Stratified shear flows in a model of turbulence-shear flow interaction

In magnetically confined plasmas there is evidence of localized regions of improved confinement. These regions are usually associated with shear flows with radial structure, and an important problem is to understand how such flows emerge. To address this problem a reaction-diffusion type model of turbulence-shear flow interaction that incorporates the mechanism of turbulence suppression by shear, and parameterizes turbulent transport as a nonlinear diffusivity is considered. The fixed points of the model correspond to the L (low confinement) and H (high confinement) modes of the system, and it is shown that for a range of parameter values the H-mode fixed point has a finite-k instability. Numerical results show that this instability leads, in the nonlinear regime, to the formation of stratified shear layers and jets in which bands of intense shear and suppressed turbulence alternate with bands of low shear and enhanced turbulence. Approximate analytical solutions of the model corresponding to high-confinement modes with radial structure are presented. (C) 2002 American Institute of Physics.