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Unsteady flow in a rotating torus after a sudden change in rotation rate

Published online by Cambridge University Press:  20 October 2011

R. E. Hewitt ,
A. L. Hazel ,
R. J. Clarke  and
J. P. Denier
R. E. Hewitt*
Affiliation:
School of Mathematics, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
A. L. Hazel
Affiliation:
School of Mathematics, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
R. J. Clarke
Affiliation:
Department of Engineering Science, University of Auckland, New Zealand
J. P. Denier
Affiliation:
School of Mathematical Sciences, University of Adelaide, Adelaide 5005, Australia
*
†Email address for correspondence: richard.e.hewitt@manchester.ac.uk
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Abstract

We consider the temporal evolution of a viscous incompressible fluid in a torus of finite curvature; a problem first investigated by Madden & Mullin (J. Fluid Mech., vol. 265, 1994, pp. 265–217). The system is initially in a state of rigid-body rotation (about the axis of rotational symmetry) and the container’s rotation rate is then changed impulsively. We describe the transient flow that is induced at small values of the Ekman number, over a time scale that is comparable to one complete rotation of the container. We show that (rotationally symmetric) eruptive singularities (of the boundary layer) occur at the inner or outer bend of the pipe for a decrease or an increase in rotation rate respectively. Moreover, on allowing for a change in direction of rotation, there is a (negative) ratio of initial-to-final rotation frequencies for which eruptive singularities can occur at both the inner and outer bend simultaneously. We also demonstrate that the flow is susceptible to a combination of axisymmetric centrifugal and non-axisymmetric inflectional instabilities. The inflectional instability arises as a consequence of the developing eruption and is shown to be in qualitative agreement with the experimental observations of Madden & Mullin (1994). Throughout our work, detailed quantitative comparisons are made between asymptotic predictions and finite- (but small-) Ekman-number Navier–Stokes computations using a finite-element method. We find that the boundary-layer results correctly capture the (finite-Ekman-number) rotationally symmetric flow and its global stability to linearised perturbations.

JFM classification

Boundary Layers: Pipe flow boundary layer
Type
Papers
Information
Journal of Fluid Mechanics , Volume 688 , 10 December 2011 , pp. 88 - 119
Copyright
Copyright © Cambridge University Press 2011

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