Turbulent entrainment at the density interface of a stable two-layer stratified fluid is studied in the laboratory, a constant surface stress being applied at the free surface. Conservation of mass requires that the overall Richardson number Ri = Dgδρ/ρu*2 is constant in each experiment, where D is the depth of the mixed layer, gδρ/ρ the buoyancy difference and u* the friction velocity. If the entrainment rate E = ue/u* is a function only of Ri, it is therefore constant in each experiment and can be measured with a greater accuracy than has previously been attained. The functional dependence of ue/u* on Ri is established over the range 30 < Ri < 1000; it is found not to follow any simple power law. The entrainment rates are considerably higher than those measured by Kato & Phillips (1969), for which the fluid below the mixed layer was linearly stratified. Such a condition allows internal gravity waves to be radiated downwards and the reduction in entrainment rate is consistent with that found by Linden (1975).

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ABSTRÀCTAn experimentar study of the structure of turburence in a conical diffuser having a total divergence angle of Bo and an area ratio of 4:r with futly developed frow at entry is described.

Turbulent flow in a conical diffuser has been examined through determination of the mean pressures, mean strain rates, energy, shear stress, triple products, length scale, and balances of energy and shear stress. Some quantities are more revealing than others in pointing out the complexity of a flow subjected to an adverse pressure gradient. Sudden application of extra strain rate to the flow seems to affect conditions so drastically that a final picture of the flow in terms of mean and turbulent fields becomes unrecognizable in relation to the initial condition. Initially all the measured parameters show rapid changes in their values and thus characterize a region of importance. Dynamics of the flow reflected by some parameters such as kinetic energy attain asymptotic values in the outlet region of the diffuser and, in turn, show a type of equilibrium of the flow. Additionally, the expanding wall region toward the exit has a low mean velocity and very high turbulence intensity. Finally, there is a growth of core region in the last one-third of the diffuser; this layer shows exceptionally high values of the higher moments.

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