Generation, propagation, and breaking of an internal 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. The oscillatory forcing leads to the development of turbulence in the region bounding the cylinder. This turbulence is found to be the primary source of the observed quasimonochromatic wave beams, whose characteristics at early times differ from theoretical predictions and experimental investigations of waves generated by small-amplitude cylinder oscillations. In particular, their wavelength is set by the Ozmidov scale rather than the size of the cylinder. The wave frequency is set by the buoyancy frequency N if the cylinder frequency is larger or much less than N. Otherwise it is set by the cylinder oscillation frequency. Over long times the finite-amplitude waves that have propagated away from their source are observed to break down and the process is examined quantitatively through conductivity probe measurements and qualitatively through unprocessed synthetic schlieren images. From an analysis 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, although not spatially, monochromatic internal wave beam. © 2010 American Institute of Physics. doi:10.1063/1.3455432