Simultaneous optical velocity and density measurements of resonant nonlinear internal waves in pycnocline

We report on an experimental study of two-dimensional resonant internal standing waves excited by an immersed oscillating cylinder in a narrow rectangular basin. The basin is filled by two miscible liquids of close densities separated by a thin pycnocline. Simultaneous measurements of both velocity components by Particle Image Velocimetry and of the density field Background-Oriented Schlieren revealed complex spatiotemporal structure of the internal wavefield; each temporal harmonic contains multiple coexisting spatial modes. The study emphasizes the contribution of the nonlinear effects. A theoretical model is suggested that guides the data analysis procedure developed to identify individual modes and to reveal interactions between them. The model reproduces adequately the decrease of about 10% in the amplitude of the density variation due to cubic nonlinearity.