Laboratory generation of internal waves from sinusoidal topography

In the first of a multistage process to understand the generation of internal waves from rough topography, we have performed laboratory experiments to study wave generation over and in the lee of small- and large-amplitude sinusoidal topography. The model hills are towed at a range of speeds along the surface of a uniformly salt-stratified fluid. The experiments show that internal waves are generated not only by flow over the hills but also by flow over “boundary-trapped” lee waves and turbulent structures in the lee. Waves are visualized and their characteristics measured using a nonobtrusive optical technique called “synthetic schlieren”. Experimental results are compared with the predictions of linear theory and Longʹs model. For low values of the excitation frequency, the internal wave frequencies are consistent with those predicted by linear theory. The wave amplitudes, however, are significantly lower than the hill amplitude, even for the small hills that have a maximum slope of 0.3. This indicates the importance of nonlinear processes, such as boundary layer separation, which act even for moderate hill slopes. When the excitation frequency exceeds the buoyancy frequency, internal waves are still excited in the lee of the topography, with frequency an approximately constant fraction of the buoyancy frequency. In these cases, boundary-trapped lee waves and turbulent structures are observed and couple with vertically propagating internal waves.