Dynamics of Edge Shear Layer Collapse and the Density Limit

Density limit phenomenology has been associated with the collapse of edge shear layers at high density. Theoretical work has suggested that the onset of such collapse can occur when adiabaticity α  drops below α ≈ 1. Here, we explored shear flow dynamics in a spatially varying density profile in a channel flow configuration. The gradient in adiabaticity triggers the formation of a barrier shear layer, which separates the region of isotropic turbulence from a zonal flow. The barrier is pinned to the location of α_crit and does not propagate. We  observe that this spontaneously generated shear layer forms for α = α_cr, and disappears when α < α_cr, throughout the domain. This behavior is suggestive of that observed at the density limit, when high edge density forces a drop in the edge layer value of α. The intensity, flux, and zonal profiles are calculated. Inhomogeneous mixing of density is observed, suggesting the development of an E × B staircase in the edge layer. Emphasis will be on neutral drag effects, but we will also explore neutral entrainment and its impact on transport.

More generally, we report on some interesting differences between zonal flow phenomena in the oft-used doubly periodic box, and in channel flows.