Interaction of sound from supersonic jets with nearby structures

A model of sound generated in an ideally expanded supersonic (Mach 2) jet is solved numerically. Two configurations are considered; (i) a free jet and (ii) an installed jet with a nearby array of flexible aircraft type panels, in the later case the panels vibrate in response to loading by sound from the jet and the full coupling between the panels and the jet is considered, accounting, in particular, for panel response and radiation. In both cases the jet is initially excited by a source of transient mass injection, however only the long time behavior of the jet is considered, i.e., after the initial wave due to the source has propagated out of the computational domain. In a shock free (ideally expanded) jet, the evolution of unsteady disturbances in both the free and the installed jet is dominated by a pronounced intense radiation of sound, termed Mach wave radiation, at roughly the Mach angle (30° for the jet considered here). The spectrum associated with this radiation has a well defined peak at a Strouhal number of approximately 0.2 together with its harmonics, consistent with observations for jets in this Mach number range. In the near field, in addition to the fundamental and its harmonics, subharmonics are present. There is a strong and intense radiation of sound at the Mach angle. This sound is generated by supersonically propagating disturbances forming a cellular structure in the jet shear layer. The cells compress and expand in a roughly periodic fashion, generating Mach wave radiation which propagates into the far field along the Mach angle. The pressure from the Mach wave radiation is superimposed on a low level, nearly continuous spectrum radiation pattern. The sound propagated upstream is at a lower level and exhibits a more continuous spectrum. When the the jet is installed with flexible panels nearby, the panel loading and response crucially depends on the location of the panels. Panels located upstream of the Mach cone are subject to a low level, nearly continuous spectral excitation and consequently exhibit a low level, relatively continuous spectral response. In contrast, panels located within the cone are subject to a significant loading due to the intense Mach wave radiation of sound and exhibit a large, relatively peaked spectral response centered around the peak frequency of sound radiation. The panels radiate in a similar fashion to the sound in the jet, in particular exhibiting a relatively peaked spectral response at approximately the Mach angle from the bounding wall.