Whispering gallery like modes along pinned vortices

TL;DR

This paper investigates sound modes in vortex flows around a cylinder, revealing whispering gallery-like modes that could impact the stability of such fluid systems, especially in supersonic regimes.

Contribution

It introduces the existence of whispering gallery-like modes in vortex flows using an analogy to gravity, highlighting their potential role in stability analysis.

Findings

Discovery of bound whispering gallery-like modes along vortices

Modes can have zero frequency in supersonic flows

Implications for stability of vortex configurations

Abstract

Employing Unruh's analogy to gravity, we study sound propagation in stationary and locally irrotational vortex flows where the circulation is wound around a long (rotating) cylinder. 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. For supersonic flow velocities (corresponding to an effective ergoregion), these modes can even have zero frequency. Thus they should be relevant for the question of stability or instability of this set-up.