An Experimental Study of Local Self-Similarity in the Mixing Transition of a Turbulent Free Jet

Abstract

Turbulent free jets play an important role to understand turbulence and momentum transport in free shear layers. The characteristic nature of this type of flow has attracted the focus of many scientists within the past century, and a large body of literature describes the dynamics in the near-field region as well as the self-similar region. Recent investigations attempt to understand the intermediate fields, called the mixing transition or the route to self-similarity. In light of this mixing transition hypothesis for jets, an apparent gap is recognized among the scientific community with two main conjectures being put forth. First, the flow will always asymptotically reach a fully self-similar state if boundary conditions permit. The second proposes partial and local self-similarity within the mixing transition. In the present work we address this topic with an experimental investigation of the intermediate-field turbulence dynamics in a nonconfined free jet with a nozzle diameter of 12.7 mm. The outer-scale Reynolds number is 15 000, and high-speed particle image velocimetry is used to record the velocity fields with a final spatial resolution of 194 × 194 µm². The analysis focuses on higher-order moments and two-point correlations of velocity variances in space and time. We observe interactions among turbulent structures that show local self-similarity and partial coherence.

Keywords:

• Turbulent round free jet
• mixing transition
• coherent structures
• local self-similarity
• high-speed particle image velocimetry

Acknowledgments

We are indebted to colleagues at the ECMFL Group of the University of Michigan for many helpful discussions as well as assistance with various aspects of the experiments. The work was made possible by the generous support of Nuclear Energy University Program of the U.S. Department of Energy under grant number 14-6552.