Nearcancellation of up and downgradient momentum transport in forced magnetized shearflow turbulence
Abstract
Viscoresistive magnetohydrodynamic turbulence, driven by a twodimensional unstable shear layer that is maintained by an imposed body force, is examined by decomposing it into dissipationless linear eigenmodes of the initial profiles. The downgradient momentum flux, as expected, originates from the largescale instability. However, continual upgradient momentum transport by largescale linearly stable but nonlinearly excited eigenmodes is identified and found to nearly cancel the downgradient transport by unstable modes. The stable modes effectuate this by depleting the largescale turbulent fluctuations via energy transfer to the mean flow. This establishes a physical mechanism underlying the longknown observation that coherent vortices formed from nonlinear saturation of the instability reduce turbulent transport and fluctuations, as such vortices are composed of both the stable and unstable modes, which are nearly equal in their amplitudes. The impact of magnetic fields on the nonlinearly excited stable modes is then quantified. Even when imposing a strong magnetic field that almost completely suppresses the instability, the upgradient transport by the stable modes is at least twothirds of the downgradient transport by the unstable modes, whereas for weaker fields, this fraction reaches up to 98%. These effects are persistent with variations in magnetic Prandtl number and forcing strength. Finally, continuum modes are shown to be energetically less important, but essential for capturing the magnetic fluctuations and Maxwell stress. A simple analytical scaling law is derived for their saturated turbulent amplitudes. It predicts the falloff rate as the inverse of the Fourier wavenumber, a property which is confirmed in numerical simulations.
 Publication:

Physics of Plasmas
 Pub Date:
 September 2022
 DOI:
 10.1063/5.0101434
 arXiv:
 arXiv:2208.03342
 Bibcode:
 2022PhPl...29i2301T
 Keywords:

 Physics  Fluid Dynamics;
 Astrophysics  Solar and Stellar Astrophysics;
 Physics  Computational Physics;
 Physics  Plasma Physics
 EPrint:
 Accepted for publication in Physics of Plasmas