Richard Collins: Could fish and geese teach us how to save on plane fuel?
Two fins are better than one: fish synchronise tail fins to save energy
By ‘shielding' or ‘sucking in’ behind other riders, cyclists in the Tour de France peloton take advantage of the slipstreams other competitors generate.
Likewise, according to the aircraft manufacturer Airbus, significant energy savings could be had by flying planes in tandem, rather than on their own. Aircraft create turbulence. A tailgating plane can exploit the vortices from one ahead of it. ‘Wake energy retrieval’ would reduce fuel consumption and CO2 emissions.
In 2021, in a ‘Fellow-fly’ experiment, two aircraft flew in tandem from Toulouse to Montreal. The fuel consumption of the planes was compared. The Airbus engineers reported: "The uplift from the wake has shown it can drive at least a 5% reduction in CO2 per trip."
This may not seem much of a saving but several tonnes of fuel are burned during long-haul flights; the reduction in greenhouse gas released is therefore significant. About 14% of transport CO2 emissions come from aviation, even though fewer than 20% of people worldwide have ever flown.
The engineers took their cue from the flight patterns of geese. Many greylags — the ‘wild geese’, of Patrick Sarsfield fame — fly 1,400km from Iceland to Ireland each autumn. The Greenland white-fronted geese, of the Wexford slobs, travel even more. The brent geese of Irish estuaries, breed in the Canadian Arctic 6,000km away. The birds cover these distances in V-formations. The leading goose cuts through the air, generating eddies and vortices behind its wingtips. These provide extra lift for the pair immediately behind the leader. Their turbulence in turn helps the next pair and so on. When a leader tires, another takes over.
But geese may not have been the first creatures to exploit such turbulence when travelling. Many fish move about in shoals. The vortices generated by a fish in a shoal might help its immediate neighbours to swim more efficiently. But do fish shoals resemble gooseflocks in exploiting this possibility?
Researchers have produced a theoretical model that demonstrates the underlying mechanisms behind how fish will synchronize their fin movements to ride each other’s vortices, thereby saving energy. #fish #fluidphysics #tohokuuniversityhttps://t.co/dBJnesA5He
— Tohoku University (@TohokuUniPR) November 6, 2023
Scientists at Tohoku University in Japan have developed a model that explains how individual fish might work both for their own benefit and that of the shoal.
Although fish are moving in water, rather than air, the laws of fluid mechanics are similar for both media. Most of a fish’s swimming power is delivered through its tailfin.
Synchronised swimming depends mainly on tailfin configuration, so the researchers modelled tail configuration and movement. Then they ran a series of computer simulations. These showed that, by working cooperatively, fish can reduce their energy demands by swimming together. The optimum separation between fish was below half of body-length. Fish, therefore "tend to be distributed at short distances from each other, where they adjust their phase difference to reduce energy dissipation", the researchers concluded.
Even military planes don’t fly together at half-body length; that would be suicidal. But they don’t need to. For optimum efficiency, Fellow-fly airliners would need to be about three kilometres apart with a 1,000-foot altitude separation. This might seem a great distance compared to what fish need. However, at the speeds at which airliners fly, the following aircraft encounters a leader’s turbulence within seconds.
Bon voyage!
