Adrian Fraser, University of Wisconsin

"Role of Stable Eigenmodes in Driven Shear-Flow Turbulence"

Abstract: In a variety of fusion and astrophysical systems, sheared flows are found that may become unstable and drive turbulence. In many cases, while the effect of physical parameters on the underlying linear instability is well-understood, their effects on the resulting turbulence are much less clear. I will discuss ongoing efforts towards understanding how turbulence driven by the Kelvin-Helmholtz (KH) instability, the canonical shear flow instability, can be understood in terms of unstable and stable (damped) eigenmodes of the linear instability. Analytical calculations show that these stable modes, universally neglected in reduced KH models, are in fact nonlinearly pumped to significant amplitudes, where they present a large-scale energy sink for turbulent fluctuations, and can alter Reynolds stress profiles. I will additionally discuss GENE simulations of a driven, KH-unstable flow, where observations of unstable and stable mode amplitudes in the fully-developed turbulent state have informed successful reduced models for how the mean flow depends on physical parameters.