Experimental investigation of internal tide generation by two-dimensional topography using synthetic Schlieren

Abstract

An experimental investigation of internal tide generation at two-dimensional topography was carried out using the synthetic Schlieren experimental technique. Two linear models were tested: Balmforth, Ierley and Young's [1] subcritical solution for a Gaussian ridge and Hurley and Keady's [15] super-critical solution for a knife-edge. The former was modified to account for the effects of viscosity in the propagating wave beams. The experiment set up comprised a wave-tank with a linear salt-water stratification and a sliding stage on which the topography oscillated horizontally to simulate tidal flow. The wave field was measured by capturing the distortion of a pattern of random dots placed on a light sheet behind the tank using a CCD camera, and using the synthetic Schlieren processing of the movies obtained. Four experiments were performed, for wave beams propagating close to 25 ° and 56 from the horizontal for each topographic feature. The subcritical theory over-predicted the peak disturbance over the Gaussian ridge by a maximum of 50%, and it correctly predicted the profile shape and evolution along the wave beam and throughout one period of the oscillation.

(cont.) The supercritical knife-edge theory predicted the disturbance amplitude, shape and evolution within experimental error. The results showed that at Reynolds numbers below 0(105), viscosity suppresses nonlinear effects and smoothes out instabilities predicted by inviscid models that would lead to overturning. These experiments have motivated the construction of a larger wave-tank to achieve higher Reynolds numbers. Future experiments will investigate nonlinear internal tide generation, overturning and mixing in unstable wave beams and flow separation over topography.