Panel-Structure Response to Acoustic Forcing by a Nearly Sonic Jet

Abstract

A model of a high subsonic jet with a nearby array ofexible, aircraft-type panels is studied numerically in two dimensions. The jet is excited by a limited duration, spatially localized starter pulse in the potential core. The long time evolution of unsteady disturbances in the jet, the responses of the panels, and the ensuing radiation are computed and compared with those of a lower Mach number jet. The results show that, for high subsonic Mach numbers, the spectral response of both the jet (near and far ® eld) and of the panels is concentrated in a relatively narrow frequency band centered at a Strouhal number (based on jet exit velocity) of approximately 0.25 and associated harmonics. Thisbehaviorof thejet and the panelsiscaused by nearly periodic vortex shedding from the nozzlelip.In contrast, forlowerMach numbersvortex sheddingisverymuch weakerand thepanelsactasnarrow- band ® lters emphasizing the natural frequencies of the panels. Radiation from the panels is weakest in upstream directions and exhibits zones of silence due to destructive interference of radiation from the different panels. HIS paper describes the results of a numerical simulation of jet noise in the presence of fourexible aircraft-type panels in a panel-stringer assembly. The simulation is based on a model that fully couples theuid dynamics of the jetow to the panel mo- tion and the resulting acoustic radiation. The primary objective is to determine the role played by near sonic jet exit conditions on instal- lation effects (i.e., the mounting of the jet near aexible structure) and on the response and the acoustic radiation from the structure. Thelongtimeevolutionofunsteadydisturbancesinanearlysonic jet exiting from a converging nozzle, together with panel responses and ensuing radiation from the panels, is considered in this paper. In previous work, jet acoustics, panel response, and radiation have been considered for lower-speed jets exiting from straight pipes, bothatrest 1 andinforwardmotion. 2 The presentresults showquali- tativelydifferentbehaviorforhighsubsonicjets,namely,arelatively narrow-bandjet and panel responseconcentratedata Strouhal num- ber of approximately 0.25. The mechanism driving this behavior is vortex shedding from the nozzle lip, which dominates jet and panel behavior for high subsonic Mach number jets. In contrast, for lower Mach number jets as considered in Ref. 1, there is a low-frequency jetresponsewithoutpronouncedspectralpeaks.In thiscasethepan- els actasnarrow-band® ltersconverting the relativelybroadband in- cident pressure into selected low-frequency bands associated with the natural frequencies of the panels, whereas the vortex shedding frequency is considerably less evident. In previous analyses, the exact sources of jet noise have been identi® ed from the basic equations ofuid dynamics. 3±6 Generally,