Numerical Simulations of Sound from Confined Pulsating Axisymmetric Jets

Abstract

Numerical simulations of the near-field flow and far-field sound radiation in confined pulsating axisymmetric low-Mach-number jets were performed. The geometry resembled a round duct with either a converging or diverging rigid orifice separating the upstream and downstream regions. A sinusoidal inlet velocity time history was imposed to generate the pulsating flow. The objectives were to 1) investigate flow patterns and sound sources, 2) assess Lighthill's acoustic analogy for far-field sound predictions in ducts, and 3) examine the effects of orifice shape on the flow and acoustic behavior. The diverging orifice case featured flow separation and reattachment resulting in a complex vortical flow within the orifice, in contrast to the converging orifice case. Both orifices produced unstable pulsating jets with vortex roll up and pairing downstream of the orifices. Agreement between directly computed sound and Lighthill's acoustic analogy was excellent for both orifices. Using the Lighthill acoustic analogy, three sound-generating mechanisms were identified. The first was a monopole source mainly caused by the fluctuating volume velocity. The second was a dipole source caused by the unsteady forces exerted on the duct walls. The third was a quadrupole sound source caused by the presence of vortex pairing. The main difference between the acoustic signal in the diverging and converging orifice cases was related to the frequency content of the quadrupole source caused by vortex pairing, which was higher in the diverging case because of the more intense vortical flow.