An experimental study of local self-similarity in the mixing transition of a turbulent free jet

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 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. Firstly 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 non-confined free jet with a nozzle diameter of 12.7mm. Outer scale Reynolds numbers range from 5'000 to 25'000 and high speed Particle Image Velocimetry is used to record the velocity fields with a final spatial resolution of 194×194µm². The analysis focusses 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.