Investigation of turbulent/non-turbulent interface and flow topology in turbulent temporally evolving jets

The turbulent/non-turbulent interface (TNTI) plays a critical role in scalar mixing. Despite extensive previous investigations, outstanding gaps persist in understanding the role of the TNTI and flow topology in turbulent mixing. Those include (i) how to reasonably identify the TNTI using scalar criteria and (ii) what is the driving mechanism of the scalar mixing process across the interface. This study aims to partly address these two issues by investigating the interaction between the TNTI and flow topology in turbulent temporally evolving jets. It is found that the scalar criterion can reasonably identify the TNTI when the Schmidt number of the scalar is Sc = 0.1, compared to the classical vorticity criterion. Among all topological structures, the Unstable Node/Saddle/Saddle (UN/S/S) structure exhibits the highest strain rate. A negative correlation is observed between the curvature of the TNTI and the conditional strain rate. The interface curvature is further found to exhibit a negative correlation with the scalar dissipation rate that characterizes the scalar mixing rate. These results collectively suggest that the multi-scalar turbulent mixing process is driven by the following mechanism: the UN/S/S topological structure promotes the generation of larger compressive strain rates at the TNTI, which flattens the interface and further leads to an increase in the scalar dissipation rate, thus enhancing the turbulent mixing.