Mechanism of Drag Reduction byGolf Ball Presentation

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Colin Smith

ME 801

Nov 23, 2010

Mechanism of Drag Reduction by

Dimples on a Sphere

Picture F.N.M. Brown


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10/18/2011 Colin Smith Slide 2 of 20

The Big Ideas

Previous experiments have found:Dimpled spheres to have up to 50%

reduction of drag of smooth spheres

The drag on a ball to become constant

above certain Reynolds' Numbers (BallSpeeds)


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Basics of Drag

Skin Friction:Viscous shear

stresses on surface

of the object

Form Drag:Pressure difference

on the object

PP

FF

F


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Pressure Coefficient over a Sphere

http://qm-aerospace.blogspot.com/2007/03/why-do-golf-balls-have-dimples.html

Potential flow solution predicts no

drag due to pressure [DAlemberts

Paradox] (dotted line)

When viscosity is accounted for,

separation occurs and the flow is no

longer symmetric (solid line)


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Boundary Layer Separation

Separation occurs when the pressure gradient

overcomes the momentum


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Laminar vs Turbulent Boundary Layers


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Motivation

Dimples induce a turbulent boundary layer, which has

higher momentum and thus delays separationAt Re>104, the majority of drag on a sphere is due to

pressure difference, not skin friction

http://www.sciencebuddies.org/science-fair-projects/project_ideas/Sports_p012.shtml



Dimples reduce drag on a sphere as much as

50% when compared to a smooth surface

The drag coefficient remains constant over a

range of Reynolds numbers

Turbulent boundary layer is caused by

separation bubbles in dimples

Findings



Definitions

Drag CoefficientReynolds Number

D= Drag Force

A=Cross Sectional Area= Density

Uo= Free Stream Velocity

d= sphere diameter

Uo= Free Stream Velocity= kinematic viscosity


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Experimental Setup

Tiger Woods ball speed

185mph=83m/s

Free stream velocities varied

from 5-28 m/s

Reynolds numbers 0.5x105-

2.8x105

Maintains laminar boundary

layer over smooth sphere

(Choi J, Jeon WP, Choia H)(Jeon S, Choi J, Jeon WP , Choi H, Park J)


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Flow Over a Smooth Cylinder

The separation angle over a smooth golf ball sized sphere

was measured at 82o for 0.5x105 Re 2.8x105


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Visualization of Flow Separation

Separation is delayed to

=110o

Separation angle constant for Re 0.9x105

The trailing edge of the tested sphere is smooth to better show

separation

(Choi J, Jeon WP, Choia H)


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Measured Drag Coefficient

Drag Coefficient constant for Re 0.9x105



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Effect of Dimples



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Smoke Wire Test

Shows no vortices are ejected



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Velocity Profile at Re=1.0x105



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Re=1.5x105


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Velocity Profiles at Increasing Re

Re= 1.0x105

First Separation at

Dimple III (80O)

Re= 1.5x105

First Separation atDimple II (71O)

Re= 2.0x105

First Separation atDimple I (63O)

If Re> 0.9x105flow always separates from the surface after dimple V



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Conclusion

(Choi J, JeonWP, Choia H)


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References

Choi J, Jeon WP, Choia H. Mechanism of Drag Reduction by Dimples on a Sphere. Physics

of Fluids. Vol.18 4 041702. 2006

Jeon S, Choi J, Jeon WP , Choi H, Park J, Active control of flow

over a sphere at a sub-critical Reynolds number, J. Fluid Mech. 517, 113

2004.

Olson, A. A Cure for Hooks and Slices? Asymmetric Dimple Patterns and Golf Ball flight.

2007. http://www.sciencebuddies.org/science-fair-projects/project_ideas/Sports_

p012.shtml

Scott, Jeff. Why do Golf Balls Have Dimples. 2005. http://qm-

aerospace.blogspot.com/2007/03/why-do-golf-balls-have-dimples.html


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Mechanism of Drag Reduction byGolf Ball Presentation