By Eric Hester, The University of Sydney
For a long time, sailors noticed that their boats would get ‘stuck’ near the glaciers and fords of the far north. Not knowing the origin of the drag, they named it ‘dead water’.
No one bothered to investigate, however, until the famous 1893-96 voyage of the great explorer Dr Fridtjof Nansen, who planned to freeze his boat Fram in the ice and ride it to the north pole. Unfortunately for Nansen, he didn’t quite get that far, but he did notice this strange ‘dead water’, and saw that it only occurred “where a layer of fresh water rests upon the salt water of the sea” – explaining its presence near melting ice and river mouths.
After returning, he relayed these observations to the scientist Dr Vagn Walfrid Ekman, who found that the key to ‘dead water’ was the generation of internal waves at the boundary of the lighter fresh water and heavier salt water. But that’s basically as far as Ekman got, and not much more investigation his taken place since.
The aim of my project has been to simulate the dead water phenomenon using the flexible ‘dedalus’ computer code, and figure out exactly what’s causing the increased drag.
Now, this investigation is by no means finished, but much progress has already been made. During the project, I ran 2D simulations of a boat both in uniformly dense water, as well as two layered ‘dead water’ (pictured in the first figure of the fluid density). What I found was that boats in dead water experience three to four times the drag of those in normal water, just as Nansen and Ekman found over 100 years ago.
Exploring further, I saw marked differences in the pressure between the simulations, seen in the next two figures. While there is a region of lower pressure immediately behind the boat in both cases, it is much larger in the stratified case. Importantly, it actually reaches the rear of the boat, allowing it to ‘pull’ on it in a way that it can’t in the one layer case, suggesting the origin of the dead water effect.
Further investigation into the origin of this low pressure region is planned for the near future, with several improvements to the simulations currently being implemented. We are also working to get dedalus on to the Artemis High Performance Computer at the University of Sydney, which will allow us to investigate more realistic parameter regimes and, hopefully, start running three dimensional simulations.
For more information on the epic story of Nansen’s three year expedition, see Farthest North (1897), written by Fridtjof Nansen, whose account of the voyage and his two-man trek to the pole remains fascinating over a century later.
For those interested in the origins of the phenomenon, take a look at Ekman’s doctoral thesis On Dead Water, published in volume 5 of The Norwegian North Polar Expedition 1893-1896. Ekman’s ability to explain the qualitative aspects of dead water is remarkable, drawing from experiment, theory, and even sailors’ first hand experiences (expect some colourful stories from Nansen).
For publicly available footage of the dead water effect, see the experimental investigation by Mercier, Vausseur and Dauxious, on youtube at https://www.youtube.com/watch?v=bzcgAshAg2o.
And finally, for anyone interested in dedalus, check out http://dedalus-project.org. If computational fluid dynamics is your thing, dedalus is an extremely flexible framework for solving partial differential equations, and might be exactly what you’re looking for.
Eric Hester was one of the recipients of a 2015/16 AMSI Vacation Research Scholarship.