Entrainment across a density interface inside a cylindrical tank with a concentric base opening

The entrainment rate across a density interface formed inside a cylindrical tank by a fresh water layer floating on top of sea water is investigated. The tank has a concentric opening at its base, and floats in a body of sea water. Mixing across the interface is induced by a uni-directional current flow below the tank. The study was carried out to investigate the feasibility of storing freshwater in a bottomless tank in the sea. From dimensional and regression analyses of laboratory measurements over a range of current speeds and sizes of the tank and base opening, empirical equations for estimating the mixing rate in such a tank system are derived. The results are compared with those for a rectangular compartment from a previous study by Foo (1995). An approximate unified equation for estimating the mixing rate in both circular and rectangular tanks is derived. Preliminary field measurements in a pilot storage tank show much higher mixing rates than the laboratory results. This is attributed to the different dynamic conditions experienced by the freely floating field tank, as it is subjected to constant impacts of wind, tides, currents and waves.