Panel-structure response to acoustic forcing by a nearly sonic jet

A model of a high subsonic jet with a nearby array of flexible, 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 numer jet. The results show that, for high subsonic Mach numbers, the spectral response of both the jet (near and far field) and of the panels is consentrated in a relatively narrow frequency band centered at a Strouhal number (based on jet exit velocity) of approximately 0.25 and associated harmonics. This behavior of the jet and the panels is caused by nearly periodic vortex shedding from the nozzle lip. In contrast, for lower Mach numbers vortex shedding is very much weaker and the panels act as narrowband filters 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.