Sound Production due to Swirl-Nozzle Interaction: Model-Based Analysis of Experiments

Abstract

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 abovementioned experiment is due to the acceleration of vorticity waves through the nozzle.

Aeroacoustics research in Europe: The CEAS-ASC report on 2020 & 2021 highlights

“…Entropy inhomogeneities and vorticity patches induce so-called indirect combustion noise when passing through a choked nozzle referred to as entropy noise and vorticity noise, respectively. It was found that vorticity noise depends on the orientation of the vorticity viz., oriented normal or parallel to the axial main flow [130][131][132]. At the DLR's Engine Acoustics laboratory -using a dedicated experimental setup [131,132] -an experimental investigation of parallel component vorticity noise was performed [131,132].…”

“…In the experiment, a time-dependent swirling flow was induced by unsteady tangential injection in a pipe upstream of a choked convergent-divergent nozzle. Theoretical analysis [130,132] found that as the resulting swirling flow passes through the nozzle, the axial stretching of the fluid causes an increase in rotation energy. The steady energy conservation in an isentropic flow (Bernoulli) implies a Mach number higher than unity at the throat and an associated reduction of density.…”

On the scattering of entropy waves at sudden area expansions

“…These are silent while advecting in the combustor but generate noise at the exhaust when interacting with the turbine blade rows [15]. This source of noise is termed indirect combustion noise and can be further decomposed as vortical [16][17][18], entropy [4,5] and, compositional noise [19][20][21] after the name of the flow perturbation which creates them. Entropy noise is believed to be its main component [9].…”