Turbulent mixing at a shear-free density interface

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 R_j, defined in terms of an integral lengthscale, buoyancy jump and turbulence intensity. The spectra of the internal waves decay as f^(−3) at frequencies below the maximum Brunt-Väisälä 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_j range, the thickness of the interfacial layer (normalized by the integral lengthscale of the turbulence) is a decreasing function of R_j. At sufficiently high R_j the interfacial thickness becomes limited by diffusive effects. Finally, we discuss a simple model for entrainment at a density interface in the presence of shear-free turbulence

Turbulence structure near a sharp density interface

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

Thermoclinic Assessment Of A Preliminary Circulation Model For Lake George In The Jefferson Project

The Jefferson Project is a collaboration between the Rensselaer Polytechnic Institute, IBM, and the FUND for Lake George aimed at understanding and managing complex factors (road salt, storm water runoff, invasive species) threatening Lake George, New York. Lake George is located about 80 km north of Albany in upstate New York and is known internationally for its water clarity. Understanding the hydrodynamics of the lake is fundamental for creation and maintenance of a research and monitoring program for the early detection of and response to adverse environmental and biological change. In this work a 3D circulation model of the lake is developed to better understand the hydro-environmental conditions of the lake; forcing is by a combination of local public survey data for the water budget and atmospheric data from the NWS (NOAA National Weather Service). The model is validated by a combination of water chemistry data collected by Darrin Fresh Water Institute (DFWI) over the last three decades, and known empirical relationships of the lake\u27s structural profile. Numerical simulations run over several years to capture the seasonal progression of thermocline depth throughout the lake, the south to north salt and surface thermal gradients and the timing of the spring and fall overturn events. Validation is by comparison with physical and chemical measurements collected over the last three decades. The study presents a novel combination of observational data, numerical modelling and empirical relationships to better understand and predict the lake circulation, and consequently the natural ecosystem

Turbulent Mixing in Stably-Stratified Fluids Subjected to Zero-Mean Shear

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 a tracer that was initially located in the less dense layer. A laser-Doppler velocimeter was used to measure the vertical component and a horizontal component of velocity near the interface, and also at a point where tracer concentration was measured. Potential refractive index fluctuation problems were avoided using solutes that provided a homogeneous optical environment. The study consists of two major parts. In the first part of the investigation, energy spectra, velocity correlations, and kinetic energy fluxes were measured. Amplification of the horizontal turbulent velocity fluctuations, coupled with a sharp reduction in the vertical velocity fluctuation level, was observed near the density interface. Moreover, the experiments indicate that the density interface acts in a manner qualitatively similar to a rigid flat plate inserted in the flow. These findings are in agreement with previous results pertaining to shear-free turbulence near rigid walls (Hunt and Graham 1978). In the second part of the investigation, internal wave spectra, wave amplitudes and velocities, and the interfacial mixing layer thickness were measured. The results indicate that mixing occurs in intermittent bursts and that the local gradient Richardson number J remains constant for a certain range of the overall Richardson number Rj. The spectra of the internal waves decay as f-3 at frequencies below the maximum Brunt-Väisälä frequency. These findings give support to a model of oceanic turbulence 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 Rj range, the thickness of the interfacial layer (normalized by the integral lengths scale of the turbulence)is a decreasing function of Rj. At sufficiently high Rj the interfacial thickness becomes limited by diffusive effects. A simple model for entrainment at a density interface in the presence of shear-free turbulence is presented and compared with the observations.</p