Generation, propagation and breaking of an internal gravity wave beam

We report upon an experimental study of internal gravity waves generated by the large-amplitude vertical oscillations of a circular cylinder in uniformly stratified fluid. Quantitative measurements are performed using a modified synthetic schlieren technique for strongly stratified solutions of NaCl or NaI. Oscillatory turbulent patches that develop around the cylinder are found to be the primary source of the observed quasi-monochromatic wave beams whose characteristics differ from theoretical predictions and experimental investigations of waves generated by small-amplitude cylinder oscillations. Over long times the waves break down into turbulence that is examined quantitatively through conductivity probe measurements and qualitatively through unprocessed synthetic schlieren images. Based on observations of the location of wave breakdown we determine that the likely mechanism for breakdown is through parametric subharmonic instability. This conclusion is supported by fully nonlinear numerical simulations of the evolution of a temporally monochromatic internal wave beam.