Presentation
The Merriam-Webster's Collegiate Dictionary states that a whirlpool is "water moving rapidly in a circle so as to produce a depression in the centre into which floating objects may be drawn". Basically a whirlpool is a vortex of vertical axis, with a downward velocity component near its centre. A good example is the bathtub vortex. VAN DYKE ([1], p. 59) presented a superb illustration. A related example is the vortex dropshaft design.

In coastal zones, whirlpools are produced by the interaction of rising and falling tides. They are often observed at the edges of straits with large tidal currents. (At Naruto, currents of up to 9 knots were observed.) The vortex (whirlpool) is a coherent structure typical of turbulent shear flows where there is a velocity difference across the shear layer. It affect the surrounding flow and water can be seen going back and forth across the shear layer between vortices.

Notable oceanic whirlpools include those of Garofalo along the coast of Calabria in southern Italy, and of Messina in the strait between Sicily and peninsular Italy, the Maelstrøm (from Dutch for "whirling stream") located near the Lofoten Islands off the coast of Norway (Table 1). Whirlpools near the Hebrides and Orkney islands, and in the Naruto strait between Awaji and Shikoku islands, are also well known.
 

Whirlpools in the world
Whirlpools are sometimes called Maelstrom, after the Norwegian current. The Maelstrøm is a strong tidal current of the Norwegian Sea in the Lofoten islands [2]. Flowing between the islands of Moskenesøya (North) and Mosken (South), it has a treacherous current. Strong local winds make the passage additionally dangerous. The word maelstrom entered the English language via fiction novelists who exaggerated the current of the channel into a great whirlpool (1). In English, the word "maelstrom" designates a large, fatal whirlpool, engulfing vessels and men, or a figurative application of the idea.

In Western Scotland, the Corryvreckan whirlpools are said to be "one of the most notorious stretches of water anywhere around the British Isles" (2).  The whirlpools are caused by a sudden rise of the seafloor in the strait.

Naruto whirlpools
The Naruto Strait (Naruto kaikyo) connects the Awaji and Shikoku islands. The strait is 1.3 km wide. The tide is semi-diurnal and the tidal range may be up to 1.7 m. The differences in tidal levels across the strait may reach 1.5 m with current speds exceeding 9 knots (4.6 m/s) on the Northern part of the Strait [3]. The notorious "whirlpools" (uzu-maki) take place on the Southern side of the Strait at the peak of the flow and ebb currents. The flow and ebb are also called southward and northward currents respectively.

Hubert CHANSON visited the Naruto whirlpools on 17 October 2001 [4] ad 15 December 2015. On Wednesday 17 October 2001, the meteorological conditions were poor because the centre of Typhoon No. 21 was located few hundreds kilometres South of Japan. The ebb current was maximum around 12:20 pm (3). The waters flowed from the Inland Sea (Setona Kai) to the Pacific Ocean. The whirlpools were best seen next to the Southern bridge pier (4). He experienced the whirlpools between 10:45 am and 12:45pm, first from the Ohnaruto bridge walkway (uzu-no-michi) (5) (6) and later in a boat. The bridge walkway is about 45 m above the sea level and reaches 450 m from the bridge abutment. From the bridge, the writer saw a freighter which became trapped in the whirlpools and vortices, and got stranded (grounded) beneath the bridge around 11:00 am. (The boat was still grounded 2 hours later, waiting for the next high tide.)

Figure A shows whirlpools and vortices on the Southern end of the Strait (right side when looking into the flow direction). The main current flows on the left of the vortices. The photograph was taken from 45 m above the sea, about 450 m from the bridge abutment. Figure B shows a street of vortices with a freighter in the background. Although the turbulence appears less intense, the ship was subjected to a faster current and to the effects of large-size cauldrons. Figure C presents the same freighter with a large size eddy (diameter larger than 50 m) downstream of the whirlpools. Figure D (Naruto12.jpg) shows a single whirlpool, view from upstream and above. Figure E & Figure F are views from downstream. In Figure E and Figure F, a whirlpool is visible in the foreground of the photograph. A grounded freighter (green hull), stranded next to the bridge pier, is visible in the background. In Figure E, a white tourist boat is coming around the Cape to view the whirlpools.
 
Discussion
Whirlpools are basically two-dimensional vortical structures occurring in a turbulent developing shear layer. Such a flow may be approximated by a series of vortices of identical rotational direction advected downstream [6]. (In first approximation the advection velocity is equal to half the main stream velocity.) Each vortex is affected by the movement of the fluid due to the other vortices. Considering a pair of vortices, they will turn around each around while being advected downstream. (Far away the vortex pair is seen as an unique vortex of strength equal to twice the strength of one vortex.) In real turbulent flows, the vortices may collapse and form a new larger vortex of stronger strength and identical rotation direction (i.e. vortex pairing). Vortex pairing may be repeated downstream and could yield very-large scale eddy structures. The process is sometimes called an inverted cascade of vortices (7). It may be repeated further downstream and this yields large vortical structures (e.g. Photo No. 2).

Footnotes

Detailed photographs

Related links

References

Acknowledgments

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Hubert CHANSON is aProfessor in Civil Engineering, Hydraulic Engineering and Environmental Fluid Mechanics at the University of Queensland, Australia. His research interests include design of hydraulic structures, experimental investigations of two-phase flows, applied hydrodynamics, hydraulic engineering, water quality modelling, environmental fluid mechanics, estuarine processes and natural resources. He has been an active consultant for both governmental agencies and private organisations. His publication record includes over 1200 international refereed papers and his work was cited over 10,000 times (WoS) to 26,000 times (Google Scholar) since 1990. His h-index is 47 (WoS), 51 (Scopus) and 79 (Google Scholar), and he is ranked among the 150 most cited researchers in civil engineering in Shanghai’s Global Ranking of Academics. Hubert Chanson is the author of twenty books, including "Hydraulic Design of Stepped Cascades, Channels, Weirs and Spillways" (Pergamon, 1995), "Air Bubble Entrainment in Free-Surface Turbulent Shear Flows" (Academic Press, 1997), "The Hydraulics of Open Channel Flow: An Introduction" (Butterworth-Heinemann, 1st edition 1999, 2nd editon 2004), "The Hydraulics of Stepped Chutes and Spillways" (Balkema, 2001), "Environmental Hydraulics of Open Channel Flows" (Butterworth-Heinemann, 2004), "Tidal Bores, Aegir, Eagre, Mascaret, Pororoca: Theory And Observations" (World Scientific, 2011), "Applied Hydrodynamics: an Introduction" (CRC Press, 2014). He co-authored three further books "Fluid Mechanics for Ecologists" (IPC Press, 2002), "Fluid Mechanics for Ecologists. Student Edition" (IPC, 2006) and "Fish Swimming in Turbulent Waters. Hydraulics Guidelines to assist Upstream Fish Passage in Box Culverts" (CRC Press 2021). His textbook "The Hydraulics of Open Channel Flows: An Introduction" has already been translated into Spanish (McGraw-Hill Interamericana) and Chinese (Hydrology Bureau of Yellow River Conservancy Committee), and the second edition was published in 2004. In 2003, the IAHR presented him with the 13th Arthur Ippen Award for outstanding achievements in hydraulic engineering. The American Society of Civil Engineers, Environmental and Water Resources Institute (ASCE-EWRI) presented him with the 2004 award for the Best Practice paper in the Journal of Irrigation and Drainage Engineering ("Energy Dissipation and Air Entrainment in Stepped Storm Waterway" by Chanson and Toombes 2002), the 2018 Honorable Mention Paper Award for  "Minimum Specific Energy and Transcritical Flow in Unsteady Open-Channel Flow" by Castro-Orgaz and Chanson (2016) in the ASCE Journal of Irrigation and Drainage Engineering, the 2020 Outstanding Reviewer Award, and the 2021 Outstanding Reviewer Award. The Institution of Civil Engineers (UK) presented him the 2018 Baker Medal. In 2018, he was inducted a Fellow of the Australasian Fluid Mechanics Society. Hubert Chanson edited further several books : "Fluvial, Environmental and Coastal Developments in Hydraulic Engineering" (Mossa, Yasuda & Chanson 2004, Balkema), "Hydraulics. The Next Wave" (Chanson & Macintosh 2004, Engineers Australia), "Hydraulic Structures: a Challenge to Engineers and Researchers" (Matos & Chanson 2006, The University of Queensland), "Experiences and Challenges in Sewers: Measurements and Hydrodynamics" (Larrate & Chanson 2008, The University of Queensland), "Hydraulic Structures: Useful Water Harvesting Systems or Relics?" (Janssen & Chanson 2010, The University of Queensland), "Balance and Uncertainty: Water in a Changing World" (Valentine et al. 2011, Engineers Australia), "Hydraulic Structures and Society – Engineering Challenges and Extremes" (Chanson and Toombes 2014, University of Queensland), "Energy Dissipation in Hydraulic Structures" (Chanson 2015, IAHR Monograph, CRC Press). He chaired the Organisation of the 34th IAHR World Congress held in Brisbane, Australia between 26 June and 1 July 2011. He chaired the Scientific Committee of the 5th IAHR International Symposium on Hydraulic Structures held in Brisbane in June 2014. He co-chaired the Organisation of the 22nd Australasian Fluid Mechanics Conference held as a hybrid format in Brisbane, Australia on 6-10 December 2020.
His Youtube channel is: {https://www.youtube.com/@Hubert_Chanson}. His Internet home page is http://www.uq.edu.au/~e2hchans. He also developed a gallery of photographs website {http://www.uq.edu.au/~e2hchans/photo.html} that received more than 2,000 hits per month since inception.

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