December 2, 2010 | 3
Humpback whales are impressively agile swimmers—thanks in no small part to the rows of bumps, called tubercles, on the leading edges of their flippers. Tubercles generate swirling water formations, called vortices, which help the massive mammals maintain lift and delay stall, an aerodynamic phenomenon in which the flow of fluid over the top of the flipper becomes separated from the flow underneath, causing increased drag.
Previous research has shown that adding tubercle-like bumps to wind turbine blades could make the blades better able to harvest energy, especially at low speeds. Engineers have already applied the principle to industrial fans and continue to develop whale-inspired wind turbine technology. Now, researchers at the U.S. Naval Academy (USNA) have shown that adding bumps to underwater tidal turbines improves their performance, too. In a laboratory experiment, the results of which they presented November 22 at the annual meeting of the American Physical Society’s Division of Fluid Dynamics, bumpy turbines produced significantly more energy at low speeds, when compared to a standard turbine with smooth-edged blades.
Ocean tides represent a large potential source of renewable, nonpolluting energy. But the tidal power industry has been slow to emerge, in large part due to technical challenges. One important obstacle facing engineers is the difficulty of designing turbines that do not stall in slow-moving water. "A lot of the turbine designs, at very slow speeds, aren’t producing lift because the flow on the upper surface is separated," explains USNA mechanical engineering professor Mark Murray. So Ensign Timothy Gruber, aware of how tubercles had worked for wind turbines, decided to test them on marine tidal turbine blades. Murray and David Fredriksson, a professor in the academy’s naval architecture and ocean engineering department, advised Gruber, now a graduate student at the Massachusetts Institute of Technology, on the project.
Gruber used a computer-aided design program to manufacture a standard tidal turbine blade, and then used the program to build versions with added tubercles. In the lab, the experimental designs performed better than the control, decreasing cut-in speed (the threshold speed at which the turbine starts turning) and producing more power at lower speeds. At higher speeds, the experimental turbines performed similar to the control.
Although the research is preliminary, the tubercle-blade design has the advantage of simplicity, Murray says. So far, attempts to overcome low and varying flow speeds have involved complex mechanisms. Some wind turbine blades, for example, can change shape depending on the speed of the flow. But that is not ideal for tidal turbines, Murray says, since the underwater environment is less hospitable. "Underwater, you’ve got to be very careful, because the more complex you make it the more chance it has to break," he notes.
Murray says the next step is to test the design against blades that engineers have already attempted to optimize, using different approaches, for low flow speeds. These designs may have drawbacks at higher speeds, he says, but the previous wind turbine research showed that adding tubercles did not negatively affect production at faster flow rates. For tidal turbines, “if you can come up with ways to harvest low-energy flows, yet not hurt yourself at higher-velocity flows, that’s a definite advantage," he says.
Of the turbine blades: Courtesy Professor Mark Murray, U.S. Naval Academy
Flipper inset: Flickr/cheesy42