Abstract Details
Abstracts
Author: D. Gogichaishvili
Requested Type: Poster
Submitted: 2017-03-17 17:08:30
Co-authors: G. Mamatsashvili,W. Horton, G. Chagelishvili G. Bodo
Contact Info:
University of Texas at Austin
RLM 11.222 Univ of Texas
Austin, TX 78712
United States
Abstract Text:
Nonuniform flows are ubiquitous both in nature and in the laboratory: they occur in atmospheres, oceans, stars, galaxies, pipe flows, and tokamak reactors etc. The problem of the onset and self-sustenance of turbulence in spectrally stable nonuniform/shear flows is a challenge to fluid/plasma dynamics research, since in such flows perturbations (of certain spatial characteristics) undergo only linear transient growth leading to short perturbation life times. The imperfect linear growth must be compensated by the nonlinear positive feedback to repopulates the transiently growing perturbations. Subtle interplay of the linear transient and nonlinear processes can self-organize (chaotic or coherent) perturbations and ensure their self-sustenance. In spectrally stable shear flows, the transient growth is strongly anisotropic in spectral (k-)space. This, in turn, leads to anisotropy of nonlinear processes in k-space. Specifically, as revealed in HD and MHD smooth shear flows, the main nonlinear process is not a direct or inverse, but rather, so-called, nonlinear transverse cascade, that is angular (transverse) redistribution of perturbation harmonics in k-space. It is noteworthy that the central, small wavenumber area of k-space is crucial in the self-sustenance process and is called the vital area.
Our goal is the description of the interplay of the linear transient growth and the nonlinear transverse cascade in the vital area that self-organizes perturbations. The self-organization is quite simple for 2D HD and MHD plane shear flows and permits a vivid schematization. The turbulence driven by the magnetorotational instability in a vertically stratified Keplerian disk flow threaded by a nonzero net azimuthal magnetic field using local shearing box simulation. In the LAPD, Helimak and mirror machine geometries a strong radial electric fifield, Er(r,t), induces an ExB plasma sheared rotation. In these lab plasmas a sustained subcritical turbulence is measured.
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