Intermittent event evaluation through a multifractal approach for variable density jets

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Variable-density jets occur in many systems, including geophysical flows and industrial applications, exhibiting a large range of scales of Reynolds and Richardson numbers. A series of jets with varying densities was ejected vertically into a large ambient region. Using particle image velocimetry, the near-exit velocity fields were measured for three different gases exhausting into air: helium, air and argon. Experiments considered relatively low Reynolds numbers from approximately 1500 to 5500 with Richardson numbers near 0.001 in magnitude. These included a variety of flow responses, notably nearly laminar, turbulent and transitioning jet flows. Flows were examined through a multifractal framework, and the singularity spectrum showed the characteristics of the flow based on the evolution in the streamwise and wall-normal direction. The variation of the Hölder exponent displayed the asymmetry and intermittency of the flow. Similar to the Reynolds shear stress, the development of intermittent behavior is a function of downstream location with respect to changes in the Reynolds number. The density of the exiting jet also influences the location of high intermittency within the flow signal. Lower density jets provide increased variability of the signal within the ambient air and the shear layer close to the exit of the jet. Specifically, the highest degree of multifractality is observed within the mixing layer of the helium jet at a transitioning Reynolds number. Conditional averaging with respect to the fluctuating velocity components and the pointwise Hölder exponent reveals high velocity-intermittency interactions at the inside of the jet mixing layer when fluid is entrained and at the turbulent/non-turbulent interface when fluid is ejected. Finally, line integral convolution illustrates the impact of turbulent/non-turbulent interface on the jet dynamics.