Density current propagation over horizontal boundaries: An experimental study.
Paez-Rivadeneira, Diana Lucia
1997
Abstract
Density currents are typified by flows along the bottom of oceans or lakes. Such currents occur because the water flowing into the receiving water is colder, saltier or contains more suspended sediment. Numerous studies of density currents have been carried out over several years, and still inconsistencies between predictions and experimental results have not been resolved. Previous experimental studies showed density currents with slower propagation speeds and larger density current thicknesses than those prescribed by a momentum balance analysis. Modifications to the basic momentum balance analysis have been developed to account for the observed slower propagation speeds; such modifications are not predictive in that there are required empirical coefficients to describe the experimental observations. In this study various analytical models that describe density currents were evaluated in terms of their ability to reproduce the experimental results. A formulation based on energy considerations, a minimum energy condition, was studied and its applicability over the whole range of flow conditions was tested. This formulation allows for density currents with larger thicknesses and slower propagation speeds than those prescribed by any momentum balance analysis. The minimum energy formulation predicts density current characteristics in an absolute frame of reference while the formulations based on the momentum balance analysis express their results in a frame of reference moving with the current front. Therefore significant differences in predicted propagation speeds arise among the various theories in situations where the receiving fluid is allowed to move in counter or coflowing conditions. In this investigation an experimental study of density currents flowing over horizontal boundaries was performed. The experimental work consisted of density currents produced by a continuous release of a heavy fluid (salt water flow) into a lighter fluid (fresh water ambient flow). The experimental setup provided flexibility to impose arbitrary flow conditions in the receiving fluid. The experiments were performed over a wide range of flow configurations to test conditions in which significant differences between various theoretical predictions were present. Extreme conditions such as very large coflows and counterflows were tested. Measurements of frontal velocity, density profiles, total water depth and flow rates were made during the experiments. It was found in this investigation that the minimum energy formulation was able to describe density current characteristics such as front propagation speed and current thickness better than any of the other formulations reviewed. Limits on current propagation were established in terms of the maximum possible relative current thickness as described by the minimum energy condition. Counterflows were limited by a maximum intrusion flow rate and coflows were limited by a maximum propagation speed.Subjects
Boundaries Current Density Experimental Horizontal Over Propagation Saline Intrusions Study Turbulence
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