Internal shear layers generated by a vertically oscillating cylinder in unbounded and bounded rotating fluids

TL;DR

This paper investigates the formation and structure of internal shear layers in rotating fluids caused by a vertically oscillating boundary, analyzing different configurations and deriving asymptotic solutions that match numerical results.

Contribution

It introduces a new analysis of internal shear layers generated by inviscid forcing in rotating fluids, extending previous work on viscous forcing and providing asymptotic solutions for various geometries.

Findings

Inviscid shear layer amplitudes diverge as Ekman number decreases.

Global solutions match numerical results well away from critical lines.

Viscous solutions perform excellently near critical lines.

Abstract

In rotating fluids, the viscous smoothing of inviscid singular inertial waves leads to the formation of internal shear layers. In previous works, we analysed the internal shear layers excited by a viscous forcing (longitudinal libration) in a spherical shell geometry (He \textit{et al.}, \textit{J. Fluid Mech.} {\bf 939}, A3, 2022; {\bf 974}, A3, 2023). We now consider the stronger inviscid forcing corresponding to the vertical oscillation of the inner boundary. We limit our analysis to two-dimensional geometries but examine three different configurations: freely-propagating wave beams in an unbounded domain and two wave patterns (a periodic orbit and an attractor) in a cylindrical shell geometry. The asymptotic structures of the internal shear layers are assumed to follow the similarity solution of Moore \& Saffman (\textit{Phil. Trans. R. Soc. Lond. A}, 264 (1156), 1969, 597-634) in the small viscous limit. The two undefined parameters of the similarity solution (singularity strength and amplitude) are derived by asymptotically matching the similarity solution with the inviscid solution. For each case, the derivation of the latter is achieved either through separation of variables combined with analytical continuation or the method of characteristics. Global inviscid solutions, when obtained, closely match numerical solutions for small Ekman numbers far from the critical lines, while viscous asymptotic solutions show excellent performance near those lines. The amplitude scalings of the internal shear layers excited by an inviscid forcing are found to be divergent as the Ekman number $E$ decreases, specifically $O(E^{-1/6})$ for the critical point singularity and $O(E^{-1/3})$ for attractors, in contrast to the convergent scalings found for a viscous forcing.