Coherent Motions and Mixing Dynamics of Single-Stream Shear Layers: Insights from 4D-PTV Measurements*

The present study addresses the need for comprehensive volumetric measurements within a single-stream shear layer (SSSL), with the aim of advancing our understanding of intrinsic flow dynamics. A 4D-PTV technique is employed to capture the separation of a wall boundary layer and its subsequent development into a self-similar SSSL. The separation region is characterized by viscous domination and a negative production rate, which is further corroborated by a turbulent kinetic energy budget analysis. An evaluation of the statistical flow descriptors shows self-similarity achieved at a significantly shorter distance ≈60θ₀ (θ₀: initial momentum thickness), contrasting with the past reports. Moreover, a comparison of shear layer width, momentum, and vorticity thickness growth rates with literature reports raises intriguing questions about the potential universality of the growth rate scaling for single- and two-stream shear layers. Phase-averaged maps underline the importance of rollup-braid intersection regions as high-production regions. Additionally, the outer edges of coherent regions exhibit peak production, vortex cores favor higher dissipation rates, and the braid region acts as an energy conduit between contiguous roll-ups.