Internal Waves in Laboratory Experiments

Recent laboratory methods developed to generate and to analyze internal waves have revealed important insights that sometimes guide and sometimes challenge theoretical predictions and the results of numerical simulations. Here we focus upon observations of internal waves generated by oscillatory or uniform flow over a solid boundary in a continuously stratified fluid. In the laboratory such flows are often examined by moving the boundary in an otherwise stationary ambient. This can be done by oscillating or towing an object, or by sequentially oscillating a series of plates to simulate a translating or standing-wave-like boundary. The generated waves are sometimes visualized and measured using a non-intrusive optical method called synthetic schlieren. This takes advantage of the changes of the refractive index with salinity which differentially bends light passing through salt-stratified fluid, a property that hampers the use of more standard non-intrusive observation methods such as Particle Image Velocimetry (PIV). From schlieren images, filtering methods can be applied to distinguish between leftward and rightward as well as upward and downward propagating two-dimensional waves. Inverse tomographic techniques can be employed to measure the structure and amplitude of axisymmetric and relatively simple three-dimensional disturbances. Increasingly PIV has been used, sometimes in connection with synthetic schlieren, to measure the velocity field associated with vertically propagating internal waves, internal wave modes and internal solitary waves. Implications and applications to oceanographic internal waves are discussed.