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An investigation of the instability mechanism present in a laboratory rocket combustor is performed using computational fluid dynamics (CFD) simulations. Three cases are considered which show different levels of instability experimentally. Computations reveal three main aspects to the instability mechanism, the timing of the pressure pulses, increased mixing due to the baroclinic torque, and the presence of unsteady tribrachial flame. The stable configuration shows that fuel is able to flow into the combustor continuously allowing continuous heat release. The unstable configuration shows that a disruption in the fuel flow into the combustor allows the heat release to move downstream and new fuel to accumulate in the combustor without immediately burning. Once the large amounts of fuel in the combustor burn there is rapid rise in pressure which coincides with the timing of the acoustic wave in the combustor. The two unstable cases show different levels of instability and different reignition mechanisms.

Combustion instability arises from the coupling between unsteady combustion and acoustic modes in a combustion chamber. A study comprising concurrent experiment and LES simulation of a single element rocket combustor was conducted. The goal was to evaluate the a priori predictive ability of the computational model with regards to self-excited combustion instability, and to use the detailed results from the simulation to interpret the mechanism of self-excitation. Pressure modes and chemiluminescence from CH* were measured at high sampling rates. Direct comparisons between the experimental and computational results were made on the basis of instability frequency, pressure mode shapes, and limit cycle amplitude. All indicated generally good agreement. The light emission representing heat release from the experiment was qualitatively similar to the simulation. The detailed results from the simulation provided much greater insight into the complex phenomena, and showed the likely mechanism that drove high amplitude instability was a periodic ignition in the recirculation zone just downstream of the dump plane. The ignition occurred nearly simultaneously with the arrival of a compression wave traveling from downstream.

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Proper Orthogonal Decomposition (POD) has been implemented in processing of highspeed movies of combustion light emission from two combustion instability experiments. Using the method of singular value decomposition, analysis produces a series of POD modes -orthogonal bases of linked spatial and temporal modes which are sorted based on decreasing singular value or "energy," which indicates the variation and descriptivity contained within each POD mode. Without initially specified conditions or functional forms beyond raw data, the POD modes identify the measured frequencies of the chamber acoustic modes and show the physical behavior of the light emission associated with each POD mode. The major behaviors of the combustion light emission can be reconstructed using select POD modes, serving as an effective low-order approximation of high-order data. Based on this effectiveness, POD analysis shall be extended to the determination of flame describing functions from the temporal POD modes and measured pressure signals. Nomenclature= M-term approximation = temporal POD mode component = spatial POD mode component = singular value or "energy" of POD mode = number of POD modes used in approximation