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Spin-down to rest in a cylindrical cavity

Published online by Cambridge University Press:  26 April 2006

Ö. Savaş
Ö. Savaş
Affiliation:
School of Aerospace and Mechanical Engineering, The University of Oklahoma, Norman, OK 73019, USA Permanent address: Department of Mechanical Engineering, University of California, Berkeley, CA 94720, USA.
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Abstract

The nature of the flow during impulsive spin-down to rest in a cylindrical cavity is studied experimentally. Flow visualization using reflective flakes and laser-Doppler velocimetry are the tools of this investigation. The velocimeter is configured to measure simultaneously the azimuthal velocity component at two arbitrarily separated locations within the cylinder. The Ekman number is about 10−5 and the flow is unstable. The mean angular velocity decreases non-uniformly and monotonically. The velocity fluctuation amplitudes and frequencies decrease steadily. A novel data analysis is used to study the velocity fluctuations, which are neither stationary nor uniform. The assumptions of this analysis are the validity of Taylor's hypothesis of frozen-eddy transport and the ergodicity of the process following that rescaling. The fluctuations are equally dominant during all phases of the spin-down process when scaled with the current mean velocity. The root-mean-squared intensity measurements in the core (r/R < 0.4) suggest an r−1 dependence while a uniform value is observed in the buffer region (0.4 < r/R < 0.8). Flow visualizations and spatial velocity correlations indicate that the flow in the core consists of vortices having axes parallel to the rotation axis and extending throughout the height of the cylinder. The power spectra of the velocity fluctuations, after amplitude scaling with the current mean velocity and Taylor's scaling in time, suggest a –2.6 power dependence on the wavenumber k. The flow in the latter phases tends to a single vortex.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 234 , January 1992 , pp. 529 - 552
Copyright
© 1992 Cambridge University Press

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