A shallow-water theory for annular sections of Keplerian Disks

TL;DR

This paper develops a shallow water theoretical framework for analyzing non-axisymmetric disturbances in thin Keplerian disks, connecting vortex dynamics to three-dimensional disk behavior and revealing potential vorticity conservation and instabilities.

Contribution

It introduces a novel shallow water model for Keplerian disks using asymptotic scaling, linking geophysical fluid dynamics concepts to astrophysical disk phenomena.

Findings

Derivation of a radially geostrophic, vertically hydrostatic model

Identification of a conserved potential vorticity quantity

Support for the existence of Rossby edgewaves and strato-rotational instability

Abstract

A scaling argument is presented that leads to a shallow water theory of non-axisymmetric disturbances in annular sections of thin Keplerian disks. To develop a theoretical construction that will aid in physically understanding the relationship of known two-dimensional vortex dynamics to their three-dimensional counterparts in Keplerian disks. Using asymptotic scaling arguments varicose disturbances of a Keplerian disk are considered on radial and vertical scales consistent with the height of the disk while the azimuthal scales are the full $2\pi$ angular extent of the disk. The scalings lead to dynamics which are radially geostrophic and vertically hydrostatic. It follows that a potential vorticity quantity emerges and is shown to be conserved in a Lagrangian sense. Uniform potential vorticity linear solutions are explored and the theory is shown to contain an incarnation of the strato-rotational instability under channel flow conditions. Linearized solutions of a single defect on an infinite domain is developed and is shown to support a propagating Rossby edgewave. Linear non-uniform potential vorticity solutions are also developed and are shown to be similar in some respects to the dynamics of strictly two-dimensional inviscid flows. Based on the framework of this theory, arguments based on geophysical notions are presented to support the assertion that the strato-rotational instability is in a generic class of barotropic/baroclinic potential vorticity instabilities. Extensions of this formalism are also proposed. The shallow water formulation achieved by the asymptotic theory developed here opens a new approach to studying disk dynamics.