Scaling of the turbulent/non-turbulent interface in boundary layers

Scaling of the interface that demarcates a turbulent boundary layer from the non-turbulent free stream is sought using theoretical reasoning and experimental evidence in a zero-pressure-gradient boundary layer. The data-analysis, utilising particle image velocimetry (PIV) measurements at four different Reynolds numbers (delta u(tau)/v = 1200-14 500), indicates the presence of a viscosity dominated interface at all Reynolds numbers. It is found that the mean normal velocity across the interface and the tangential velocity jump scale with the skin-friction velocity u(tau) and are approximately u(tau)/10 and u(tau), respectively. The width of the superlayer is characterised by the local vorticity thickness delta(omega) and scales with the viscous length scale v/u(tau). An order of magnitude analysis of the tangential momentum balance within the superlayer suggests that the turbulent motions also scale with inner velocity and length scales u(tau) and v/u(tau), respectively. The influence of the wall on the dynamics in the superlayer is considered via Townsend's similarity hypothesis, which can be extended to account for the viscous influence at the turbulent/non-turbulent interface. Similar to a turbulent far-wake the turbulent motions in the superlayer are of the same order as the mean velocity deficit, which lends to a physical explanation for the emergence of the wake profile in the outer part of the boundary layer.