Convective generation of a vortex street in plasma shear flow

Abstract

By numerical calculation, it is shown that vortices are sequentially generated on the background charged layer forming parallel shear flows in a two-dimensional low beta incompressible plasma when the charged layer is locally disturbed by a small amount of additional charge. Numerical experiments are performed in a rectangular domain with 0<x<Lx and −Ly/2<y<Ly/2 and the background charged layers are distributed to form the plasma flow U in the x direction with a velocity shear of ∂U/∂y≠0. The boundaries at x=0 and Lx are to supply plasmas identical to the background plasma initially loaded inside the domain. The plasma particles reaching the boundaries are freely allowed to leave the experimental domain. A vortex (named the mother vortex) first develops the local charge disturbance initially given, and subsequently new vortices (named the daughter vortices) are generated on the charged layer. This can be thought of as a result of propagation of the initial disturbance: A relative plasma flow as seen from the mother vortex can carry downstream the disturbances originating from the vortex, which can trigger growth of the daughter vortex. The numerical experiments in our nonperiodic system show that the intervals between the vortices are relatively close to the wavelength of the most unstable mode predicted by the linear analysis for periodic vortices. It is also shown that the temporal growth rate γ of the vortex in its early growing period is comparable to the corresponding linear growth rate of the unstable periodic vortices with the same wavelength.