Interior Duct Wall Pressure Downstream of a Low-Speed Rotor

Abstract

The region downstream of a ducted rotor has been experimentally investigated in terms of its wake characteristics and the duct wall pressure fluctuations. The motivation for the measurements was to document and understand the sources of structural excitation that would lead to shell vibration. The wake measurements indicated a swirling hydrodynamic flow field that decayed rapidly in the streamwise direction. The interior duct wall pressure similarly indicated strong decay in the fluctuation levels in the streamwise direction. The spectral features of the wall pressure at various speeds and axial positions were described, and found to be influenced by the relative interactions of the blade rate harmonics and the duct cut-on modes. A circumferential modal analysis was used to describe the specific features of the hydrodynamic and acoustic pressure components. I. Introduction HE prediction of sound generated by turbomachines operating within a cylindrical duct represents one of the canonical aeroacoustic problems. A significant source of radiated sound can be structure-borne when the duct material is elastic. Direct mechanical forces as well as fluid pressure at the wall can lead to excitation of the structure. The unsteady surface pressure can be characterized in terms of both acoustic pressure and the convected, or hydrodynamic, pressure fluctuations. Prediction of the shell motion and sound radiated through the duct walls can only be made if an adequate space-time (alternatively, wavenumber-frequency) description of the wall pressure is known 1 . T