Flow past the opening of a cavity can excite acoustic resonance. The periodic velocity field in the region of the cavity opening has been measured for various flow conditions and the vorticity distribution has been computed from the measured data using numerical differentiation. The results indicate that the shear layer rolls up into discreet vortices, which travel across the cavity opening. Two resonances were found. The first is characterized by a single vortex being present and results in the greatest amplitude. The second is characterized by the presence of two vortices which excite a distinct but lesser amplitude resonance. As the flow velocity changes, the position of these vortices shifts relative to the phase of the acoustic cycle. The timing of the interaction between the moving vortices and the acoustic particle velocity determines, to a large extent, the intensity of the excitation, and therefore also the oscillation amplitude. The measurements also indicate that the boundary layer upstream of the leading edge pulsates considerably.


Mechanical Engineering



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