LOCALIZED MIXING DUE TO A BREAKING INTERNAL WAVE RAY AT A SLOPING BED

A laboratory experiment was conducted to investigate the characteristi cs of turbulence generated by an internal wave ray breaking on a slopi ng bed. The width of the incident wave ray was small compared to the b ed length, so that an isolated turbulent patch was generated by the br eaking process, a configuration unique to the present study. The param eter range covered subcritical, critical and supercritical frequencies . Flow visualization and velocity measurements revealed that near crit ical conditions the flow was confined to a narrow region above the bed and, contrary to expectations, critical waves showed a weak turbulenc e field. Subcritical and supercritical reflection resembled wave-wave interaction between the incident and the reflected waves and showed co mparable centred displacement lengthscales. As the incident waves beca me progressively supercritical instabilities were first initiated away from the bed. For supercritical waves the centred displacement length scale and the turbulent Reynolds number both increased steadily up to about gamma approximate to 2, after which they started to decrease (ga mma = omega/omega(c), where omega is the frequency of the incident wav e 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 su bcritical waves an increase in the centred displacement lengthscale an d the turbulent Reynolds number was also observed. The mixed fluid gen erated at the boundary collapsed into the fluid interior in the form o f a horizontal two-dimensional viscous-buoyancy intrusion: the efficie ncy of mixing was, however, very small and no measurable change in the mean density gradient was observed over the duration of the experimen ts.