LAUSR

We consider the scalar concentration field in the proximity of the turbulent/non-turbulent interface (TNTI) of a round momentum-driven turbulent jet at Re = 10 600. Orthogonal cross sections of the jet are taken at 50 nozzle diameters from the nozzle exit using planar laser-induced fluorescence. The conditional scalar concentration is evaluated along the interface-normal direction, identifying the thickness of the TNTI region as 0.64λ (where λ is the Taylor microscale). Conditioning the scalar concentration within the TNTI revealed higher values of the passive scalar in the vicinity of the boundary elements shaped by large vorticity structures, i.e., isosurface points with low curvature (flat regions), small interface angle, and large radial distance from the jet centerline. In contrast, small vorticity structures near the boundary manifesting with high interface curvature, high interface angle, and small radial distance are associated with lower concentration values. Using the current experimental resolution, we find that high concentrations near the far boundary points persist up to a distance of 0.40λ–0.48λ into the TNTI region, after which boundary points closer to the jet centerline exhibit larger concentration values along the interface-normal direction, similar to the fully turbulent region. The cross correlation analysis showed that in regions characterized by low streamwise momentum, there are positive, albeit small, scalar correlations between the non-turbulent and the TNTI regions. The latter may imply local detrainment of the fluid particles containing the scalar at far radial positions.