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Measurements of lift in oscillatory flow

Published online by Cambridge University Press:  21 April 2006

G. N. Rosenthal  and
J. F. A. Sleath
G. N. Rosenthal
Affiliation:
Department of Engineering, University of Cambridge Present address: Department of Civil Engineering, University of Cape Town.
J. F. A. Sleath
Affiliation:
Department of Engineering, University of Cambridge
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Abstract

Measurements are reported of the lift on a sphere in oscillatory flow. Two different oscillating-tray rigs were used and measurements were made for a sphere at various distances from both smooth and rough beds. For the rough bed, the roughness consisted of a single close-packed layer of similar spheres glued to the surface of the tray. Both smooth and rough beds showed somewhat similar trends. At sufficiently low Reynolds numbers the lift varied smoothly during the course of the cycle with maxima near the point at which fluid velocity was greatest and minima near velocity reversal. However, at higher Reynolds numbers a secondary peak began to appear near the point of velocity reversal. It is suggested that this secondary peak may be associated with vortex formation around the sphere. When the sphere diameter D was large compared with the viscous-boundary-layer lengthscale 1/β(= (2ν/ω)½), the maximum value of CL during the course of a half-cycle initially increased with increasing Reynolds number, reached a maximum, and then declined steadily. The curves were similar for smaller βD-values except that the tests did not extend to low enough Reynolds numbers to show the initial rise. For the smaller values of βD the lift record became unstable when the Keulegan–Carpenter number exceeded approximately 244. At higher βD-values it was not possible to observe any significant instability for the range of Reynolds numbers investigated.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 164 , March 1986 , pp. 449 - 467
Copyright
© 1986 Cambridge University Press

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