Linear and nonlinear decay of cat's eyes in two-dimensional vortices, and the link to Landau poles

• Published in: Journal of fluid mechanics Vol. 593; pp. 255 - 279
• Main Authors: TURNER, M. R., GILBERT, ANDREW D.
• Format: Journal Article
• Language: English
• Published: Cambridge, UK Cambridge University Press 25.12.2007
• Subjects: Convection, turbulence, diffusion. Boundary layer structure and dynamics; Decay; Earth, ocean, space; Exact sciences and technology; External geophysics; Fluid dynamics; Fluid mechanics; Fundamental areas of phenomenology (including applications); Inviscid flows with vorticity; Laminar flows; Linear equations; Meteorology; Nonlinear systems; Physics; Reynolds number; Strain; Topology; Large Reynolds number; Atmospheric dynamics; Streamlines; Non viscous fluid; Digital simulation; Vortices; Decay; Vorticity; Modelling
• ISSN: 0022-1120; 1469-7645
• DOI: 10.1017/S0022112007008944

This paper considers the evolution of smooth, two-dimensional 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 strain field is smoothly switched off, the cat's eyes may persist, or they may disappear as the vortex relaxes back to axisymmetry. A numerical study obtains criteria for the persistence of cat's eyes as a function of the strength and time scale of the imposed strain field, for a Gaussian vortex profile. In the limit of a weak external strain field and high Reynolds number, the disturbance decays exponentially, with a rate that is linked to a Landau pole of the linear inviscid problem. For stronger strain fields, but not strong enough to give persistent cat's eyes, the exponential decay of the disturbance varies: as time increases the decay slows down, because of the nonlinear feedback on the mean profile of the vortex. This is confirmed by determining the decay rate given by the Landau pole for these modified profiles. For strain fields strong enough to generate persistent cat's eyes, their location and rotation rate are determined for a range of angular velocities of the external strain field, and are again linked to Landau poles of the mean profiles, modified through nonlinear effects.