July 4, 2003 FAYETTEVILLE, Ark. -- Tornadoes have a reputation for being unpredictable. However, the pattern of destruction that occurs when a tornado interacts with a building is predictable, and that makes it preventable, according to University of Arkansas researcher Panneer Selvam.
Outbreaks like the one in May, 2003, reinforce the dangers tornadoes pose. In the 10 days from May 2-11, 583 tornadoes struck in 17 states. The results include 42 deaths and more than $2.2 billion in property damage.
Selvam, a wind engineer and professor of civil engineering, has studied the behavior of tornadoes for more than 20 years. His model shows that when a tornado encounters a building, the force has no choice but to lift up. In fact, Selvam's research has shown that a tornado exerts an upward force on a building up to 10 times as strong as the force of gravity.
"When a straight wind encounters a wall, there are options," explained Selvam. "Some of the wind may go around to the left, some to the right and some over the top. But the cyclonic direction of the wind in a tornado does not allow it to go to either side, so there is only one direction for it to move -- it lifts up."
Selvam and graduate student Paul Millett have developed and tested a model to accurately describe the interaction between a tornado and a building to facilitate the design of tornado-resistant buildings. They presented their results recently at the 10th International Conference on Wind Engineering in Lubbock, Texas.
Earlier researchers treated tornadoes as if they were straight-line winds that remained constant. They tried to determine the tornado's force by investigating storm damage and a calculating the maximum wind speed. This process was used to produce the Fujita Scale, which is currently used to classify tornadoes. However, it assumes that the wind forces are constant when in fact they change over time.
This rapidly changing wind speed and direction make a tornado difficult, if not impossible, to study in a wind tunnel. Previous attempts have been made to make a computer model of a tornado, were limited by the extreme complexity of the tornado system and the computational power. They had to ignore turbulence or they discounted the effects of viscosity.
But in reality, tornadic winds have both high turbulence and viscosity. Selvam's model has shown that they also interact with the structures they encounter. This interaction can change the course and force of the tornado.
"Tornado-structure interaction is a complex phenomenon," Selvam said. "For example, the details of tornado wind speed and turbulence from ground level up to 100 meters are needed. In addition, the tornado is moving with respect to the building and the details of tornado wind speed at every instant of time are essential to impose the proper boundary conditions."
Selvam's research may also shed light on why tornadoes seem to take such an unpredictable path. They may move down one side of a street for blocks, then, inexplicably, hop to the other side or veer around a corner. Selvam's model shows that the dimensions of a structure can affect the path of a tornado.
"When we set up the tornado to move directly toward a building, we were surprised to find that the vortex of winds around the building actually caused the tornado to veer to the side of the structure," Selvam said.
"Because it is a 3-D model, we can change the height and shape of the building. Although we are primarily interested in exploring the tornado's impact on buildings, we can also see that the building has an impact on the tornado's path."
By using this model, builders can design tornado-resistant buildings, which can minimize the loss to life and property posed by tornadoes.
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