The entrainment interface in a stratified fluid

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Abstract
A study is made of the entrainment interface in a two-layer flow with one layer turbulent owing to an imposed surface stress. Turbulent and wave parameters were measured from films of an experiment carried out by Kantha, Phillips & Azad (1977) and are analysed to determine relationships between wave and turbulent length and velocity scales and bulk scales.A measure of interface distortion is calculated and it is found to be related to the mixed-layer depth only at small overall Richardson number. The local velocity of advance of the interface is determined and is found to vary with overall Richardson number tending to the Kolmogorov scale at small Ri and to Phillips’ (1966) quasilaminar scale at large Ri.Kelvin-Helmholtz type instabilities are observed and their wavelength, amplitude and lifetime are measured. The wavelength is not found to be related to the mixed-layer depth but some dependence on overall Richardson number is observed. An interfacial mean flow Richardson number is estimated and the variation of wave slope at maximum amplitude with this Richardson number is found to be comparable with observations of Thorpe (1973a) in a laminar two-layer flow. A relationship between the wavelength of an instability and the distortion length scale is found and is discussed.