Experiments on a jet in a crossow in the low-velocity-ratio regime

Jet-in-a-crossflow is commonly encountered in natural and engineering systems, such as combustion chambers, volcanic eruptions, or film cooling. We characterise the first oscillatory bifurcation in JICF from a stationary regime dominated by a pair of counter-rotating vortices to periodic hairpin shedding, using both flow visualisation and PIV measurements. Dominant frequency associated with hairpin shedding remains approximately constant for the investigated velocity ratio range. Based on the observed Strouhal number, we propose a threshold that differentiates the low-velocity-regime (with dynamics dominated by downstream recirculation zone) and high-velocity-ratio regime (in which the upstream recirculation zone dominates, see e.g. Mahesh, ARFM, 2013; Karagozian, PoF, 2014; Shoji et al., PRF, 2020). We determine the spatial support of the global mode and track its global spatial peak to characterise the amplitudę of hairpin shedding oscillations. We show that the hairpins are generated by a Hopf bifurcation. Using the Landau model, we determine the critical value of the velocity ratio for hairpin shedding. We also characterise how its value depends on the crossflow Reynolds number. Finally, we investigate the effect of the jet diameter. Our findings apply to film cooling to detect the optimal range of velocity ratio to minimise the mixing rate.