DYNAMICS OF FREE SURFACE GRAVITY CURRENTS IN LOCK EXCHANGE FLOWS

摘要

Preliminary results from an on-going study into the dynamics of free surface gravity currents are presented. A lock exchange experimental configuration is used to explore the bulk characteristics of the release of fresh water into a saltwater environment. The general structure of the current, the depth of the current head and its speed of propagation, as functions of Reynolds numbers, are described. The results clearly demonstrate two regimes, where the flow structure is markedly different, although the parameters that define these regimes are not altogether clear. Unlike solid boundary, and intrusive, currents, high Reynolds number free surface currents do not exhibit a constant velocity and constant head depth regime. The depth of the current decays continuously after release, while a Froude number based on the head depth and propagation speed, remains approximately constant with a value of 1.0. This value is significantly larger than that for both solid boundary and intrusive currents. The nature of the free surface boundary is characterised by the speed of the surface relative to the current nose. Experimental observations indicate the rather unexpected result that, for high Reynolds numbers, the speed of the surface is roughly 80% of the current beneath, while for low Reynolds numbers the surface moves at 95% of the current speed. This latter result is in marked contrast to the conclusions drawn by Didden and Maxworthy (1982) for viscous surface currents who deduced from their experimental results that the free surface acted as a rigid, no-slip, boundary.