Dominant Spatiotemporal Shear Layer Modes for Variable Density Jets in Crossflow*

This study explores gaseous jets in crossflow (JICF), focusing on the upstream shear layer (USL) instabilities and their transitions. It builds upon extensive prior experiments [Besnard, et. al., PR Fluids 2022; Harris, et. al., PR Fluids 2023] that examine various flow conditions and nozzle geometries to study regimes of and conditions producing convectively unstable (CU) and absolutely unstable (AU) shear layers. The present experiments explore flowfield dynamics via laser-based particle image velocimetry (PIV) and acetone planar laser-induced fluorescence (PLIF), where flowfield dynamics are extracted using proper orthogonal decomposition (POD), revealing dominant spatiotemporal characteristics. Structures associated with transverse jet vortex roll-up along the USL trajectory produced POD mode coefficients exhibiting sinusoidal patterns reminiscent of wave packets, with an onset that initiates earlier and can propagate upstream as crossflow velocity is increased and the USL instabilities transition from CU to AU. These reduced spatiotemporal variables may be used to model the JICF's complex dynamics as it transitions from CU to AU and further, to globally unstable (GU) regimes, for example, via reduced order modeling using the Counter-Current Shear Layer (CCSL) model [Shoji, et. al., JFM 2020; Souza, et. al., PR Fluids 2021] and/or through the Ginzburg-Landau amplitude equation.