Large-Eddy Simulations of Flows in a Ramjet Combustor

Abstract

A numerical simulation technique was developed for investigating the oscillatory cold flow in a ramjet combustor configuration. Some simulations have been conducted, and the results are presented and analyzed here. The main objective of the simulations is to investigate the interaction between the vorticity component and the acoustic component of the flow field when the reduced frequency of the flow based on the speed of sound is of the order of unity. In constructing the numerical model, it was found that the interior of the combustor must be isolated from the external region by a choked nozzle. The resulting numerical simulations arc able to exclude the effects of the artificially imposed boundary conditions at the outflow boundary.Flow visualizations and frequency spectra of the vorticity and pressure fluctuations were analyzed to understand the flow field. It appears that the pressure fluctuation at the base of the backward-facing step, a signal taken as acoustic in nature, contains only low-frequency components. This is in contrast to the frequency spectrum of the vorticity fluctuation in the free shear layer near the separation point, which contains both high-frequency and low-frequency components. The vorticity fluctuation at the location where the shear layer impinges on the nozzle wall also contains only low-frequency components. The unsteady flow fields near the shear layer separation point and in the nozzle region are investigated. It appears that the boundary layer immediately upstream of the separation point 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. In the nozzle region, the Mach number in the subsonic region fluctuates at a high amplitude when a vortical structure impinges on the nozzle. However, the Mach number in the supersonic region downstream of the throat remains stationary.Mean flow quantities and higher moments were computed by running time averaging to evaluate the contribution of the coherent structures to the transport of momentum. The results were compared to available experimental data for a two-dimensional backward-facing step configuration. The good agreement over the majority of the flow region shows that the large-scale structures are faithfully simulated and are the main contributor to the transport of momentum.An attempt was made to extract acoustic information from the results of the numerical simulations by computing the instantaneous dilatation field, which is shown to drive the unsteady potential flow, i.c., acoustic disturbances. It turns out that the near-field dilatation contains mainly the characteristics of the sound sources. The quadrupole nature of the sound source around each large-scale vortex in the flow field is clearly demonstratcd. The complex distributed dilatation field near the impingement point of the shear layer on the nozzle wall is considered as a compact acoustic source and is analyzed by multipole expansion of the distributed field. The results show a strong axial acoustic dipole at the nozzle. The dipole fluctuation is 180 degrees out of phase with the impinging vorticity fluctuations. This result can be applied as the impedance for the vorticity-acoustic fluctuations at the nozzle. However, the propagation aspects of the sound field cannot easily be visualized in a near field. The spectra of the pressure and vorticity fluctuations at selective points are analyzed to reveal the existence of two types of fluctuations. One is the resonant acoustic mode, in which the vortical disturbances excite the acoustic free-modes. The other is the coupled mode, in which the acoustic disturbances and the vortical disturbances are coupled through the dipole radiation at the nozzle and the acoustic susceptibility of the separating shear layer at the dump plane. A simple model for the coupled mode is proposed to explain the latter mechanism and to provide an approximate method for estimating its frequency.KeywordsMach NumberAcoustic WaveShear LayerPressure FluctuationCombustion InstabilityThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.