Conditional statistics at the turbulent/non-turbulent interface of variable viscosity jets

In nature as well as in industrial applications, turbulent mixing is ubiquitous. In most cases, these are different fluids with different physical properties (density and/or viscosity). Moreover, all important changes such as mass, momentum and scalar fluxes occur across the turbulent/non-turbulent interface, a thin and sharp layer that separates the turbulent core from the irrotational surrounding fluid. In this paper, we present statistics conditioned on the instantaneous interface position in the very near field of a variable viscosity jet to study the birth and growth of turbulence. The simultaneous scalar concentration and velocity fields are obtained from planar laser-induced fluorescence, where the images undergo an original correction and normalization process, and stereo-particle image velocimetry, respectively. We show that the turbulence is much more advanced in the variable viscosity flow (VVF), which exhibits some features that are visible much later in the constant viscosity flow (CVF). Furthermore, this study reveals a change in the nature of the mixing process between VVF and CVF, which needs to be further investigated.