Abstract
This paper presents a theoretical and experimental study of the exchange of fluids of different densities through an opening. Three types of openings are examined: a bottom opening (the opening is at the bottom of a gate), a middle opening (in the middle) and a window opening (the opening is in the middle but does not extend across the width). Simultaneous measurements of velocity field and interface position were obtained using flow visualization and image processing techniques. Experimental results confirm the predictions of the internal hydraulic theory that there are two internal hydraulic controls in the flow through bottom openings, but one control in the middle and window opening experiments. The neglect of non-hydrostatic forces and interfacial mixing in the theory, however, results in a significant underestimate of the exchange rate by more than 50 % in the middle and window opening experiments. The fluctuations in the interface position were caused by Kelvin-Helmholtz instabilities as well as basin-scale internal seiche, and the transition of internally supercritical flow to subcritical flow was caused by the mixing generated by these instabilities.