Effects of nozzle body on jet noise

J. Bridges, F. Hussain

Research output: Contribution to journalArticle

12 Scopus citations

Abstract

Most models for jet noise assume that the turbulence producing the noise is far enough removed from any solid surface that the presence of surfaces (such as the nozzle itself) has negligible effect on the sound field. This paper addresses the validity of this assumption. Experiments were performed on a low Mach number jet in which the dominant sound source, the pairing of vortex rings, was well documented by previous work [1]. The vortex rings were stabilized spatially and temporally by artificial excitation at StDj= 1·14 and became coplanar (one inside the other) at x/Dj≅ 2·5 with a frequency of occurrence StDj= 0·285. In the current study, the directivity of this source was measured for various external nozzle geometries. The external nozzle shape was changed from a conventional conic shape to a flat plate whose diameter was then changed by a factor of three to determine how external nozzle shape and size affected the sound of the vortex pairing in the jet. To explain the variations in directivity observed with the different nozzle geometries, a simple model of the vortex ring pairing was created using Biot-Savart vortex simulations. Vortex sound theory, including surface dipole terms, was applied to this estimate of the vorticity field to calculate the resulting dipole and quadrupole sound sources. The dipole sound was of the same order as the freestream quadrupole sound. When the phase-average sound field measured in the experiments was decomposed into multipole components, the relative strengths of the low frequency dipole and quadrupole components were in good agreement with those of the simulation, supporting the general conclusion that the dipole produced by the presence of the nozzle is not negligible for vortex motions within the first few diameters of the jet, and supporting the validity source of the vortex sound theory itself. The decomposition also unveiled a weaker monopole, which is seen as evidence of subharmonic feedback from the pairing to the jet nozzle, helping stabilize successive pairings even though no excitation was provided as these subharmonic frequencies.

Original languageEnglish
Pages (from-to)407-418
Number of pages12
JournalJournal of Sound and Vibration
Volume188
Issue number3
DOIs
StatePublished - Dec 7 1995

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