The effects of a sharp density interface and a rigid flat plate on oscillating-grid induced shear-free turbulence were investigated experimentally. A two-component laser-Doppler velocimeter was used to measure turbulence intensities in and above the density interface (with matched refractive indices) and near the rigid flat plate. Energy spectra, velocity correlations, and kinetic energy fluxes were also measured. Amplification of the horizontal turbulent velocity, coupled with a sharp reduction in the vertical turbulent velocity, was observed near both the density interface and the flat plate. These findings are in agreement with some previous results pertaining to shear-free turbulence near rigid walls (Hunt & Graham 1978) and near density interfaces (Long 1978). The results imply that, near the density interface, the turbulent kinetic energy in the vertical velocity component is only a small fraction of the total turbulent kinetic energy and indicate that the effects of the anisotropy created by the density interface or the flat plate are confined to the large turbulence scales.

The interaction of a sharp density interface with oscillating-grid-induced shear-free turbulence was experimentally investigated. A linear photodiode array was used in conjunction with laser-induced fluorescence to measure the concentration of dye that was initially only in the less dense layer. A laser-Doppler velocimeter was used to measure the vertical velocity in and above the density interface at a point where the dye concentration was also measured. Potential refractive-index-fluctuation problems were avoided using solutes that provided a homogeneous optical environment across the density interface. Internal wave spectra, amplitudes and velocities, as well as the vertical mass flux were measured. The results indicate that mixing occurs in intermittent bursts and that the gradient (local) Richardson number remains constant for a certain range of the overall Richardson number R3, defined in terms of an integral lengthscale, buoyancy jump and turbulence intensity. The spectra of the internal waves decay as f -' a t frequencies below the maximum Brunt-Vaisala frequency. These findings give support to a model for oceanic mixing proposed by Phillips (1977) in which the internal waves are limited in their spectral density by sporadic local instabilities and breakdown to turbulence. The results also indicate that, for a certain R, range, the thickness of the interfacial layer (normalized by the integral lengthscale of the turbulence) is a decreasing function of R3. At sufficiently high R, the interfacial thickness becomes limited by diffusive effects. Finally, we discuss a simple model for entrainment a t a density interface in the presence of shearfree turbulence.

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Lake Mead, the largest-volume man-made reservoir in the United States, faces a variety of challenges, including increasing demands for municipal water, 10 years of drought in the Colorado River system, lower water surface elevations, discharges of highly treated wastewater effluent, invasive mussels, and climate change. Lake Mead is an important source of water for 25 million people in the southwest U.S. and is also a National Recreation Area. Thus, it is imperative that the lake be adequately protected and managed to meet the often competing needs of the multiple users. A well-calibrated and validated three-dimensional hydrodynamic and water quality model of Lake Mead has been a key component of this management strategy, enabling hydrodynamics and water quality within the reservoir to be predicted and assessed for a wide range of anticipated conditions. The model was developed using the ELCOM and CAEDYM simulation codes, and has been calibrated and validated for the 2000-2008 period using measured field data for temperature, conductivity, perchlorate, bromide, chlorophyll a, nutrients (phosphorus and nitrogen), total organic carbon, pH, and dissolved oxygen. The model captured the hydrodynamics and water quality of this complex system well, and the standard errors of the model results for selected parameters were found to be larger than, but of the same order of magnitude, as the accuracy of the measured field data.

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