. Ex) Trains, Airplanes, Objects with high length to width ratios

Pressure Drag Part of the drag that is due directly to the pressure on an object. This drag

strongly depends on the shape or form of the object. Ex) Cars, Balls, Blunt Bodies

The dominant drag force on a golf ball is due to pressure drag.

Flow Separation

Boundary layer separation is the detachment of the flow of the fluid near the surface into a broader wake. Boundary layer separation occurs when the flow closest to the wall or leading edge reverses in direction. The separation point is defined as the point between the forward and backward flow, where the shear stress is zero.

When the flow separates, it modifies the outside potential flow and pressure field. In the case of golf balls, the pressure field modification results in an increase in pressure drag.

CFD- Smooth Ball

- Flow is Laminar around the ball. - Simulation shows the large flow separation in the wake (the more blue

region behind the ball). - The early flow separation causes a large amount of drag on the smooth ball.- Bigger pocket of low pressure air behind the ball that creates an adverse

pressure gradient

Separation Point

CFD- Golf Ball

- The dimples trip the flow to be turbulent at a lower Reynold’s numbers than in a smooth ball.

- The separation occurs much later in the golf ball creating a much narrower wake and resulting in less drag.

- The reason for the later separation has to due with the turbulence in the boundary layer. - The boundary layer momentum of a turbulent flow is greater than in the laminar flow of a

smooth golf ball allowing for the flow to remain attached longer.

Separation Point

Wind Tunnel- Smooth Ball𝑅𝑒=

𝜌𝑉𝑙𝜇

𝐶𝑑=𝐹𝐷

.5 𝜌𝑉 2𝐴

We matched the Reynolds Number and then measured the drag force to find

Wind Tunnel- Golf Ball𝑅𝑒=

𝜌𝑉𝑙𝜇

𝐶𝑑=𝐹𝐷

.5 𝜌𝑉 2𝐴

We matched the Reynolds Number and then measured the drag force to find

Coefficient of Drag Values @ Re= 2E5 Smooth Ball

CFD- .515 Wind Tunnel- 0.491 Book- .510

Golf Ball CFD- .316 Wind Tunnel- .303 Book- .250

Comparison

Green- CFDGray- Wind Tunnel

Results

A golf ball has 43% less drag as a smooth ball. Thus the range of speed in which a golf ball travels, the drag is significantly lower for a golf ball compared to a smooth ball.

A professional golfer can hit a golf ball up to twice as far as a smooth ball. Even though there is an increase in surface friction due to the dimples, it

decreases the wake size because of later flow separation allowing for a decrease in pressure drag. The decrease in pressure drag is much greater than the increase in friction resulting in a lower drag coefficient.

Dimples cause the flow to become turbulent which decreases the overall drag force for the given range of Reynolds Numbers.

The pocket of low pressure air behind the ball is much smaller for a dimpled ball thus there is less drag.

Errors in result due to different dimple patterns for golf balls

References

Munson, B., & Okiishi, T. (2013). Fundamentals of fluid mechanics (7th ed.). Hoboken, NJ: John Wiley & Sons.

(n.d.). Retrieved November 13 , 2015, from https://en.wikipedia.org/wiki/Flow_separation

Naruo, T., & Mizota, T. (n.d.). The Influence of Golf Ball Dimples on Aerodynamic Characteristics. Procedia Engineering, 780-785.