Abstract Details
Abstracts
Author: Adrian E. Fraser
Requested Type: Pre-Selected Invited
Submitted: 2019-02-22 12:28:58
Co-authors: M.J.Pueschel, P.W.Terry, E.G.Zweibel
Contact Info:
University of Wisconsin-Madison
1150 University Ave
Madison, WI 53706
USA
Abstract Text:
Reduced transport models are of interest in shear-flow turbulence, particularly for quick predictive characterization of discharges where strongly sheared zonal flows may become unstable. Here, we present a series of investigations into how shear-flow instabilities saturate and drive turbulence. Particularly, we show that large-scale, linearly stable (damped) modes play an important role, and that properly accounting for them can significantly improve reduced transport models. Unlike previous work in systems that were gyroradius-scale or quasi-homogeneous, this work is an investigation into stable modes in a macroscopic, fully inhomogenous instability.
Like other systems, stable modes in shear flows decay in the linear regime – and thus are generally neglected in reduced models – but can be shown to be nonlinearly driven to large amplitudes in the saturated state [Fraser et al. PoP (2017)]. Here, the modes are inviscid, and the linear decay corresponds to reversible energy transfer to the mean flow, reflected in their significant modifications to the Reynolds stress. These findings are consistent with gyrokinetic simulations of a driven, shear-unstable flow, where the amplitudes of stable and unstable modes are nearly equal in the turbulent state [Fraser et al. PoP (2018)]. The relative amplitudes can be controlled by a large-scale damping term that is observed to preferentially suppress stable modes over unstable ones. By comparing regimes with and without significant stable mode activity, it is shown that stable modes are a crucial ingredient in reduced models of Reynolds stress when they are present. Ongoing work towards modeling a shear layer with a flow-aligned magnetic field, where stable mode activity is expected to depend on the equilibrium field, will also be presented.
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