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On the origin of the drag force on dimpled spheres

Published online by Cambridge University Press:  20 September 2019

Nikolaos Beratlis Open the ORCID record for Nikolaos Beratlis [Opens in a new window] ,
Elias Balaras Open the ORCID record for Elias Balaras [Opens in a new window]  and
Kyle Squires Open the ORCID record for Kyle Squires [Opens in a new window]
Nikolaos Beratlis
Affiliation:
School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, AZ 85281, USA
Elias Balaras*
Affiliation:
Department of Aerospace and Mechanical Engineering, The George Washington University, Washington, DC 20052, USA
Kyle Squires
Affiliation:
School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, AZ 85281, USA
*
†Email address for correspondence: balaras@gwu.edu
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Abstract

It is well established that dimples accelerate the drag crisis on a sphere. The result of the early drag crisis is a reduction of the drag coefficient by more than a factor of two when compared to a smooth sphere at the same Reynolds number. However, when the drag coefficients for smooth and dimpled spheres in the post-critical regime are compared, the latter is higher by a factor of two to three. To understand the origin of this behaviour, we conducted direct numerical simulations of the flow around a dimpled sphere, which is similar to commercially available golf balls, in the post-critical regime. By comparing the results to those for a smooth sphere, it is found that dimples, although effective in accelerating the drag crisis, impose a local drag penalty, which contributes significantly to the overall drag force. This finding challenges the broadly accepted view that dimples only indirectly affect the drag force on a sphere by energizing the near-wall flow and delaying global separation.

JFM classification

Boundary Layers: Boundary layer separation Flow Control: Drag reduction
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 879 , 25 November 2019 , pp. 147 - 167
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Copyright
© 2019 Cambridge University Press 

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