Title: Another look at zonal flows: Resonance, shearing, and frictionless saturation

Abstract

We show that shear is not the exclusive parameter that represents all aspects of flow structure effects on turbulence. Rather, wave-flow resonance enters turbulence regulation, both linearly and nonlinearly. Resonance suppresses the linear instability by wave absorption. Flow shear can weaken the resonance, and thus destabilize drift waves, in contrast to the near-universal conventional shear suppression paradigm. Furthermore, consideration of wave-flow resonance resolves the long-standing problem of how zonal flows (ZFs) saturate in the limit of weak or zero frictional drag, and also determines the ZF scale. We show that resonant vorticity mixing, which conserves potential enstrophy, enables ZF saturation in the absence of drag, and so is effective at regulating the Dimits up-shift regime. Vorticity mixing is incorporated as a nonlinear, self-regulation effect in an extended 0D predator-prey model of drift-ZF turbulence. This analysis determines the saturated ZF shear and shows that the mesoscopic ZF width scales as L _ZF~f ^3/16(1-$$f$$) ^1/8ρ $$^{5/8}_s$$ $$l^{3/8}_0$$in the (relevant) adiabatic limit (i.e., τ _ckk$$2\atop{||}$$ D _||$$\gg$$1). $$f$$ is the fraction of turbulence energy coupled to ZF and l ₀ is the base state mixing length, absent ZF shears. We calculate and compare the stationary flow and turbulence level in frictionless, weakly frictional,more » and strongly frictional regimes. In the frictionless limit, the results differ significantly from conventionally quoted scalings derived for frictional regimes. To leading order, the flow is independent of turbulence intensity. The turbulence level scales as E~(γ _L/ε _c) ², which indicates the extent of the “near-marginal” regime to be γ _Lc, for the case of avalanche-induced profile variability. Here, ε _c is the rate of dissipation of potential enstrophy and γ _L is the characteristic linear growth rate of fluctuations. The implications for dynamics near marginality of the strong scaling of saturated E with γ _L are discussed.« less

Authors:

Li, J. C.  ^[1];

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Diamond, P. H. ^[1]

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1. Univ. of California, San Diego, CA (United States). Center for Astrophysics & Space Sciences (CASS)

Publication Date:

2018-04-23

Research Org.:

Univ. of California, San Diego, CA (United States)

Sponsoring Org.:

USDOE Office of Science (SC)

OSTI Identifier:

1540198

Alternate Identifier(s):

OSTI ID: 1434357

Grant/Contract Number:  

FG02-04ER54738

Resource Type:

Journal Article: Accepted Manuscript

Journal Name:

Physics of Plasmas

Additional Journal Information:

Journal Volume: 25; Journal Issue: 4; Journal ID: ISSN 1070-664X

Publisher:

American Institute of Physics (AIP)

Country of Publication:

United States

Language:

English

Subject:

70 PLASMA PHYSICS AND FUSION TECHNOLOGY; Physics