On the structure of the turbulent/non-turbulent interface in a fully developed spatially evolving axisymmetric wake

In this work, we numerically study the structure of the turbulent/nonturbulent (T/NT) interface in a fully developed spatially evolving axisymmetric wake by means of direct numerical simulations. There is a continuous and contorted pure shear layer (PSL) adjacent to the outer edge of the T/NT interface. The local thickness of the PSL ${\delta }_{PSL}$ exhibits a wide range of scales (from the Kolmogorov scale to the Taylor microscale) and the conditional mean thickness $〈{\delta }_{PSL}〉{}_I/{\eta }_c\approx 6$ with ${\eta }_c$ being the centerline Kolmogorov scale is the same as the viscous superlayer. In the viscous superlayer, the pure shear motions without rotation are overwhelmingly dominant. It is also demonstrated that the physics of the turbulent sublayer is closely related to the PSL with a large thickness. Another significant finding is that the time averaged area of the rotational region $〈A_R〉$, and the pure shear region $〈A_S〉$ at different streamwise locations scale with the square of the wake-width ${b_U}^2$. This study opens an avenue for a better understanding of the structures of the T/NT interface.