Passive scalar dynamics near the turbulent/nonturbulent interface in a jet

The present work uses several direct numerical simulations (DNS) of turbulent planar jets at Reynolds number ranging from $Re_\lambda=120$ to $Re_\lambda=160$ and Schmidt numbers raging from $Sc=0.7$ to $7.0$ to analyze the nature and properties of the \textit{''scalar interface''} and to investigate the dynamics of turbulent mixing of a passive scalar. Specifically, we employ conditional statistics in relation to the distance from the T/NT interface in order to eliminate the intermittency that affects common turbulence statistics close to the jet edge. The physical mechanisms behind scalar mixing near the T/NT interfaces and their associated turbulent scales and topology are investigated. A sharp scalar interface exists separating the Turbulent and the irrotational flow regions. The thickness of this scalar interface $\delta_{\theta}$ is also of the order of the Taylor micro-scale, $\lambda$. However, the thickness of the scalar gradient variance $\left< \theta^2 \right>_I$ (where $G_j = \partial \theta/\partial x_j$) is much smaller. Very intense scalar gradient sheet structures along regions of intense strain, in particular at the T/NT interface. The scalar gradient transport equation is analyzed in order to further investigate the physical mechanism of scalar turbulent mixing at the jet edge. Almost all mixing takes place in a confined region close to the interface, beyond which they become reduced to an almost in perfect - balance between production and dissipation of scalar variance.