PROJECT SUMMARY REPORT

Project 2

Project Title	Experiments on Gravity-Driven Surface Currents Simulating Fresh-Water River Discharges into the Ocean
Name of Group Leader	Dr. Peter J. Thomas
Home Laboratory	Fluid Dynamics Research Centre, School of Engineering, University of Warwick, United Kingdom
E-mail address	pjt@eng.warwick.ac.uk
Telephone	44 (0)24 76522200

1. Project objectives (no more than 10 lines)

The objective of the project was to carry out a large-scale experimental laboratory study investigating the influence of background rotation on the flow physics of gravity-driven surface currents. Features such as the current width, their depth, their speed of propagation as well as their internal velocity and density structures were studied. The goal was to obtain results supplementing similar data collected in a previous small-scale study. The analysis of the small-scale data had revealed discrepancies which are believed to have arisen as a consequence of the limited size and the limited parameter regime feasible in small-scale experiments. For the detailed data analysis, following our present experiments, corresponding results from large- and small-scale experiments will be compared to each other and to results from computational models. The goal is to resolve the open questions which emerged from the small-scale data and, ultimately, to improve understanding by comparisons of different modeling approaches.

2. Main achievements and difficulties encountered (no more than 20 lines)

The goals of the experimental study have been fully achieved. The experiments proceeded as anticipated and according to schedule. No major, unforeseen difficulties were encountered. Extensive new data sets suitable for comparison with the results from small-scale experiments and computational models were obtained.

The preliminary data analysis has revealed similarities as well as significant differences between the results of large-scale and small-scale experiments. For instance, the depth of large-scale currents as well as the current width visible on the fluid surface appear to scale similar to the behaviour observed in small-scale experiments. However, the results for the propagation speed of the currents in small- and large-scale experiments reveal differences. Nevertheless, these differences are consistent with observed trends which had already indicated that the small-scale experiments might be biased by scale effects.

The measurements of the internal velocity and density structure of the large-scale currents have confirmed our speculation that the current width, as identified on the fluid surface through dye visualization experiments is not the appropriate quantity to characterize the current dynamics. Our new experimental data show that the bulk of the fluid transport takes place in a wall-near region much narrower than the current width identified on the fluid surface. This wall-near region appears to be in geostrophic balance. However, the data reveal the existence of a thin and substantially wider top layer which is probably not in geostrophic balance.