Sound production due to swirl-nozzle interaction: Model-based analysis of experiments

Indirect noise due to the interaction of flow inhomogeneities with a choked nozzle is as an important cause of combustion instability in solid rocket motors and is believed to be important in aircraft engines. A previously published experiment demonstrated that interaction of the nozzle with time-dependent axial swirl can also be a source of sound. This axial swirl was generated by intermittent circumferential mass injection upstream from a choked nozzle in a so-called Vortex Wave Generator. The present work discusses the impact of swirl-nozzle interaction in this experiment on the acoustic waves detected downstream of the nozzle. The main source of sound appears to be the reduction in mass flux through the choked nozzle, which depends quadratically on the swirl number. This effect is quantitatively predicted by a quasi-steady and quasi-cylindrical analytical model. The model, combined with empirical data for the decay of axial swirl in pipe flows, predicts the observed influence of the distance between the Vortex Wave Generator and the nozzle. The findings presented here contradict the hypothesis found in the literature, which presumes that sound production in the above-mentioned experiment is due to the acceleration of vorticity waves through the nozzle.