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Bumpy humpback flippers inspire new tidal turbine design

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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.

Photo credits:
Of the turbine blades: Courtesy Professor Mark Murray, U.S. Naval Academy
Flipper inset:

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  1. 1. jongolften 5:24 pm 12/2/2010

    Why would the design not locate some of the tubercules near the base of the turbine blade? That is the area of the blade that is naturally located in the slower flow region during normal rotational velocities.

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  2. 2. superlosch 8:34 pm 12/2/2010

    I’m sorry but this just seems like common sense to me. A complete waste of time researching the topic. We, as in the human race, have already learned about this facet of fluid dynamics. Bumpy Vs. Smooth, Does anyone researching this topic play golf? Because if you had you would have realized that the dimples on the balls are using this very principle. The question of why this happens the way it does is however, somewhat of a mystery. I would assume that the smooth wing, ball, flipper, etc. would reduce friction causing less drag but the bumpy one breaks up the surface tension and causes less friction. Like jumping off a bridge, if you try to make yourself into a bullet to reduce friction you will hit the water with full force and most likely break something, but if you drop a rock into the water right before you jump, you wont get hurt as severely.

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  3. 3. aksfung 3:05 am 12/10/2010

    Stall occurs at high angles of attack, which are due to a combination of low flow speed and high blade speed. The blade speed is higher towards the blade tip than towards the base.

    Any twist in the geometry of the blade would modify the angle of attack, so it is possible to design a blade with a higher angle of attack at the base than the tip and stall at the base before the tip. But that is not the case for the blades in the photo, which appear to have zero twist.

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