April 4-6

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Author: Bindesh Tripathi
Requested Type: Consider for Invited
Submitted: 2022-03-04 10:16:45

Co-authors: A.E. Fraser, P.W. Terry, E.G. Zweibel, and M.J. Pueschel

Contact Info:
University of Wisconsin-Madison
Thomas C Chamberlin Hall, 1150
Madison, Wisconsin   53706
United States

Abstract Text:
Owing to the ubiquity of instability-driven turbulence, a two-dimensional magnetohydrodynamic (MHD) turbulence driven by a Krook-forced unstable shear flow is examined, to study nonlinear interactions of linear eigenmodes. Continuous up-gradient momentum transport by linearly stable but nonlinearly excited modes is identified that nearly cancels the down-gradient transport by unstable modes, as opposed to earlier observations of only transient up-gradient transport in freely evolving shear layers. Impact of magnetic fields is then quantified. In the unforced shear-layer, stronger fields with larger stresses, can quasilinearly flatten the flow shear, making it stable, before affecting nonlinear interactions of the eigenmodes. Forcing the layer, a clear impact of magnetic fields on nonlinearly coupled modes is identified. The stable modes, even with the strongest magnetic field that almost shuts off the instability, are found to drive up-gradient transport that is two-thirds of down-gradient transport by the unstable modes. These results are robust with respect to variation in magnetic Prandtl number and forcing strength. In addition, one of the most extensively studied phenomena, cross-scale energy transfer via straining by shear flow, which in MHD leads to enhanced magnetic energy cascade to small scales, is significantly inhibited due to stable modes. These effects are quantified numerically via linear and nonlinear energy transfer rates, decomposed by eigenmodes, and visco-resistive dissipation rates. Visualizations of straining of magnetic field by artificially removing the large-scale stable modes show unbridled straining by unstable modes, leading to an order-of-magnitude enhancement in small-scale fluctuations and the associated dissipation. Guided by these analyses and numerical calculation of nonlinear coupling coefficients between eigenmodes, triadic interactions involving stable modes are suggested to improve quasilinear models that rely on unstable modes.