Localized mixing due to a breaking internal wave ray at a sloping bed

A laboratory experiment was conducted to investigate the characteristics of turbulence generated by an internal wave ray breaking on a sloping bed. The width of the incident wave ray was small compared to the bed length, so that an isolated turbulent patch was generated by the breaking process, a configuration unique to the present study. The parameter range covered subcritical, critical and supercritical frequencies. Flow visualization and velocity measurements revealed that near critical conditions the flow was confined to a narrow region above the bed and, contrary to expectations, critical waves showed a weak turbulence field. Subcritical and supercritical reflection resembled wave-wave interaction between the incident and the reflected waves and showed comparable centred displacement lengthscales. As the incident waves became progressively supercritical instabilities were first initiated away from the bed. For supercritical waves the centred displacement lengthscale and the turbulent Reynolds number both increased steadily up to about gamma approximate to 2, after which they started to decrease (gamma = omega/omega(c), where omega is the frequency of the incident wave and omega(c), = N sin beta is the critical frequency for an ambient uniform stratification of magnitude N and a bed angle of beta). For subcritical waves an increase in the centred displacement lengthscale and the turbulent Reynolds number was also observed. The mixed fluid generated at the boundary collapsed into the fluid interior in the form of a horizontal two-dimensional viscous-buoyancy intrusion: the efficiency of mixing was, however, very small and no measurable change in the mean density gradient was observed over the duration of the experiments.