Growth of organized flow coherent motions within a single-stream shear layer: 4D-PTV measurements

Journal Article · 23 July 2024 · Experiments in Fluids

DOI: https://doi.org/10.1007/s00348-024-03846-5 · OSTI ID:2437849

Gautam, Ankit K.; Livescu, Daniel; Mejia-Alvarez, Ricardo

Abstract This study investigates the evolution of a single-stream shear layer (SSSL) originating from a wall boundary layer past a backward-facing step. Utilizing a time-resolved 3D-Particle Tracking Velocimetry (4D-PTV) technique, we track the trajectories of fluorescent particles to gain insight into the flow characteristics of the SSSL. A compact water tunnel facility ( $$$$\textrm{Re}_\tau =1\,240$$$$ ${\text{Re}}_{\tau }=1240$ ) is fabricated to obtain an SSSL with a perpendicular slow entrainment stream past the separation edge. A hybrid interpolation approach that combines ensemble binning and Gaussian weighting is implemented to derive minimally filtered mean and instantaneous lower- and higher-order flow field parameters. Spanwise-dominant coherent motion accompanied by finer flow scales is observed to grow due to flow entrainment through “nibbling” actions of small-scale vortices, “engulfing” by large-scale vortices, and vortex pairing events. Furthermore, the non-zero-speed stream edge grows relatively faster than the zero-speed stream edge, showing a strong asymmetry in mixing composition across a mixing layer. The SSSL reaches self-similarity at a streamwise distance of $$$$\approx 55\,\theta _{0}$$$$ $\approx 55{\theta }_0$ , where $$$$\theta _0$$$$ ${\theta }_0$ is the initial momentum thickness from the separation edge, i.e., considerably shorter than reported in previous studies. A literature comparison of growth rate parameters raises intriguing questions regarding a potential inclusive growth scaling unifying the free shear layers. A turbulent kinetic energy (TKE) budget analysis reveals a negative production region immediately downstream of the separation edge attributed to a large positive streamwise gradient of streamwise velocity. In the self-similar region, the phase-averaged flow mapping demonstrates a larger concentration of turbulence production rate around the outer edges of spanwise vortices, specifically at the intersection of braids and vortices. Furthermore, a spatial separation exists in the regions of peak production and dissipation rates within the vortex core region favoring dissipation. The braids exhibit a larger concentration of turbulence diffusion rates, indicating their function as a conduit for exchanging turbulence between neighboring coherent motions.

• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA)
• OSTI ID: 2437849
• Journal Information: Experiments in Fluids, Journal Name: Experiments in Fluids Journal Issue: 8 Vol. 65; ISSN 0723-4864
• Publisher: Springer Science + Business MediaCopyright Statement
• Country of Publication: Germany
• Language: English