Numerical study of internal wave-caustic and internal wave-shear interactions in a stratified fluid

The behaviour of internal waves at a caustic level, turning point and criti cal layer have been investigated numerically. At a caustic reflection, a tr iad interaction was formed within the reflection region and the internal wa ve energy was transferred to lower frequencies (subharmonics). This resulte d in a local subharmonic instability. One of the excited internal waves pen etrated the caustic level and propagated downwards. This downward propagati ng wave then produced a second caustic where further reflection could take place. At a turning point, nonlinear interaction between the incident and r eflected waves transferred energy to higher frequencies (evanescent trapped waves) which resulted in a superharmonic instability. At the critical leve l, energy was transferred to the mean flow. As the degree of nonlinearity i ncreased, more energy was found to be transferred and overturning resulted due to a shear instability.