Abstract Details
Abstracts
Author: Adrian E. Fraser
Requested Type: Poster
Submitted: 2017-03-17 16:37:40
Co-authors: P.W.Terry, E.G.Zweibel, M.J.Pueschel
Contact Info:
University of Wisconsin-Madison
1150 University Ave
Madison, Wisconsin 53706
USA
Abstract Text:
The Kelvin-Helmholtz (KH) instability is ubiquitous in astrophysical and fusion systems where turbulent momentum transport is of interest. The linear theory features both unstable and stable modes at the same scales, but the stable modes are traditionally ignored in theories of KH turbulence. We show that KH turbulence, in keeping with recent findings for plasma microturbulence, nonlinearly excites stable modes to a significant amplitude. This nonlinear energy transfer from unstable to stable modes is a departure from the usual understanding of instability saturation, where linear energy injection is balanced by the nonlinear cascade of energy to smaller scales. Exciting stable modes reduces how much energy is available to drive small-scale turbulence and can significantly change how fluctuations affect the mean flow profile.
Using a threshold parameter, we evaluate energy transfer to stable modes and its associated impact on turbulent amplitudes and transport, demonstrating the possibility of stable-mode-regulated KH systems. We show that the role of stable modes in saturation is as large as the Kolmogorov-like transfer to smaller scales, with stable modes reaching significant amplitudes relative to unstable modes at saturation. A quasilinear momentum transport calculation shows that stable modes may significantly affect the broadening of the shear layer. To investigate stable mode amplitudes in fully-developed turbulence, we perform shear-flow-driven gyrokinetic simulations -- which we show have growth rates identical to the associated fluid calculations -- where linearly stable shear-flow mirror modes are identified and their impact on turbulence is quantified.
This work is partially supported by U.S.~DOE Grant No.~DE-FG02-89ER53291.
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