The essence of onset and self-sustenance of three-dimensional magnetohydrodynamic subcritical turbulence in plane shear astrophysical flows

Author: Mariami Kavtaradze
Co-authors: George Mamatsashvili, George Chagelishvili
Keywords: Astrophysical shear flows, magnetohydrodynamics, nonmodal growth, nonlinear transverse cascade, turbulence.
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To understand the mechanism of the self-sustenance of subcritical turbulence in spectrally stable shear flows, we performed direct numerical simulations of turbulence in 3D MHD homogeneous plane shear flow in the local shearing-box approximation with subsequent analysis of the dynamical processes in spectral/Fourier space. We show that a main nonlinear process in shear flows is transverse (angular) redistribution of harmonics in Fourier space - nonlinear transverse cascade - rather than usual direct or inverse cascades. We demonstrated that the turbulence is sustained by a subtle dynamical interplay between the linear nonmodal growth and the nonlinear transverse cascade. As it appeared, in the considered plane 3D MHD case the leading linear process energetically supplying turbulence is due to the kinematics (Reynolds stress), controversy to Keplerian flows where energy injection into perturbation harmonics is due to magnetic field (Maxwell stress). It was shown that in the turbulent regime the transverse cascade prevails over the direct one, so the perturbation harmonics, staying in, so-called, vital area, draws sufficient shear energy to ensure the maintenance of the turbulence. We propose the basic cycle of the turbulence sustenance in the considered case, which clearly show the synergy of linear and nonlinear processes in the self-organization of the flow. Performing numerical simulations for different values of the background magnetic field, we show that with the increase of the field, the onset of turbulence occurs at larger times and the power of turbulence reduces. Finally, at definite threshold background magnetic field the flow completely stabilizes.