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Unsteady Taylor-vortex dynamo is fast

Liam O’Connor1,2, Daniel Lecoanet1,2, Geoffrey M. Vasil3, Kyle C. Augustson4, Florentin Daniel2, Evan H. Anders5, Keaton J. Burns6,7, Jeffrey S. Oishi8, and Benjamin P. Brown9

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Phys. Rev. Research 8, 023176 – Published 18 May, 2026

DOI: https://doi.org/10.1103/62yn-rdmn

Abstract

Astrophysical and geophysical fluids commonly generate organized magnetic fields, despite having enormous magnetic Reynolds numbers Rm and abundant small-scale turbulence. Flow-induced dynamo action produces these fields, with the “kinematic dynamo problem” devoted to determining the rate at which a flow exponentially amplifies weak magnetic fields. However, previous studies on high-Rm kinematic dynamos have generated flows via imposed volumetric forcing or oscillatory boundary conditions. In this article, we investigate a system with three important attributes: realistic flow conditions, fast dynamo action (operational for Rm→∞), and a subharmonic spatiotemporal structure. We show that unsteady Taylor-vortex flow, a regime observed in laboratory experiments, gives rise to fast dynamos with timescales and length scales twice those of the flow at high Rm. By numerically integrating a Floquet system driven by periodic oscillations of Taylor vortices, we solve the kinematic dynamo problem up to Rm=3.2×106, calculating the dynamo's growth rate as a function of Rm and streamwise wave number. We find the onset of instability and compute finite-time Lyapunov exponents, which identify the regions of Lagrangian chaos required for fast dynamo action. To our knowledge, unsteady Taylor-vortex flow produces the most physically motivated fast dynamo to date.

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Phys. Rev. Research 8, 023176– Published 18 May, 2026
  • Received 12 June 2025
  • Accepted 25 February 2026
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