Excitation and resonant enhancement of axisymmetric internal wave modes

TL;DR

This paper introduces a novel wave generator for axisymmetric internal waves, explores resonance phenomena, and observes non-linear transitions near resonance conditions, advancing understanding of wave mode excitation and behavior.

Contribution

It presents a new wave generation method for arbitrary axisymmetric internal wave modes and investigates resonance effects with experimental validation.

Findings

Resonant peaks match theoretical predictions.

Vertical confinement induces resonance.

Near-resonance, wave fields become non-linear.

Abstract

To date, axisymmetric internal wave fields, which have relevance to atmospheric internal wave fields generated by storm cells and oceanic near-inertial wave fields generated by surface storms, have been experimentally realized using an oscillating sphere or torus as the source. Here, we use a novel wave generator configuration capable of exciting axisymmetric internal wave fields of arbitrary radial form to generate axisymmetric internal wave modes. After establishing the theoretical background for axisymmetric mode propagation, taking into account lateral and vertical confinement, and also accounting for the effects of weak viscosity, we experimentally generate and study modes of different order. We characterize the efficiency of the wave generator through careful measurement of the wave amplitude based upon group velocity arguments. This established, we investigate the ability of vertical confinement to induce resonance, identifying a series of experimental resonant peaks that agree well with theoretical predictions. In the vicinity of resonance, the wave fields undergo a transition to non-linear behaviour that is initiated on the central axis of the domain and proceeds to erode the wave field throughout the domain.