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

Hewitt, R.E. and Hazel, A.L. and Clarke, R.J. and Denier, J.P. (2011) Unsteady flow in a rotating torus after a sudden change in rotation rate. Journal of Fluid Mechanics, 688. pp. 88-119. ISSN 1469-7645

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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.

Item Type: Article
Additional Information: © 2011 Cambridge University Press
Uncontrolled Keywords: Unsteady finite-time singularity separation boundary layer rotating torus
Subjects: MSC 2010, the AMS's Mathematics Subject Classification > 76 Fluid mechanics
PACS 2010, the AIP's Physics and Astronomy Classification Scheme > 40 ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID MECHANICS > 47 Fluid dynamics
Depositing User: Dr Richard E. Hewitt
Date Deposited: 14 Nov 2013
Last Modified: 20 Oct 2017 14:13
URI: http://eprints.maths.manchester.ac.uk/id/eprint/2044

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