The Aerodynamic Excitation of Trapped Diametral Acoustic Modes in Rectangular Ducted Cavities

Abstract

The excitation mechanism of trapped diametral acoustic modes within a rectangular cavity-duct system is investigated both numerically and experimentally. The asymmetry inherent within the rectangular geometry introduces a preferred orientation, ensuring the excited diametral modes remain stationary. Three separate cavities are manufactured and tested. This included two asymmetric rectangular cross-sections and one symmetric square cavity. Experimental results indicate that the aeroacoustic responses of the three cavities are dominated by the strong excitation of trapped diametral modes. Numerical simulations indicate that the resolved radial acoustic particle velocity distributions are non-uniform at the upstream separation edge where the formation of vortical structures is initiated. As the cavity became smaller, and more asymmetric, the trapped nature of the acoustic modes decreased with an accompanied increase in the radiation losses and reduction in pulsation amplitude. Observations of the aeroacoustic measurements show evidence of three unique modal behaviours. The first case is the independent excitation of a single stationary mode where specific circumferential sections of the shear layer were excited and initiating the formation of vortical disturbances. These circumferential sections, and distribution of disturbances, were akin to the excited mode shape. The second case involved simultaneous excitation of two stationary modes. This suggested that the shear layer was exciting two modes simultaneously. Neighbouring circumferential sections, at the initial region of the shear layer, were being excited independently and at different resonant frequencies. Finally, a spinning trapped acoustic mode was observed in the symmetric square cavity. Due to the spinning nature, the excited circumferential portions and formation of vortices were non-uniform and rotated with the spinning acoustic mode. This resulted in the formation of a three-dimensional helical structure.