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Many effluents are released into water bodies or into the atmosphere, and dilution is relied on to minimize the toxic effects of the pollutants on the environment. Dilution occurs due to entrainment and subsequent mixing of the “clean” (or cleaner) ambient fluid and the effluent stream. In the near field, dilution occurs due to momentum or buoyancy driven entrainment into the effluent stream, which is proportional to a characteristic velocity of the jet or plume. Whereas in the far field, dilution occurs due to turbulent diffusion at a rate dependent on the level of turbulence in the receiving fluid. In order to predict the expected dilution of an effluent stream, it is necessary to answer the question, at what point does the dilution mechanism change and how does it affect dilution rates. An experimental study examining the velocity and concentration decay of plane jets released into shallow coflows with increasing levels of external turbulence, indicated that levels of external turbulence just large enough to disrupt the jet structure reduced dilution rates significantly. This indicates that models, in which the jet dilution and turbulent diffusion are superimposed, will not always give a conservative estimate of effluent dilution.

Ocean outfalls are modelled as buoyant jets. The final behaviour in a deep ocean is the advected line thermal, which is dominated by the jet buoyancy and the ambient velocity. The dilution of the effluent along its path is of interest, in some cases the traditional time averaged information is needed and in others the instantaneous or peak concentrations are needed. Laser induced fluorescence (LIF) is used to study the instantaneous flow structure and the mixing dynamics of the advected line thermal. Large scale strucmres in the flow are identified as subthermals and their development influences the flow behaviour. Statistical measures of the magnitude of the concentration fluctuations and the intermittency of the flow are obtained from LIF images.