Characteristics of the viscous superlayer in shear free turbulence and in planar turbulent jets

Direct numerical simulations of a planar jet and of shear free turbulence at Reλ = 115–140 using very fine resolutions allow the first direct identification and characterisation of the viscous superlayer (VSL) that exists at the edges of mixing layers, wakes, jets, and boundary layers, adjacent to the turbulent/non-turbulent interface. For both flows the VSL is continuous with higher local thicknesses forming near the larger intense vorticity structures. The mean thickness of the VSL is of the order of the Kolmogorov micro-scale and agrees well with an estimate based on the Burgers vortex model.