Centre for Geophysical and Astrophysical Fluid Dynamics
Recent Submissions

Ribbon turbulence
(AIP, 2014)We investigate the nonlinear equilibration of a twolayer quasigeostrophic ow in a channel with an initial eastward baroclinically unstable jet in the upper layer, paying particular attention to the role of bottom ... 
Thresholds for the formation of satellites in twodimensional vortices
(Cambridge University Press, 2008)This paper examines the evolution of a twodimensional vortex which initially consists of an axisymmetric monopole vortex with a perturbation of azimuthal wavenumber m = 2 added to it. If the perturbation is weak, then the ... 
The influence of periodic islands in the flow on a scalar tracer in the presence of a steady source
(American Institute of Physics, 2009)In this paper we examine the influence of periodic islands within a time periodic chaotic flow on the evolution of a scalar tracer. The passive scalar tracer is injected into the flow field by means of a steady source term. ... 
Neutral modes of a twodimensional vortex and their link to persistent cat's eyes
(American Institute of Physics, 2008)This paper considers the relaxation of a smooth twodimensional vortex to axisymmetry after the application of an instantaneous, weak external strain field. In this limit the disturbance decays exponentially in time at a ... 
Effective diffusion of scalar fields in a chaotic flow
(American Institute of Physics, 2008)The advection of a tracer field in a fluid flow can create complex scalar structures and increase the effect of weak diffusion by orders of magnitude. One tool to quantify this is to measure the flux of scalar across contour ... 
Nonlinear equilibration of a dynamo in a smooth helical flow
(Cambridge University Press, 1997)We investigate the nonlinear equilibration of magnetic fields in a smooth helical flow at large Reynolds number Re and magnetic Reynolds number Rm with Re >> Rm >> 1. We start with a smooth spiral Couette flow driven by ... 
The spiral windup of vorticity in an inviscid planar vortex
(Cambridge University Press, 1998)The relaxation of a smooth twodimensional vortex to axisymmetry, also known as `axisymmetrization', is studied asymptotically and numerically. The vortex is perturbed at t = 0 and differential rotation leads to the windup ... 
The spiral windup and dissipation of vorticity and a passive scalar in a strained planar vortex
(Cambridge University Press, 1999)The response of a Gaussian vortex to a weak timedependent external strain field is studied numerically. The cases of an impulsive strain, an on–off step function, and a continuous random strain are considered. Transfers ... 
Accelerated diffusion in the centre of a vortex
(Cambridge University Press, 2001)The spiral windup and diffusive decay of a passive scalar in circular streamlines is considered. An accelerated diffusion mechanism operates to destroy scalar fluctuations on a time scale of order P1/3 times the turnover ... 
Kinematic dynamo action in large magnetic Reynolds number flows driven by shear and convection
(Cambridge University Press, 2001)A numerical investigation is presented of kinematic dynamo action in a dynamically driven fluid flow. The model isolates basic dynamo processes relevant to field generation in the Solar tachocline. The horizontal plane ... 
The onset of thermal convection in Ekman–Couette shear flow with oblique rotation
(Cambridge University Press, 2003)The onset of convection of a Boussinesq fluid in a horizontal plane layer is studied. The system rotates with constant angular velocity Ω, which is inclined at an angle ϑ to the vertical. The layer is sheared by keeping ... 
Vortex motion in a weak background shear flow
(Cambridge University Press, 2004)A point vortex is introduced into a weak background vorticity gradient at finite Reynolds number. As the vortex spreads viscously so the background vorticity becomes wrapped around it, leading to enhanced diffusion of ... 
Nonlinear dynamo action in rotating convection and shear
(Cambridge University Press, 2005)Magnetic field amplification by the motion of an electrically conducting fluid is studied, using a rotating planelayer geometry. The fluid flow is driven by convection, and by a moving bottom boundary, which leads to an ... 
Linear and nonlinear decay of cat's eyes in twodimensional vortices, and the link to Landau poles
(Cambridge University Press, 2007)This paper considers the evolution of smooth, twodimensional vortices subject to a rotating external strain field, which generates regions of recirculating, cat's eye stream line topology within a vortex. When the external ... 
Nonlinear solutions of the amplitude equations governing fluid flow in rotating spherical geometries
(University of ExeterMathematical Sciences, 20080908)We are interested in the onset of instability of the axisymmetric flow between two concentric spherical shells that differentially rotate about a common axis in the narrowgap limit. The expected mode of instability takes ... 
Energycrossing number relations for braided magnetic fields
(American Physical Society, 1993)This Letter derives lower bounds on the energy of braided magnetic fields, based on crossing number techniques pioneered by Freedman and He. Mean crossing numbers are defined for magnetic fields inside a cylinder; for ... 
Photospheric flux density of magnetic helicity
(EDP Sciences, 2005)Several recent studies have developed the measurement of magnetic helicity flux from the time evolution of photospheric magnetograms. The total flux is computed by summing the flux density over the analyzed region. All ... 
What is the spatial distribution of magnetic helicity injected in a solar active region?
(EDP Sciences, 2006)Context. Magnetic helicity is suspected to play a key role in solar phenomena such as flares and coronal mass ejections. Several investigations have recently computed the photospheric flux of magnetic helicity in active ...