Cavities in supersonic combustors serve as flame stabilizers, but at a large length-to-depth ratio (L/H = 8.5), they develop low-frequency oscillations that can damage the structure. Previous studies mostly focused on single cavities, leaving the interaction of two shear layers and shock systems in an opposed twin-cavity configuration unexplored. This work fills that gap by showing that the 'breathing' mechanism involves periodic mass exchange between the cavity and the external flow.
Experiments were conducted in a high-enthalpy direct-connect facility at the Indian Institute of Science. We used high-speed schlieren imaging (20 kHz), CH* chemiluminescence, and high-frequency pressure transducers (40 kHz). To understand the recirculation-zone dynamics, we performed two-dimensional numerical simulations of non-reacting flow in OpenFOAM. From pressure and schlieren data, we built a reduced-order model with two pressures and a shear-layer displacement coordinate, whose parameters were identified by least-squares fitting.
At low enthalpy, a self-sustaining 'breathing' mode was found at about 156 Hz with a large phase difference between the cavity-floor and ramp pressures (about 135°). Numerical simulation showed that periodic mass exchange between the cavity and the external flow causes cyclic pressure drops and rises, creating a feedback loop. The reduced-order model reproduced the measured frequency, phase relationship, and marginal stability. Nitrogen injection only temporarily damped the oscillations, whereas ethylene injection followed by ignition completely suppressed the mode: the attractor in phase space collapsed to a stable equilibrium, and the dominant eigenvalue became stable. Heat release altered density, compressibility, and pressure response times, weakening the feedback.
This work shows for the first time that in an opposed twin-cavity configuration, oscillations are sustained not only by geometry but also by the ramp-pressure dynamics, which acts as an active feedback element. This distinguishes it from previous studies of single cavities, where the Rossiter acoustic mechanism dominated. The resulting two-pressure reduced-order model offers a simple tool for predicting cavity stability under varying conditions.
The results open the way to developing model-based active control systems for oscillations in scramjet combustors. One can investigate the influence of cavity geometry, fuel composition, and injection distribution on stability. Extending the reduced-order model to reacting flows with chemical kinetics is also promising.
The findings will help design more stable combustors for hypersonic vehicles, reducing the risk of vibration damage and improving combustion efficiency.
A natural next step is a parametric study of the effects of L/H ratio, aft-wall angle, and injected-gas flow rate on the stability of the breathing mode. It is also important to test the model on other cavity configurations.
This work links low-frequency cavity oscillations to the unresolved problem of predicting unsteady loads in supersonic combustors. It shows that simple acoustic models are insufficient and that the hydrodynamic coupling between pressure and the shear layer must be considered.
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What the work does
This study investigates low-frequency 'cavity breathing' oscillations in an opposed twin-cavity supersonic combustor. Using synchronized schlieren imaging, pressure measurements, numerical simulations, and a reduced-order model, it is established that the oscillations are sustained by a feedback loop between pressure and the mixing layer. It is shown that nitrogen injection only temporarily weakens the oscillations, whereas ethylene injection with ignition completely suppresses them through heat release.
Where its strength is
For the first time for an opposed twin-cavity configuration, the role of ramp pressure as an active feedback element is demonstrated, which distinguishes the mechanism from the classical Rossiter acoustic mechanism. The reduced-order model with two pressures provides a simple tool for predicting cavity stability.
Where it could go next
Worth trying
Why this may matter
Understanding the mechanism of low-frequency oscillation suppression in supersonic combustors is important for developing stable scramjets and preventing destructive vibrations.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.51, against a usual 0.64. It looks like few have drilled nearby.
The cavity 'breathing' frequency (156 Hz) is close to the idle rotation frequency of an automotive crankshaft, but the pressure oscillation amplitude reaches 30 kPa, comparable to the sound wave from a firecracker blast.