Study of scalar macro- and microstructures in a confined jet

The scalar structures in a confined jet are studied at high Reynolds and Schmidt numbers. Both flow modes without the recirculation zone (jet mode) and with the massive separation and creation of the recirculation zone (r-mode) are considered. Despite of the big difference in the flow modes, the fine scale scalar structures gained from highly resolved PLIF measurements possess similar statistical properties such as the normalized cumulative distributions and probability densities of the dissipation rate. The fine scalar structures are distributed nearly uniformly in space with the scalar gradient vector having a slight preference to align with the most compressive mean strain axis. The scalar field exhibits small-scale intermittency which is strongly dependent on the flow mode. The intermittency is most pronounced in the front of the recirculation zone and becomes weaker on the centerline and downstream. The most contribution to the scalar variance is made by large scale motions whereas the contribution of fine scales smaller than typical inertial range scales is negligible. Examination of the multiplier distributions has not supported the concept of the multifractal nature of the scalar dissipation field.