1R               ,661                        (GLWRULQ&KLHI7DNHR501DNDJDZD-RXUQDORI)ORZ        

2Q WKH 2ULJLQ DQG *HQHUDWLRQ 0HFKDQLVP RI
/DUJH6FDOH9RUWLFHVLQ6WURQJ7LGDO(VWXDULHV
7DNHR501DNDJDZDDQG$L1DNDJDZD7KLQN7DQN)ORZ3UHVV*ØWWLQJHQ‫כ‬0HOERXUQH‫כ‬3UDJXH‫כ‬.LQJVWRQ1Journal of Flow, 2022-10-2‫ ׉‬IFEMS 2001
Printed in JapanOn the Origin and Generation Mechanism of Large-Scale
Vortices in Tidal Estuaries
Takeo R.M. Nakagawa
Institute of Environmental Fluid Mechanics, Advanced College of Symbiosis Hakusan,
Ai Nakagawa
Department of Environmental Sciences, Academy of Hakusan
2-14, Hakusan 920-2152 Japannakagawatakeo2@gmail.com

Abstract: Large scale vortices in strong tidal currents have been found on the tranquil
sea surface at Western Port, Melbourne, Australia.

Aerial observation of dye

patches reveals sometimes remarkably regular honeycomb arrangement in a mannerin 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 experiment has been conducted todemonstrate 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.  Ingeneral, 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 tothese ordinary origins of vortices, it has been concluded that they are generated byVJGKPVGTCEVKQP of tidal currents and sand dunes at singular time: When tidal currentschange from acceleration to deceleration and YLFH YHUVD, the fluid body in the recirculatory flow region (cavity) behind each dune crest is changed into a pair ofvortices by being ejected.

Then, the paired vortices are transported to the sea

surface in experiencing a series of change in the structure, and finally form the cellularvortices on the sea surface.

It is inferred that this new finding is critical to understandany oscillating flow over the roughness with the separation in nature and laboratoryaccompanying the cyclical change from acceleration to deceleration and ᶔᶇᶁᶃᴾᶔᶃᶐᶑᵿ.

2Keywords: Cellular Vortex, Tidal Current, Cavity, Oscillating Flow, Dune, Turbulence.
Introduction
Well over the last 50 years, it has been known by fishermen, seamen,oceanographers and others in Japan, USA, Italy, UK, and/or Norway that intriguing
vortices similar to Bpnard cell appear on the tranquil sea surface of estuaries. Thisenhances us to investigate into those vortices at Western Port, Australia.
Observation made during dye release has shown the existence of large-scalevortices in strong tidal currents as shown in Fig.1(a). These observations, made intidal estuaries in Western Port, Australia by Hinwood and his students (Pollock, 1973;
Chandler & Berzkalns, 1977: Nakagawa & Hinwood, 1978; Hinwood, 1978; Nakagawa,1979a) have been extended by the study of aerial photographs of bays and tidal inlets
in other Australian States, Delaware Bay, U.S.A, Naruto Strait in Seto-inland Sea, Japan(Fig.1(b)) and so forth.

These vortices were coined by anonymous Japanese as‘Uzushio’ (Tidal Vortex,₻) in earlier than the tenth-century: In the historical book,

named “Tosa Nikki”, Tsurayuki Kino(1912) referred to “Uzushio” observed during hisjourney in 935 returning to Kyoto from Tosa province across the Naruto Strait. Veryrecently, it is realized that vortices observed at Western Port are quite similar to thoseat Naruto Strait in their origin and mechanism of generation.

This prompts us toinvestigate the vortices with a renewed interest. Despite the widespread occurrenceof these vortices, very little is known about their origin and mechanism of generation.
It is evident that the vortices alter the rate of diffusion or dispersion of effluents within
the water and may influence the rate of sediment transport and of dissipation of tidal
energy, hence a sound understanding of the vortices is essential to the study of flowin estuaries and tidal inlets.

3Fig.1(a) Aerial photograph of large-scale vortices in Western port, Victoria, Australia.
After Pollock, 1973.