Hairpin vortex structures in a supersonic, separated, longitudinal cylinder wake

The near-wake flow structure behind a blunt-based cylinder aligned with a Mach 2.49 freestream is studied experimentally using tomographic particle image velocimetry (TPIV), which acquires three-component velocity measurements throughout a volumetric region. TPIV measurements were acquired in three different volumetric regions throughout this flow, including two regions in the separated shear layer and one in the high-speed portion of the trailing wake, with a large ensemble of measurement volumes (approximately 2500) acquired in each region. The quality of these data is validated using point-by-point comparisons to previous experimental data with known uncertainty estimates. Hairpin vortex structures were observed to exist commonly throughout this flow field (in both subsonic and supersonic regions), and they induced strong turbulent fluctuations aligned with a consistent direction. Inverted hairpin vortex structures were also observed to commonly exist in this flow, inducing strong turbulent fluctuations in a direction opposite to that of the upright hairpins, but their presence was limited to subsonic flow regions. These coherent structures are demonstrated to be significant drivers of kinematic Reynolds shear stress and turbulent kinetic energy throughout this flow. A planar turbulent quadrant analysis was used to provide a measure of the spatial dependencies of these structures within the flow, and a linear stochastic estimation was used to provide robust statistical evidence of their frequent existence in various subregions. Upright hairpins were demonstrated to statistically grow with streamwise progression, and the strength of their induced velocity fluctuations increased in the presence of the adverse pressure gradient associated with flow field reattachment.