| Laboratory experiments are performed to study internal wave generation from finite-amplitude periodic topography. Model hills of varying steepness are towed at constant 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 by vigorous turbulence created in the lee of steep hills. Waves are visualized and their characteristics measured using the non-intrusive optical technique, "synthetic schlieren". Experimental results are compared with the predictions of linear theory and Long's model. For low values of the excitation frequency, linear theory predicts well the internal wave frequencies but significantly overestimates the wave amplitudes. This indicates the importance of nonlinear processes such as boundary layer separation. When the excitation frequency exceeds the buoyancy frequency, turbulence is created and results in the excitation of internal waves within a narrow frequency range. |
