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April 30, 2004

Scientists put new spin on tornado research

Fred Haan, Partha Sarkar and Bill Gallus
Researchers Fred Haan, Partha Sarkar and Bill Gallus have attracted the likes of NBC News and National Geographic as they pursue tornado research. Photo by Bob Elbert.
by Debra Gibson
Growing up in India, Partha Sarkar was a championship runner with an affinity for building his own versions of long bridges, tall chimneys and towering buildings. His father's work as an electrical engineer at a coal-fired power plant fascinated and motivated the young Sarkar, but he was even more in awe of the building that housed the power plant, or the dam and reservoir that supplied its water.

Halfway around the globe in Johnstown, Penn., Bill Gallus was scratching out a weather journal by age 7. By third grade, he was firming up plans to become a meteorologist, and he'd evolved into an expert tornado chaser by early adulthood.

But it wasn't until Sarkar observed firsthand the devastation of a Tennessee tornado that the men's paths crossed and their interests merged into a university research project that may well reduce the destruction wrought by nature's twisters.

Now both ISU faculty members, the men have collaborated on the largest tornado simulator ever constructed for engineering purposes (Hollywood technically possesses the largest such machine), a project borne of Sarkar's determination to put his academic expertise to real-world use.

"When I used to read and see news coverage of total devastation in tornadoes, I told myself that I need to do something about it," he explained. "I am a structural engineer who works in wind tunnels and understands the effects of the wind."

Strength in numbers
In spite of the havoc they wreak, tornadoes have received little attention from wind engineers, Sarkar said. While it may be economically feasible to design structures to resist F2 tornadoes (those involving wind speeds of up to 157 miles per hour), Sarkar argues that essential facilities such as power plants, hospitals and airports should be built to withstand tornado winds of higher intensity.

"Any such design work requires accurate information about the nature of the wind loads on structures during tornadoes," he explained. "But determining tornado-induced wind loads is difficult for two reasons: quantifying wind velocity magnitudes in tornadoes is complicated, and simulating tornadoes in a laboratory while measuring wind pressures on structures has not been attempted systematically."

After joining the ISU faculty in 2000 as the department of aerospace engineering's Wilson Chair and director of wind tunnel operations, Sarkar contacted Gallus, an associate professor of geological and atmospheric sciences known as the campus authority on tornadoes.

For two years, they worked to merge Gallus' knowledge of the latest research on tornado formation with Sarkar's expertise in wind tunnel design and testing and the effects of wind loads on buildings. Sarkar also collaborated with Fred Haan, assistant professor in aerospace engineering, on the instrumentation needed to characterize the wind speeds and ground pressures in this 3-D simulator, and Ryan Kardell, a graduate student in engineering mechanics. It took about one month and the work of many undergraduate students to actually construct it.

tornado simulated in ISU's
The tornado simulator in action. Photo by Bob Elbert.
Batten down the hatches
The result? A simulator that resembles the top one-third of a grain bin, 18 feet in diameter and 11 feet high, affixed to a 5-ton crane. When activated, it moves horizontally above a large wooden ground plane. The "tornado," best visualized through the use of dry ice, can achieve heights from 4 to 8 feet and grow up to 4 feet in diameter. Its size can emulate tornadoes from an F1 to an F5 rating. (An F5 tornado can be more than one mile wide, with estimated wind speeds of 261 to 318 miles per hour.)

Though Sarkar isn't the first engineer to test scaled models in a laboratory tornado simulator (predecessors have included former ISU President Martin Jischke, while on faculty at the University of Oklahoma), he has pioneered a unique design. While almost all previous simulators have generated stationery "tornadoes," the ISU simulator is designed to generate a moving vortex using a mechanism that resembles what occurs in nature.

The simulator creates a tornado by moving the vortex entirely from above, rather than from the ground up as in most other simulators. This leaves the large ground plane below for models to be "hit" by the tornado. Sarkar and Haan plan to test 25 models, which will be scale-versions (1/150 to 1/300) of homes, churches, schools, bridges, water tanks and other public buildings. The models will be fixed to a turntable on the ground plane and equipped with sensors that will record the tornadoes' effects.

Wind warriors
"As an engineer, I understand the detrimental effects of wind on these structures," Sarkar explained. "I want to contribute to the better design of all kinds of civil engineering structures, or to problems like trucks blowing over on the highways in high crosswinds. There are no design guidelines for winds generated in a tornado, a microburst (a strong downdraft that extends only 2.5 miles or less but that can induce damaging winds as high as 168 mph) or a gust front. With my research, we will help to create the guidelines that will improve the building codes."

Beginning this summer, Sarkar will evaluate how well a simulated tornado matches what is known about the "real-life" phenomenons. He and Haan will measure the flow generated by the simulator and study the effect of varying simulator parameters. They also will test pressure forces on buildings from tornado winds, using the small-scale models.

Down the road, Gallus would like to see the simulator used to test meteorological theories as well.

"It might be interesting to test the impacts of surface roughness (related to terrain and land use) on the tornado, and also to investigate the impact of the temperature of the downdraft on the tornado," Gallus said. "In nature, the temperature of the downdraft is believed to determine how likely a tornado is, and how strong and long-lasting it might be."

Until then, Sarkar, Haan and Gallus are more intent on learning all they can about the vicious vortex and its impact on the globe.

"Average citizens will not be sitting ducks when tornadoes hit their homes in the future," Sarkar predicted. "Maybe 20 years from now, we will not have tornado sirens going off because we will feel safe in our homes."

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