Internal wave tunnelling: Laboratory experiments

Heuristics based upon ray theory are often used to predict the propagation of internal gravity waves in non-uniform media. In particular, they predict that waves reflect from weakly stratified regions where the local buoyancy frequency is less than the wave frequency. However, if the layer of weak stratification is sufficiently thin, waves can partially transmit through it in a process called tunnelling. The first laboratory evidence of internal wave tunnelling through a weakly stratified region is analysed using the synthetic schlieren technique and the Hilbert transform is applied to filter the wavefield into upward- and downward-propagating components. Transmission is calculated as the squared ratio of transmitted and incident wave amplitude and using an appropriate superposition of plane waves to reproduce the structure of the incident wave beam, a corresponding weighted sum of transmissions can be used to predict the beam transmission. These transmission predictions are compared with experimental measurements.