Numerical Simulations of the Flowfield in Central-Dump Ramjet Combustors. 2. Effects of Inlet and Combustor Acoustics

摘要

A potentially important source of large pressure oscillations in compact ramjets is a combustion instability induced by the interaction of large-scale vortex structures with the acoustic modes of the ramjet. To study these interactions,we have performed time-dependent, compressible numerical simulations of the flow field in an idealized ramjet consisting of an axisymmetric inlet and a combustor. The simulations indicate strong coupling between the flow field and the acoustics of both the inlet and the combustor. For the cases studied, forcing at the first longitudinal acoustic mode of the combustor induces vortex-rollup near the entrance to the combustor at that frequency. A low frequency oscillation is also observed in all the simulations. Pressure and velocity fluctuations in the inlet indicate that the low frequency corresponds to a quarter-wave mode in the inlet. Changing the length of the inlet appropriately changes the observed low frequency. The merging pattern of the inlet is changed. These merging patterns are explained on the basis of an interaction between the vortex-rollup frequency and the acoustic modes of the inlet and combustor. Keywords: Ramjet, Combustion instability, Turbulence. (mjm)