Internal waves revisited

We describe a new laboratory technique that has been developed to examine the structure and amplitude of internal waves. As well as being relatively inexpensive to set up, the technique is sensitive to small density fluctuations: heat rising from a hand can easily be seen. If the internal wave field is uniform across the span of the tank, then the density gradient field can be measured non-intrusively everywhere in space and time. We use this technique to measure the amplitude of internal waves generated by a circular cylinder that oscillates at an angle with the vertical, and we examine how the amplitude and phase of the waves changes as a function of the angle of oscillation. The experimental results are compared with analytic theory. Generally, the theory agrees well with experimental results although some consistent discrepancies exist which in part we attribute to the effects of a viscous boundary layer surrounding the cylinder.