A shear instability induced by a composite internal wave

Abstract

Experiments have been performed in a thermally stratified two-layer shear flow with a nonzero mean velocity to observe the effects of artifically introduced low frequency internal waves on higher frequency, Kelvin–Helmholtz waves growing spatially on the density interface. Phillips’ model for wave-induced shear across an interface is extended to the two first-mode wave trains which can propagate in such a flow. It is shown that the Richardson number becomes a function of the phase of the internal composite waves and that its minimum value is either at the front or at the back face of the waveform depending on the sign of the background vorticity. Measurements are presented of the phase-averaged temperature and velocity fields of generated internal waves and show the waves breaking down on the face where the Richardson number was found to be at a minimum. The breakdown is characterized by the rapid growth of high frequency waves which appear to be phase locked on the parent internal gravity wave. Possible instances where this type of breakdown could occur in nature are discussed.