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

• Published in: Experiments in fluids Vol. 65; no. 8
• Main Authors: Gautam, Ankit K., Livescu, Daniel, Mejia-Alvarez, Ricardo
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.08.2024; Springer Nature B.V; Springer Science + Business Media
• Subjects: Backward facing steps; Boundary layers; Braiding; Concentration gradient; Diffusion rate; Dissipation; Engineering; Engineering Fluid Dynamics; Engineering Thermodynamics; Entrainment; Flow characteristics; Flow mapping; Fluid dynamics; Fluid flow; Fluid- and Aerodynamics; Fluorescence; Heat and Mass Transfer; Kinetic energy; Parameters; Particle tracking; Particle tracking velocimetry; Research Article; Self-similarity; Separation; Shear layers; Turbulence; Turbulent flow; Vortices
• ISSN: 0723-4864; 1432-1114
• DOI: 10.1007/s00348-024-03846-5

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 ( Re τ = 1 240 ) 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 ≈ 55 θ 0 , where θ 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.