Transitioning Instabilities for Variable Density Jets in Crossflow

This experimental-theoretical study investigates variable-density gaseous jets in crossflow (JICF), focusing on the nature of transitions in the upstream shear layer (USL) and sensitivity to the constituents that make up the jet mixture. Prior studies demonstrate that jet-to-crossflow density ratio (S) and flux momentum ratio (J) influence global/absolute instability transitions in the USL. Building on these results, the present work analyses the JICF flow field using acetone PLIF and stereo PIV measurements with complementary hotwire anemometry along the USL trajectory to identify trends in the critical J values (J_cr) associated with USL transition. The proper orthogonal modes of the PIV flow field yield systematic mode transitions, also represented in spectral contour maps of the hotwire-based measurements. Characteristic signatures suggest that the transition can be gradual under slight changes to the crossflow, but for a fixed jet Reynolds number and prescribed S and J values, the jet’s gas constituents can affect J_cr and hence affect the transition in the USL instability and its dynamical character. As done in recent equidensity JICF studies, comparisons with linear stability analysis aid in interpreting these results utilizing a counter-current shear layer analogy.