Modes of oscillation in a nonreacting ramjet combustor flow

Abstract

The results of the numerical simulation of cold flows in a ramjet are used to identify the mechanism that leads to the observed pressure fluctuations. The acoustic disturbance is defined as the unsteady part of the potential field and is shown to be driven by the instantaneous dilatation field. The quadrupole nature of the sound source around each vortex in the flowfield is demonstrated. The dilatation field in the vortex impingement region on the nozzle wall is considered a compact acoustic source and is analyzed by multipole expansion of the distributed field, revealing a strong axial acoustic dipole 180 deg out of phase with the impinging vorticity fluctuations. This dipole response to the vortical fluctuation is applied as the impedance for the vorticity/acoustic fluctuations at the nozzle. The spectral analysis of pressure and vorticity fluctuations reveals both a resonant acoustic mode in which the vortical disturbances excite the acoustic free modes, and a coupled mode in which the acoustic and vortical disturbances are coupled through dipole radiation at the nozzle and the acoustic susceptibility of the separating shear layer at the dump plane. A model for the coupled mode is proposed that provides a method for estimating its frequency.