Flow-excited acoustic resonance of trapped modes in ducts has been reported in different engineering applications. The excitation mechanism of these modes results from the interaction between the hydrodynamic flow field and the acoustic particle velocity, and is therefore dependent on the mode shape of the resonant acoustic field, including the amplitude and phase distributions of the acoustic particle velocity. For a cavity-duct system, the aerodynamic excitation of the trapped modes can generate strong pressure pulsations at moderate Mach numbers (M>0.1). This paper investigates numerically the effect of mean flow on the characteristics of the acoustic trapped modes for a cavity-duct system. Numerical simulations are performed for a two-dimensional planar configuration and different flow Mach numbers up to 0.3. A two-step numerical scheme is adopted in the investigation. A linearized acoustic perturbation equation is used to predict the acoustic field. The results show that as the Mach number is increased, the acoustic pressure distribution develops an axial phase gradient, but the shape of the amplitude distribution remains the same. Moreover, the amplitude and phase distributions of the acoustic particle velocity are found to change significantly near the cavity shear layer with the increase of the mean flow Mach number. These results demonstrate the importance of considering the effects of the mean flow on the flow-sound interaction mechanism.

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