Abstract
We study sound propagation in stationary and locally irrotational vortex flows where the circulation is wound around a long (rotating) cylinder, using Unruh’s formalism of acoustic space-times. Apart from the usual scattering solutions, we find anomalous modes which are bound to the vicinity of the cylinder and propagate along its axis—similar to whispering gallery modes. These modes exist for subsonic and supersonic flow velocities. In the supersonic case (corresponding to an effective ergoregion in the acoustic space-time), they can even have zero frequency ω = 0 and thus the associated quasiparticles with E = ħω = 0 are easy to excite from an energetic point of view. Hence they should be relevant for the question of stability or instability of this setup.
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