The behaviour of the scalar gradient across the turbulent/non-turbulent interface in jets

The dynamics of a passive scalar field near a turbulent/non-turbulent interface is analysed through direct numerical simulations of turbulent planar jets, with Reynolds numbers ranging from 142≤Reλ≤246, and Schmidt numbers from 0.07≤Sc≤7.0. A scalar-gradient turbulent/non-turbulent interface (SG-TNTI) forms at the outer edge of the jet, which does not coincide with the vorticity turbulent/non-turbulent interface (VO-TNTI) for the lower Schmidt number cases (Sc = 0.07 and 0.7). Specifically, for Sc = 0.07 and 0.7, the scalar gradient maxima, and thus the bulk of the mixing takes place in the irrotational region, between 10 and 30 Kolmogorov micro-scale distances from the start of the VO-TNTI. For these moderate Schmidt number cases, the SG-TNTI exhibits an irrotational-diffusive superlayer, where the scalar gradient diffusion dominates, while the production is negligible, followed by an irrotational-straining sublayer where the scalar gradient production dominates. In contrast for Sc = 7.0, the SG-TNTI consists of a viscous-convective superlayer that closely matches the viscous superlayer from the VO-TNTI and an inertial-convective sublayer, where scalar gradient production dominates, which is much smaller than the turbulent sublayer of the VO-TNTI. The scaling laws and mean thicknesses of each one of these (sub)layers are briefly discussed. This work presents a systematic study of the effects of the Schmidt number on the scalar gradient evolution and of the SG-TNTI characteristics.