Distinguishing Among Transverse Jet Instabilities with Data-Based Reduced Order Models

Experiments and theory are used to explore upstream shear layer (USL) instabilities associated with the jet in crossflow (JICF), focusing on approaches to enable prediction of transition from convective to absolute to global instability as crossflow magnitude is increased. Experiments explore flowfield dynamics via laser-based diagnostics, with flowfield dynamics that are extracted using proper orthogonal decomposition (POD), revealing dominant spatiotemporal characteristics. Transverse jet vortex roll-up along the USL trajectory produces POD mode coefficients exhibiting oscillatory patterns with wave packet-like behavior, with an onset that initiates closer to the jet exit as crossflow velocity is increased, consistent with convective to absolute instability transition, and potentially further transition to global instability. Theoretical explorations of the USL use both linear stability analysis associated with a Counter-Current Shear Layer (CCSL) model [Souza, et al., PR Fluids 2021] as well as modeling of the dynamics via the Ginzburg-Landau disturbance amplitude equation, with relevant parameters extracted from experimental datasets. These approaches form a strategy for development of a data-based reduced order model (ROM) for prediction of transitions in USL instabilities, with important implications for jet flow control.