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Experimental study of flow around polygonal cylinders

Published online by Cambridge University Press:  22 December 2016

S. J. Xu ,
W. G. Zhang ,
L. Gan ,
M. G. Li  and
Y. Zhou
S. J. Xu*
Affiliation:
School of Aerospace Engineering, Tsinghua University, 100084, China
W. G. Zhang
Affiliation:
School of Aeronautics, Northwestern University, Xi’an 710072, China
L. Gan*
Affiliation:
School of Engineering and Computing Sciences, Durham University, DH1 3LE, UK
M. G. Li
Affiliation:
Department of Mechanical Engineering, The University of Melbourne, Victoria 3010, Australia
Y. Zhou
Affiliation:
Institute of Turbulence-Noise-Vibration Interaction and Control, Shenzhen Graduate School, Harbin Institute of Technology, 518055, China
*
†Email addresses for correspondence: xu_shengjin@tsinghua.edu.cn, lian.gan@durham.ac.uk
†Email addresses for correspondence: xu_shengjin@tsinghua.edu.cn, lian.gan@durham.ac.uk
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Abstract

The wake of polygonal cylinders with side number $N=2\sim \infty$ is systematically studied based on fluid force, hot-wire, particle image velocimetry and flow visualisation measurements. Each cylinder is examined for two orientations, with a flat surface or a corner leading and facing normally to the free stream. The Reynolds number $Re$ is $1.0\times 10^{4}\sim 1.0\times 10^{5}$, based on the longitudinally projected cylinder width. The time-averaged drag coefficient $C_{D}$ and fluctuating lift coefficient on these cylinders are documented, along with the characteristic properties including the Strouhal number $St$, flow separation point and angle $\unicode[STIX]{x1D703}_{s}$, wake width and critical Reynolds number $Re_{c}$ at which the transition from laminar to turbulent flow occurs. It is found that once $N$ exceeds 12, $Re_{c}$ depends on the difference between the inner diameter (tangent to the faces) and the outer diameter (connecting corners) of a polygon, the relationship being approximately given by the dependence of $Re_{c}$ on the height of the roughness elements for a circular cylinder. It is further found that $C_{D}$ versus $\unicode[STIX]{x1D709}$ or $St$ versus $\unicode[STIX]{x1D709}$ for all the tested cases collapse onto a single curve, where the angle $\unicode[STIX]{x1D709}$ is the corrected $\unicode[STIX]{x1D703}_{s}$ associated with the laterally widest point of the polygon and the separation point. Finally, the empirical correlation between $C_{D}$ and $St$ is discussed.

JFM classification

Turbulent Flows: Turbulent transition Wakes/Jets: Wakes
Type
Papers
Information
Journal of Fluid Mechanics , Volume 812 , 10 February 2017 , pp. 251 - 278
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Copyright
© 2016 Cambridge University Press 

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