Hydrodynamic mechanism and suppression of cavity breathing oscillations in a twin-cavity supersonic combustor

Abstract

Low-frequency cavity breathing oscillations can strongly affect flame stabilization and operability in cavity-stabilized supersonic combustors, yet their hydrodynamic origin and suppression remain insufficiently understood in opposed twin-cavity configurations. We investigate the breathing dynamics of an opposed twin-cavity supersonic combustor under low- and high-enthalpy conditions using synchronized high-speed Schlieren imaging, wall-pressure measurements, numerical simulations, nonlinear phase-space reconstruction, Hilbert-transform analysis, and reduced-order modelling. Under low-enthalpy conditions, a self-sustained breathing mode is observed at a dominant frequency of approximately 156 Hz, with large phase differences between cavity-floor and ramp pressures. Measurements and simulations show that periodic mass exchange between the cavity and freestream drives cyclic cavity depressurization and repressurization, establishing a pressure-shear-layer feedback loop that sustains the oscillation. A two-pressure-state reduced-order model, coupled with a shear-layer displacement coordinate, reproduces the measured frequency, pressure phase relationship, and stability characteristics, demonstrating that both bulk-cavity and reattachment/shock-foot pressure states are required to capture the dynamics. Upstream nitrogen injection produces only transient attenuation, with the oscillation recovering as the injected gas convects downstream. In contrast, ethylene injection followed by ignition permanently suppresses the oscillation, with the nonlinear attractor collapsing to a stable equilibrium. Heat release reorganizes the cavity pressure field and alters density, compressibility, and pressure-response timescales, thereby weakening the feedback mechanism responsible for sustaining the breathing mode.