Dynamics of the intense vorticity structures near the turbulent/nonturbulent interface in jets

Direct numerical simulations (DNS) of turbulent planar jets are used to analyse the dynamics of the intense vorticity structures (IVS) near the turbulent/nonturbulent (T/NT) interface in jets. Deep inside the jet shear layer the characteristics of the IVS are similar to the IVS found in many other flows: the mean radius, tangential velocity, and circulation Reynolds number are $R/\eta \approx 4.6$, $u_0/u' \approx 0.8$, and $Re_{\Gamma}/Re_{\lambda}^{1/2} \approx 28$, where $u_0$, and $Re_{\lambda}$ are the root-mean-square of the velocity fluctuations and the Reynolds number based on the Taylor micro-scale, respectively. Statistics conditioned on the distance from the T/NT interface are used to analyse the effect of the T/NT interface on the dynamics of the IVS and show that the mean radius $R$, tangential velocity $u_0$ and circulation $\Gamma$ of the IVS increases as the T/NT interface is approached, while the vorticity norm $|\omega|$ stays approximately constant. Unlike the IVS deep inside the shear layer, there is a small predominance of vortex diffusion over stretching for the IVS near the T/NT interface implying that the core of these structures is not stable {\it i.e.} it will tend to grow in time.