Zonalization and Induction of 3-D Kelvin–Helmholtz-Instability-Driven Turbulent Dynamo

A new mechanism of large-scale dynamo in shear-flow turbulence has been identified, whose critical steps of dynamo cycle are shown here. The 3-D instability of x-directed shear flow U_x(z) induces in the early nonlinear phase strong x-invariant zonal jets, which, due to their favorable cross phase with magnetic fluctuations, slowly generate strong z-reversed mean magnetic field. Such x-aligned field resists orthogonal fluid motions. Thus-impacted flow instability modulates the jet-field cross phase, regulating the cyclic dynamo. A reduced predictive model of mean-field dynamo incorporates dominant field-stretching by single-wavenumber (k_y) zonal jets, against subdominant magnetic-flux cancellation due to single-k_y turbulent flux transport across z. Full-scale simulations of small- (large-) Pm dynamos display more (less) frequent reversals of the mean-field polarities, as weaker fields with small-Pm allow rapid changes in dynamo cycle. An Rm²-scaling of large-scale magnetic energy observed in simulations is explained by a simple analytical model where field-stretching by zonal jets occurs coherently over a resistive diffusion time. Though z-directed flow perturbations are sensitive to Pm, zonal jets are largely unaffected by variations in Re and Rm, thus driving a robust large-scale dynamo. Variations in Alfvenic Mach number and initial mean-field orientation show that the zonal-jet dynamo is universal. The dynamo is also examined in cylindrical geometry, relevant for accretion flows.