Linear stability of a rotating liquid column revisited

TL;DR

This paper revisits the classical problem of the linear stability of a rotating liquid column, clarifying the relationship between inviscid and viscous stability criteria through detailed spectral analysis.

Contribution

It reveals the complex structure of inviscid stability in the We-k plane and clarifies the conditions under which viscous stability criteria are sufficient or necessary.

Findings

Viscous stability criterion is both necessary and sufficient.

Inviscid stability exhibits an infinite hierarchy of stable islands.

Dominant stable island persists for azimuthal wavenumber n=1.

Abstract

We revisit the somewhat classical problem of the linear stability of a rigidly rotating liquid column in this communication. Although literature pertaining to this problem dates back to 1959, the relation between inviscid and viscous stability criteria has not yet been clarified. While the viscous criterion for stability, given by $We = n^2+k^2-1$, is both necessary and sufficient, this relation has only been shown to be sufficient in the inviscid case. Here, $We = \rho \Omega^2 a^3/\gamma$ is the Weber number and measures the relative magnitudes of the centrifugal and surface tension forces, with $\Omega$ being the angular velocity of the rigidly rotating column, $a$ the column radius, $\rho$ the density of the fluid, and $\gamma$ the surface tension coefficient; $k$ and $n$ denote the axial and azimuthal wavenumbers of the imposed perturbation. We show that the subtle difference between the inviscid and viscous criteria arises from the surprisingly complicated picture of inviscid stability in the $We-k$ plane. For all $n >1$, the viscously unstable region, corresponding to $We > n^2+k^2-1$, contains an infinite hierarchy of inviscidly stable islands ending in cusps, with a dominant leading island. Only the dominant island, now infinite in extent along the $We$ axis, persists for $n= 1$. This picture may be understood, based on the underlying eigenspectrum, as arising from the cascade of coalescences between a retrograde mode, that is the continuation of the cograde surface-tension-driven mode across the zero Doppler frequency point, and successive retrograde Coriolis modes constituting an infinite hierarchy.