Numerical simulations of oscillatory cold flows in an axisymmetric ramjet combustor

A numerical simulation and analysis technique was developed to study the interaction between the vorticity component and the acoustic component of the flowfield in an axisymmetric ramjet combustor. To exclude the effects of the outflow boundary conditions, the interior of the combustor was isolated from the external region by a choked nozzle. The subsonic inflow boundary conditions are damping and thus do not force the oscillations. The shear layer separating at the step is perturbed by the low-frequency pressure fluctuation at the base of the step. The perturbation is then amplified downstream by the shear-layer instability and large-scale vortical structures are formed. When these structures impinge on the downstream nozzle wall, secondary vortices are generated, and large fluctuations in the Mach number upstream of the nozzle throat are observed. However, the Mach number in the supersonic region downstream of the throat remains stationary. The pressure and the vorticity spectra indicate the presence of the same low-frequency components, suggesting a possible vortex/acoustic wave interaction. Analysis of the computed mean-flow quantities and the higher moments indicates that, for the majority of the flow region, the large-scale structures are the main contributor to the transport of momentum. In a related study, the information obtained from these simulations is used to propose a model for one of the possible vortex/acoustic wave interactions.

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