Mechanism of Drag Reduction by Dimples on a Sphere Colin Smith ME - - PowerPoint PPT Presentation

Colin Smith ME 801 Nov 23, 2010

Mechanism of Drag Reduction by Dimples on a Sphere

Picture F.N.M. Brown

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 (Ball Speeds)

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Basics of Drag

Skin Friction: Viscous shear stresses on surface

• f the object

Form Drag: Pressure difference

• n the object

P P

F

F 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

• vercomes 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 separation

At 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 Coefficient

Reynolds Number

D= Drag Force A=Cross Sectional Area ρ= Density Uo= Free Stream Velocity d= sphere diameter Uo= Free Stream Velocity ν= 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 ϕ=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

(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)

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 at Dimple II (71O)

Re= 2.0x105

First Separation at Dimple 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

• f Fluids. Vol.18 4 041702. 2006

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

• ver 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?