Abstract Details
Abstracts
Author: Bindesh Tripathi
Requested Type: Poster
Submitted: 2024-04-12 16:08:42
Co-authors: A.E. Fraser, P.W. Terry, E.G. Zweibel, and M.J. Pueschel
Contact Info:
University of Wisconsin-Madison
1150 University Avenue
Madison, WI 53706
United States
Abstract Text:
Shear flows generate small-scale flows, which can rapidly amplify small-scale magnetic fields. Given the potency of efficient small-scale dynamo, observed large-scale fields in the Universe remain poorly understood. Using high-resolution 3D magnetohydrodynamic Dedalus turbulence simulations, we show, when turbulence is driven by an unstable shear flow, a new mechanism for large-scale field generation. Such mean fields have reversed polarity along the mean-flow-shear (z-)axis. They are slowly created via stretching of field-fluctuations at zonal wavenumbers by zonal flows whose gradient reverses along z, unlike the unidirectional mean-flow gradient. Zonal flows result from nonlinear coupling to the mean-shear-flow instability, particularly via their near-resonant coupling with stable and unstable eigenmodes of the mean flow [1]. Zonal flows tend to be suppressed when mean magnetic fields grow. We show the field-line stretching also generates zonal magnetic fields. Such fields are then advected, similar to momentum transport, across the shear layer by turbulent flow at zonal wavenumbers. This process generally depletes the reversed mean-field profile that slowly dissipates resistively. As zonal flows become dominant, they restart the dynamo; depending on the correlation between the zonal flows and the fluctuation-fields to be stretched, we find growth of an opposite mean-field profile that alternates with time. We also build 1D reduced models, and show that only a few zonal wavenumbers are sufficient in the mean turbulent electromotive force to reproduce key features, such as the evolution of the mean magnetic-field profile, of the full-scale nonlinear 3D turbulent dynamo. Such self-regulating cyclic polarity reversals of the global dynamo are observed in astrophysics, and similar current-profile adjustments are seen in sawtooth oscillations in fusion experiments.
[1] Tripathi et al., Phys. Fluids 35, 105151 (2023).
*Supported by NSF/DOE Grant No. DE-SC0022257.
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