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8/20/2019 Mechanism of Drag Reduction by Golf 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
P P
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 [D’Alembert’s
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
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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
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Definitions
Drag CoefficientReynolds Number
D= Drag Force
A=Cross Sectional Areaρ= Density
Uo= Free Stream Velocity
d= sphere diameter
Uo= Free Stream Velocityν= 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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Measured Drag Coefficient
Drag Coefficient constant for Re≥ 0.9x105
(Choi J, Jeon WP, Choia H)
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Effect of Dimples
(Choi J, Jeon WP, Choia H)
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Smoke Wire Test
Shows no vortices are ejected
(Choi J, Jeon WP, Choia H)
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Velocity Profile at Re=1.0x105
(Choi J, Jeon WP, Choia H)
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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> 0.9x105 flow always separates from the surface after dimple V
(Choi J, Jeon WP, Choia H)
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Conclusion
(Choi J, Jeon WP, 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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Questions?