googlead166c37c697d4d3.html Glenn Ashton Author Blog: Wind Power – Lessons of a Fluttering Bridge named Galloping Gertie


Wednesday, October 20, 2010

Wind Power – Lessons of a Fluttering Bridge named Galloping Gertie

The challenge to save the earth from the ravages of global warming forces us to consider all alternative ways to generate energy, rather than continue to use fossil fuels that increase the blanket of greenhouse gases covering our world. And that means we have to rely on our species' spirit of innovation. The old ways of generating energy will damage our world, so we have to find new ways.
One way to find new energy sources is to learn from past experiences, and figure out how they can help us find something new. This is what happened to a new company named Humdinger, which has invented the Windbelt.
Humdinger looked to the past – to the collapse of a bridge named Galloping Gertie on November 7, 1940. The flutters of Galloping Gertie lead Humdinger to consider what happens when you hold a blade of grass in your fingers and blow on it.
Black Cocker Spaniel, like Tubby
The collapse of Galloping Gertie did not kill anybody; the only casualty was a three-legged, paralyzed black male Cocker Spaniel named Tubby. This is the story of Tubby's last day, as seen through the eyes of some of those who were on the bridge when it collapsed.
In 1940 engineers threw a half-mile long bridge, with a central span of 2,800 feet, across the Tacoma Narrows strait of Puget Sound to link Tacoma and the Kitsap Peninsula in the state of Washington. The bridge was then the third longest suspension bridge in the world, and lasted just over 4 months. Motorists paid a 75 cent toll to cross the slender bridge, which rose 190 feet above the water, while pedestrians only paid 10 cents.
The bridge was nicknamed Galloping Gertie because of its behavior in slight winds. TIME magazine described it's motions this way at the time:
The Narrows bridge heaved like a hammock. Sometimes a car approaching
would seem to drop clear out of sight with an undulation of the roadway. Yet the bridge was strong. Heavy winds failed to shake it; but when lighter, intermittent breezes swept in from the open Sound, it was agitated by a peculiar weaving, sinuous motion that its builder said looked like the movement of a snake under a rug. Some people got seasick at once when the bridge began to sway; some enjoyed the weird sensation, high above the water, with the wind howling and the bridge throbbing as if it were alive.
On the morning of November 7, 1940 a forty-mile-per-hour wind sprang up, and Galloping Gertie began fluttering in a way never seen before, with its left side heaving more than twenty feet upwards every four to five seconds, then falling, and its right side heaving upwards in turn, while the midpoint of the twisting bridge remained still.
The last person to actually cross the bridge in her Model T Ford was Frances Carlson (click here for her photo). She drove past Leonard Coatsworth, a Tacoma News Tribune editor who was the last person to drive on the bridge. Frances made it across, and paid her toll.
Behind her, the bridge started to vibrate.
This is how Coatsworth describes his dance with death:
I drove on the bridge and started across. In the car with me was my daughter's cocker spaniel, Tubby. The car was loaded with equipment from my beach home at Arletta. Just as I drove past the towers, the bridge began to sway violently from side to side. Before I realized it, the tilt became so violent that I lost control of the car. . . . I jammed on the brakes and got out, only to be thrown onto my face against the curb. Around me I could hear concrete cracking. I started back to the car to get the dog, but was thrown before I could reach it. The car itself began to slide from side to side on the roadway. I decided the bridge was breaking up and my only hope was to get back to shore.

On hands and knees most of the time, I crawled 500 yards or more to the towers . . . . My breath was coming in gasps; my knees were raw and bleeding, my hands bruised and swollen from gripping the concrete curb . . . . Toward the last, I risked rising to my feet and running a few yards at a time . . . . Safely back at the toll plaza, I saw the bridge in its final collapse and saw my car plunge into the Narrows.
With real tragedy, disaster and blasted dreams all around me, I believe that right at this minute what appalls me most is that within a few hours I must tell my daughter that her dog is dead, when I might have saved him.
Much of the wreckage of the bridge still lies on the floor of the Tacoma Narrows, where it forms one of the world's largest artificial reefs. The remains are now protected as a historical site, and the replacement bridge enjoys the nickname Sturdy Gertie.
Before the bridge collapsed, Howard Clifford, the photographer of the Tacoma News Tribune, decided to walk onto the center span of the heaving bridge to see if he could save Tubby, trapped in Coatsworth's car:
I probably wouldn't have gone out there, if it hadn't been for the dog. I liked dogs and had seen the Coatsworth's dog at a company picnic recently. Or, if I didn't have the camera, I probably wouldn't have gone out on the bridge. I got about 10 yards from the tower and stopped. Taking another squint into the camera viewfinder, I saw the span buckle and start to break in the center. I pressed the camera trigger and started to run.
I tried to run up the yellow line in the center of the roadway, but found myself being bounced from one curb to the other and making no headway towards shore. I felt I could be tossed over the edge at any time. I was running in the air part of the time, because the bridge was moving faster than gravity. It dropped out from under me and then bounced back, knocking me down to my knees, banging the camera on the pavement. Behind me I heard rumblings and explosive sounds which scared the daylights out of me. Having played football during my junior high and high school days, I tucked my camera under my arm and charging low got that added ounce of energy from somewhere which enabled me to make some headway toward the bridge entrance.
One more person tried to rescue Tubby (from TIME):
Around 10:55 [Professor] Farquharson, who of all people should have known better, made another attempt to rescue Tubby. He retreated when the presumably terrified and disoriented dog bit his finger. Farquharson reached safety just in time, for shortly after 11:00 a.m. the structure reached the limits of its strength. The suspender cables, which hung from the main cables and supported the roadway, began to snap one by one. When the weight of the roadway exceeded what the remaining supporting cables could bear, 600 feet of it tumbled into the water below.
Fast forward now to Humdinger and its invention, the Windbelt. They realized that the cause of Galloping Gertie's failure was aeroelastic flutter. The design of the bridge used "I" beams which meant the wind was unable to pass beneath the bridge as easily as it could pass over it, and this caused a difference in pressure similar to that which lifts an airplane wing.
Humdinger's invention departs from the wind turbine design which is the only commercially viable device for harvesting wind power to date. The Windbelt relies on the flutter which caused Galloping Gertie's demise. Its design allows the power of the wind to be captured at scales and costs beyond those of conventional wind turbines. The design pulls energy from the wind by using a tensioned membrane which undergoes a flutter oscillation.
To picture how this works, think of how you held a blade of grass between your fingers as a kid and made it whistle, -- or how the strapping on a truck can be seen moving in the wind.  That is roughly how the Windbelt can pull energy from the wind – then, it’s a second step to turn that energy of the moving membrane into electricity, which is done by actuating new types of linear generators.
Humdinger hope to produce small Windbelts, to replace batteries for wireless sensors, by "sipping small airflows" such as those inside air-conditioning ducts, or at remote science stations, or on the sides of highways. They also plan to produce much larger versions, providing much higher power boosts.
All this from lessons learned from a fluttering bridge, and a dog named Tubby.

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