Albatrosses sail the Southern Oceans almost effortlessly. Even though known for centuries, only now, engineers have developed a new model to simulate dynamic soaring, and have used it to identify the optimal flight pattern that an albatross should take in order to harvest the most wind and energy. They found that as an albatross banks or turns to dive down and soar up, it should do so in shallow arcs, keeping almost to a straight, forward trajectory.

An albatross always has been a sight to watch, even in old sailing times. Almost effortlessly, it can fly for hours without a flap with its enormous wings. In former times, they were believed to take the souls of sailors to heaven. Credit: Michael Wenger
An albatross always has been a sight to watch, even in old sailing times. Almost effortlessly, it can fly for hours without a flap with its enormous wings. In former times, they were believed to take the souls of sailors to heaven. Credit: Michael Wenger

Observers have noted for centuries that these feathered giants keep themselves aloft for hours, just above the ocean surface, by soaring and diving between contrasting currents of air, as if riding a sidewinding rollercoaster -- a flight pattern known as dynamic soaring. Now engineers at MIT have developed a new model to simulate dynamic soaring, and have used it to identify the optimal flight pattern that an albatross should take in order to harvest the most wind and energy. They found that as an albatross banks or turns to dive down and soar up, it should do so in shallow arcs, keeping almost to a straight, forward trajectory. The new model, they say, will be useful in gauging how albatross flight patterns may change as wind patterns shift with changing climate. It also may inform the design of wind-propelled drones and gliders which, if programmed with energy-efficient trajectories for given wind conditions, could be used to perform long-duration, long-range monitoring missions in remote regions of the world. "The wandering albatross lives in the Southern Ocean, which is not very well-known. It's very hard to get there, and there is a lot of wind and waves," says Gabriel Bousquet, a graduate student in MIT's Department of Mechanical Engineering. "The region is extremely important for understanding the dynamics of climate change. With robots that can use the wind, you could monitor in real-time and get much denser data than we can now. This is an important step forward to actually write algorithms for robots to be able to use the wind." Bousquet is the first author of a paper reporting the team's results, published in the journal Interface.

Already more than 100 years ago, the concept of dynamic soaring was developed by Nobel Prize winner Lord John Strutt, 3. Baron Rayleigh. However, the latest model recently published refine Rayleigh’s model. Credit: Toni Bürgin, David Senn
Already more than 100 years ago, the concept of dynamic soaring was developed by Nobel Prize winner Lord John Strutt, 3. Baron Rayleigh. However, the latest model recently published refine Rayleigh’s model. Credit: Toni Bürgin, David Senn

Renowned English physicist Lord Rayleigh was the first to describe dynamic soaring in mathematical modeling terms, predicting that albatrosses should fly in a series of arcing, 180-degree half-circles as they alternately soar through layers of high wind and swoop down to layers of low wind. This has been the general understanding, even today. However, Bousquet and his colleagues came to a quite different conclusion. The team first modeled the wind field, drawing up a relatively simple equation to represent the change in wind speed with altitude. They specifically noted the thickness of the shear layer, which can be thought of as the distance between a layer of slow winds and a layer of fast winds. They then used a three-dimensional model to represent the flight of an albatross or glider. This model consists of complicated equations of motion that are extremely difficult to solve. The researchers solved those complicated equations using a method called numerical optimization. They varied the thickness of the shear layer and looked for the minimum wind needed to sustain flight. They found that the thinner the shear layer, the less wind was needed to keep a bird aloft. In other words, the closer the layers of slow and fast winds, the less energy an albatross must expend to stay in the air.

Like their large cousins, black-browed albatrosses use different wind speeds over water for dynamic soaring and long-distance flights. According to Bousquet, they turn in shallow arcs rather than half-turns. Credit: Michael Wenger
Like their large cousins, black-browed albatrosses use different wind speeds over water for dynamic soaring and long-distance flights. According to Bousquet, they turn in shallow arcs rather than half-turns. Credit: Michael Wenger

Co-Author Professor John Slotine states: "If we want to design robots that use the wind, now we know that moving forward along shallow arcs favors both travel speed and efficient energy extraction. And it turns out the albatrosses are doing it that way."

Source: Jennifer Chu, Massachusetts Institute for Technology