Sound emission by a vortex ring passing through a circular hole in a large flat plate

Acoustic emission has been studied experimentally when a vortex ring passes through a circular hole in a large flat plate, along its normal axis. The speed of the vortex ring is made high enough for the sound emission to be detectable, but can be regarded as sufficiently low in comparison to the sound speed. Substantial monopole and quadrupole components are observed in the detected wave profiles. Translational motion of the vortex ring in the presence of the flat plate with a hole has been observed optically, and its relation with the sound emission is determined. In this case, the power law of the acoustic pressure amplitude of monopolar vortex sound versus the translation speed U of the vortex ring is first measured in detail and is found to be U^2.1–U^2.4. This means that experimentally determined powers for the monopole components in the two half-spaces also agree approximately with the corresponding values predicted by the theory of vortex sound.