Numerical simulations of sound from a confined pulsating axisymmetric jet

Direct Numerical Simulations (DNS) of confined pulsating axisymmetric jets, including both the near- field flow and the far-field sound radiation, were performed. A rigid model of the human vocal tract, resembling a duct with either a converging or diverging nozzle separating the upstream and downstream regions was considered. A sinusoidal inlet velocity time history was imposed to generate the pulsating flow. The objectives were to: (1) investigate the flow patterns and sound sources, (2) assess the accuracy of Lighthill's acoustic analogy for far-field sound predictions in confined ducts, and (3) examine the effect of the nozzle shape on the flow and aeroacoustic behavior. The diverging nozzle case featured flow' separation and reattachment resulting in a complex vortical flow within the nozzle, in contrast to the converging nozzle case. Both nozzles produced unstable pulsating jets with vortex roll-up and pairing occurring downstream of the nozzles. The agreement between the directly computed sound and the Lighthill acoustic analogy was excellent for both nozzle geometries. Using the Lighthill acoustic analogy, three sound sources were identified. The first sound source was a monopole source due to the fluctuating volume velocity. The second sound source was a dipole source due to the unsteady forces exerted on the duct walls. The third was a quadrupole sound source due to the presence of vortex pairing in the axisymmetric pulsating jet. The main difference between the acoustic signal in the diverging and converging nozzle cases was related to the frequency content of the quadrupole source due to vortex pairing, which was higher in the diverging case due to the more intense vortical flow produced by the combination of flow separation/reattachment and jet instability.