Analytical model for flow-excited interior cavity resonance and its application to the Stratospheric Observatory for Infrared Astronomy (SOFIA)

Abstract

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a joint effort between NASA and the German Space Agency that has installed a 20<th>000-kg telescope in a modified 747-SP. The modifications entailed constructing bulkheads, one of which is used to provide the active mount for the telescope, and a door that rotates to open as much as one-quarter of the fuselage circumference to the atmosphere. This configuration represents a large compartment that can exhibit acoustic resonances at low frequencies. Concern arose that a Rossiter mode, which is an aerodynamic resonance in which vortices shed from the leading edge of a gap form a coherent standing pattern at certain speeds, would create a strong acoustic source for acoustic and structural modes, whose frequencies might coincide. A model consisting of a two-dimensional hard-walled waveguide having a Rossiter mode source and an elastic plate at one end was analyzed in order to understand these issues. Unlike conventional analyses of interior cabin noise, in which vibrating walls are the acoustic source, the elastic plate represents a compliant boundary that couples with the acoustic modes. The results lead to some general insights to the the possibility of a ‘‘triple resonance,’’ as well as the role of structural compliance for cavities that are excited by turbulent external flows.