On vortex rings around vortices: an optimal mechanism

• Published in: Journal of fluid mechanics Vol. 578; pp. 295 - 304
• Main Authors: ANTKOWIAK, ARNAUD, BRANCHER, PIERRE
• Format: Journal Article
• Language: English
• Published: Cambridge, UK Cambridge University Press 10.05.2007; Cambridge University Press (CUP)
• Subjects: Coriolis force; Data analysis; Engineering Sciences; Exact sciences and technology; Fluid dynamics; Fluid mechanics; Fluids mechanics; Fundamental areas of phenomenology (including applications); Hydrodynamic stability; Instability of shear flows; Mechanics; Optimization; Physics; Rotational flow and vorticity; Vortex dynamics; Stability; Modal analysis; Shear flow; Digital simulation; Disturbances; Ring vortex; Modelling; Optimization; Optimal perturbation; Vortex dynamics; Nonmodal stability analysis
• ISSN: 0022-1120; 1469-7645
• DOI: 10.1017/S0022112007005198

Stable columnar vortices subject to hydrodynamic noise (e.g. turbulence) present recurrent behaviours, such as the systematic development of vortex rings at the periphery of the vortex core. This phenomenon lacks a comprehensive explanation, partly because it is not associated with an instability stricto sensu. The aim of the present paper is to identify the physical mechanism triggering this intrinsic feature of vortices using an optimal perturbation analysis as a tool of investigation. We find that the generation of vortex rings is linked to the intense and rapid amplification of specific disturbances in the form of azimuthal velocity streaks that eventually evolve into azimuthal vorticity rolls generated by the rotational part of the local Coriolis force. This evolution thus appears to follow a scenario opposite to the classical lift-up view, where rolls give rise to streaks.