On the Origin and Generation Mechanism of Large-Scale Vortices in Tidal Estuaries

Abstract

Large scale vortices in strong tidal currents have been found on the tranquil sea surface at Western Port, Melbourne, Australia. It is found by the similitude analyses that the origin and generation mechanism of these vortices are governed by the following relation: d/ = (||/g, UD/ν), where d is the diameter of vortices,  the scale of dunes,  the absolute value of acceleration and/or deceleration of tidal current, g the gravitational acceleration, U the velocity of tidal current, D the depth of water, and  the kinematic viscosity of the sea water. Note Ж is an unknown function introduced by the analyses. Aerial observation of dye patches reveals sometimes remarkably regular honeycomb arrangement in a manner in which they are distributed over the sea surface.　 In the field, flow visualization by two color dye patches, and sea bed survey in terms of a side-scan sonar have been done, while in the laboratory, extensive towing experiments have been conducted to demonstrate the constant, accelerated and/or decelerated flows over each dune model for simulating cycle of tidal currents. On the basis of field and laboratory experiments, the nature, origin, and generation mechanism of the vortices are scrutinized. 　In general, it is possible that vortices on the sea surface have various origins such as thermal convection (Bénard cell) and wind shear stress (Langmuir cell).   Contrary to these ordinary origins of vortices, it has been concluded that they are generated by the interaction of tidal currents and sand dunes at singular time during spring tide: When tidal currents change from acceleration to deceleration, the fluid body in the re-circulatory flow region (cavity) behind each dune crest is intensified during the acceleration period, and then it is ejected upwards and changed into a pair of vortices.   Then, the paired vortices are transported to the sea surface in experiencing a series of change in the structure, and finally form the cellular vortices on the sea surface, with which diameter of each vortex is increased by the interaction.    It is inferred that this new finding is critical to understand any oscillating flow over the roughness with the separation in nature and laboratory accompanying the cyclical change from acceleration to deceleration and vice versa.