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It’s spring and the daffodils are in bloom. Glimpses of the bright yellow flowers brighten everyone’s day as they go about their business while contemplating the lessons that daffodils have to teach…the construction industry?
Yes, it turns out there really is a link between daffodils and some sectors of the construction industry. It’s called von Karman vortex shedding and it’s an important factor in the designs of such things as bridges, towers, antennae, smokestacks — anything that can produce oscillating sideways forces as wind flows across them.
Scientists studying fluid dynamics have known about them for years, but not in time to save the Tacoma Narrows Bridge in Washington State.
People were disturbed when, after the bridge opened in 1940, it sometimes seemed to swing or twist somewhat when the wind was strong enough or from the right direction. That’s what earned it the nickname Galloping Gertie. Finally, on a windy day just months after it opened, it collapsed into the water beneath it.
Now researchers from Seoul National University and Ajou University, both in South Korea, have found that a structure with a twisted, helical shape and an elliptical cross section — inspired by the stem of a daffodil — can reduce drag and eliminate side-force fluctuations.
Side forces come into play whenever wind flows across an elongated object — like your arm when you stick it out the window of a moving car. As the air flows around your arm, it forms vortices that come off the top and bottom alternately.
"You will immediately feel that your arm will be forced to move up and down," says Haecheon Choi, a researcher at Seoul National University. He says this vortex shedding affects any long structure caught in wind or water currents. That includes things like lampposts, highrise buildings and the long vertical pipes used when drilling for oil at sea.
In the case of the Tacoma Narrows Bridge the periodic forces caused by vortex shedding happened to hit the bridge’s resonant frequency.
That triggered severe twisting of the bridge’s central span, "and finally the bridge collapsed," Choi says.
Choi and his team started their quest by studying how nature solves the problem of vortex shedding. They found their inspiration in daffodils. The flower’s stem is a twisting, elliptical cylinder that enables it to turn away from wind to protect its petals.
So the researchers did computer simulations to explore the fluid dynamics around the daffodil stem’s shape.
They tested variations — some with more elliptical cross sections, or with more twists, for example, in both turbulent wind and in smooth, laminar airflow. And in both cases, they found the daffodil-shaped stem made a big difference.
For example, Choi says, some helically twisted cylinders virtually eliminated the vortex shedding. That resulted in a reduction in wind drag with no side-force fluctuations.
He said that when compared to a round cylinder, the daffodil shape reduced drag by 18 per cent for laminar airflows and by 23 per cent for turbulent flows.
Designing a bridge with daffodils in mind probably isn’t going to happen. But the knowledge gained by Choi and his fellow researchers could work for things like lampposts, chimneys and underwater drilling pipes where the vibration caused by vortex shedding isn’t wanted.
And it could be used in highrise structures as well. Chimneys of industrial buildings now often have spiral fins attached up and down the exterior to eliminate vortex shedding. But designing such chimneys with an elliptical cross section should also work.
Designers of tall buildings often taper their structures.
There is an esthetic reason, of course, but it also prevents the entire building being driven by the same resonant frequency.
Choi says that the lessons learned could even apply to leisure time.
In fact, he says the research team already has a patent for a golf club with a helical shaft.
Korky Koroluk is an Ottawa-based freelance writer. Send comments to editor@journalofcommerce.com.
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