Journal of Fluid Mechanics



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


I. P. D. DE SILVA a1c1, J. IMBERGER a1 and G. N. IVEY a1
a1 Centre for Water Research, Department of Environmental Engineering, The University of Western Australia, Nedlands, Western Australia 6907

Abstract

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 γ[approximate]2, after which they started to decrease (γ=ω/ωc, where ω is the frequency of the incident wave and ωc=Nsinβ is the critical frequency for an ambient uniform stratification of magnitude N and a bed angle of β). 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.

(Received March 18 1996)
(Revised June 13 1997)


Correspondence:
c1 Present address: Department of Mechanical Engineering, Sirindhorn International Institute of Technology, Thammasat University, PO Box 22, Thammasat-Rangsit Post Office, Pathumthani 12121, Thailand.


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