Diffusion and the formation of vorticity staircases in randomly strained two-dimensional vortices

Turner, M.R., Bassom, A.P. and Gilbert, A.D. (2009) Diffusion and the formation of vorticity staircases in randomly strained two-dimensional vortices Journal of Fluid Mechanics, 638. pp. 49-72. ISSN 0022-1120

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Abstract

The spreading and diffusion of two-dimensional vortices subject to weak external random strain fields is examined. The response to such a field of given angular frequency depends on the profile of the vortex and can be calculated numerically. An effective diffusivity can be determined as a function of radius and may be used to evolve the profile over a long time scale, using a diffusion equation that is both nonlinear and non-local. This equation, containing an additional smoothing parameter, is simulated starting with a Gaussian vortex. Fine scale steps in the vorticity profile develop at the periphery of the vortex and these form a vorticity staircase. The effective diffusivity is high in the steps where the vorticity gradient is low: between the steps are barriers characterized by low effective diffusivity and high vorticity gradient. The steps then merge before the vorticity is finally swept out and this leaves a vortex with a compact core and a sharp edge. There is also an increase in the effective diffusion within an encircling surf zone. In order to understand the properties of the evolution of the Gaussian vortex, an asymptotic model first proposed by Balmforth, Llewellyn Smith and Young (J. Fluid Mech., vol. 426, 2001, p. 95) is employed. The model is based on a vorticity distribution that consists of a compact vortex core surrounded by a skirt of relatively weak vorticity. Again simulations show the formation of fine scale vorticity steps within the skirt, followed by merger. The diffusion equation we develop has a tendency to generate vorticity steps on arbitrarily fine scales; these are limited in our numerical simulations by smoothing the effective diffusivity over small spatial scales.

Item Type: Journal article
Additional Information: © Cambridge University Press, 2009
Subjects: G000 Computing and Mathematical Sciences > G100 Mathematics
H000 Engineering
DOI (a stable link to the resource): 10.1017/S0022112009990875
Faculties: Faculty of Science and Engineering > School of Computing, Engineering and Mathematics > Engineering and Product Design Research > Automotive Engineering
Depositing User: Dr Matthew Turner
Date Deposited: 26 Nov 2009
Last Modified: 21 May 2014 11:01
URI: http://eprints.brighton.ac.uk/id/eprint/6475

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