A Vortex that Destroys Bridges, or Powers Cities

Take a look at this video. Can you guess what force it is that makes the pistons go up and down?

Anyone? Anyone? OK, here’s the baffling answer: Vortex Induced Vibration. I ran across it while researching a column on tidal energy and learned that it isn’t a gas or electric motor that moves the pistons, but strange properties of the water itself. VIV, as it’s known, is starting to be tapped as an energy source that one day might electrify entire cities. But the phenomenon has intrigued and bedeviled engineers since Leonardo da Vinci.

In the 1500s da Vinci noted “Aeolian Tones,” the sound that wind makes when passing over a wire, and that vortices swirl beneath the pilings of bridges. What was a curiosity for him has turned out to be the bane of engineers in the mechanical age. VIV is complex reaction that occurs when vortexes in a fluid (water or air) cause a structure to oscillate back and forth, causing fatigue and sometimes startling destruction. Engineers have taken great pains to design around it in everything from fishing nets to flagpoles to nuclear cooling towers and undersea oil rigs. They remember all too well the lessons that VIV taught in 1940, when the newly-built Tacoma Narrows Bridge in Washington started thrashing in the wind like a beast possessed. It collapsed four months after opening, one of the marquee design failures of all time. Ever since, footage of the bridge’s collapse has been played in engineering classes like a top-ten greatest blooper.

It wasn’t until 2004 that Professor Michael Bernitsas at the University of Michigan realized that this terrific force could be brought over from the dark side and recruited to create electricity. Along with a graduate student, he placed cylinders in a water tank similar to the one in the video above. When he started the water flowing, he recalls, the result was startling:

When the impeller was turned on and water flowed through the tank…something amazing happened. The cylinders started to move up and down through the water, and then they began to move in sequence, almost as if they were part of a four-piston reciprocating engine. The motion, first up, then down, was so forceful that it was evident that the cylinders were tapping into some large supply of energy.

A traditional tidal turbine. Source: goodcleantech.com

Recent years have seen an explosion of new designs for capturing the power of flowing water that have a much lighter footprint than your traditional hydroelectric dam, which inundates valuable land and blocks the migration of fish. But most of these designs have turbines and blades resembling those of a traditional wind turbine. Some worry that they will kill fish in the same way that wind turbines threaten birds and bats.

Bernitsas’ VIVACE (Vortex Induced Vibration for Aquatic Clean Energy) generator may avoid that problem because it has no turbine. Best of all, it can operate at speeds lower than other hydroelectric turbines, which require a water flow of five or six knots. In the video above the water is moving at a lazy 2.6 knots, which is about the speed of many of the world’s large and medium-sized rivers.

VIVACE also boasts a high power density: It produces a substantial amount of power in a small amount of space. Bernitsas compares it to the Horse Hollow wind farm in Texas, which covers 190 square kilometers and is one of the world’s largest. When he adjusts for the superior density of water (830 times more dense than air), and the fact that rivers flow all the time while wind is intermittent, he claims that VIVACE’s power density is 14,600 times greater than that of a wind farm.

A vortex as photographed in Bernitsas' lab.

One concern about renewable-energy technologies like wind and solar is that they occupy vast amounts of space, eating up much of the lovely landscape that the environmentalists hoped to spare in the first place. Bernitsas says near the end of this video that a farm of his devices 100 meters square in a river moving at three knots could create a megawatt of electricity, enough to power 1000 homes, and would be capable of producing three to 10 times more power than other marine energy generators.

The science and economics of tidal and river turbines are still just coming into focus, but VIVACE is one concept worth keeping an eye on.