Internal waves generated from a turbulent mixed region

Using two nonintrusive visualization methods, laboratory experiments are performed to examine the internal wave field underlying a turbulent region generated by a vertically oscillating grid. The first method uses dye-lines to mark the vertical motions of isopycnal layers and the second uses “synthetic schlieren” to visualize the entire wave field. In a range of experiments, the strength of the stratification is varied so that the buoyancy frequency N=0.33–1.40 s−1. In all cases, large tank-scale standing wave modes are established which last throughout the experiment. The amplitudes of the isopycnal lines, Aξ, follow a power law relation Aξ∼N−1.5. The synthetic schlieren technique allows us to visualize turbulent eddy-scale waves and to isolate the properties of the strongest downward propagating waves at the base of the turbulent layer. These waves have a surprisingly narrow range of frequencies and vertical wavenumbers. The angles of wave propagation from the vertical for the dominant waves lie in the range Θ=42°–55°. The amplitudes in the wave field follow a relation Aξ∼N−1.68. These waves are of large amplitude: their vertical displacement is from 2% to 4% of their horizontal wavelength, a significant fraction of the breaking amplitude.