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On Lagrangian and vortex-surface fields for flows with Taylor–Green and Kida–Pelz initial conditions

Yang, Yue and Pullin, D. I. (2010) On Lagrangian and vortex-surface fields for flows with Taylor–Green and Kida–Pelz initial conditions. Journal of Fluid Mechanics, 661 . pp. 446-481. ISSN 0022-1120.

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For a strictly inviscid barotropic flow with conservative body forces, the Helmholtz vorticity theorem shows that material or Lagrangian surfaces which are vortex surfaces at time t = 0 remain so for t > 0. In this study, a systematic methodology is developed for constructing smooth scalar fields φ(x, y, z, t = 0) for Taylor–Green and Kida–Pelz velocity fields, which, at t = 0, satisfy ω·∇φ = 0. We refer to such fields as vortex-surface fields. Then, for some constant C, iso-surfaces φ = C define vortex surfaces. It is shown that, given the vorticity, our definition of a vortex-surface field admits non-uniqueness, and this is presently resolved numerically using an optimization approach. Additionally, relations between vortex-surface fields and the classical Clebsch representation are discussed for flows with zero helicity. Equations describing the evolution of vortex-surface fields are then obtained for both inviscid and viscous incompressible flows. Both uniqueness and the distinction separating the evolution of vortex-surface fields and Lagrangian fields are discussed. By tracking φ as a Lagrangian field in slightly viscous flows, we show that the well-defined evolution of Lagrangian surfaces that are initially vortex surfaces can be a good approximation to vortex surfaces at later times prior to vortex reconnection. In the evolution of such Lagrangian fields, we observe that initially blob-like vortex surfaces are progressively stretched to sheet-like shapes so that neighbouring portions approach each other, with subsequent rolling up of structures near the interface, which reveals more information on dynamics than the iso-surfaces of vorticity magnitude. The non-local geometry in the evolution is quantified by two differential geometry properties. Rolled-up local shapes are found in the Lagrangian structures that were initially vortex surfaces close to the time of vortex reconnection. It is hypothesized that this is related to the formation of the very high vorticity regions.

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Additional Information:© 2010 Cambridge University Press. Received 16 February 2010; revised 31 May 2010; accepted 1 June 2010. This work has been supported in part by the National Science Foundation under Grant DMS-0714050.
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Subject Keywords:topological fluid dynamics; turbulence theory; vortex dynamics
Record Number:CaltechAUTHORS:20101206-105702294
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Official Citation:YANG, Y. and D. I. PULLIN (2010). "On Lagrangian and vortex-surface fields for flows with Taylor?Green and Kida?Pelz initial conditions." Journal of Fluid Mechanics 661(-1): 446-481.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:21181
Deposited By: Tony Diaz
Deposited On:10 Dec 2010 22:16
Last Modified:03 Oct 2019 02:20

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