Thin shear layers in high reynolds number turbulence - DNS results

Using direct numerical simulation of turbulence in a periodic box driven by homogeneous forcing, with a maximum of 4096³ grid points and Taylor micro-scale Reynolds numbers R_λ up to 1131, it is shown that there is a transition in the forms of the significant, high vorticity, intermittent structures, from isolated vortices when R_λ is less than 10² to complex thin-shear layers when R_λ exceeds about 10³. Both the distance between the layers and their widths are comparable with the integral length scale. The thickness of each of the layers is of the order of the Taylor micro-scale λ. Across the layers the velocity 'jumps' are of the order of the rms velocity u_o of the whole flow. Within the significant layers, elongated vortical eddies are generated, with microscale thickness ℓ_ν ∼ 10η << λ, with associated peak values of vorticity as large as 35ω_rms and with velocity jumps as large as 3.4u_o, where η is the Kolmogorov micro scale and ω_rms the rms vorticity. The dominant vortical eddies in the layers, which are approximately parallel to the vorticity averaged over the layers, are separated by distances of order ℓ_ν. The close packing leads to high average energy dissipation inside the layer, as large as ten times the mean rate of energy dissipation over the whole flow. The interfaces of the layers act partly as a barrier to the fluctuations outside the layer. However, there is a net energy flux into the small scale eddies within the thin layers from the larger scale motions outside the layer.