From the MGM-produced twister that swept up Dorothy’s house in 1939’s The Wizard of Oz to today’s Storm Chasers and Weather Channel exclusives, tornadoes and their destructive potential have long held the interest of many.
It’s a highly American phenomenon, this fascination with funnel clouds, largely because they occur in the central United States more frequently than anywhere else on earth.
“On the entire globe, the hot spot for tornadoes is Tornado Alley – Texas, Oklahoma, Nebraska, Kansas,” said Dr. David Lewellen, research associate professor in Mechanical and Aerospace Engineering in West Virginia University’s” College of Engineering and Mineral Resources. Lewellen has studied tornadoes and other atmospheric turbulence at WVU since 1993.
Lewellen is just finishing the first year of a three-year $397,703 National Science Foundation award to further study what has become his specialty – the interaction of a tornado’s central vortex with the ground surface.
The grant is the fifth such NSF award Lewellen has received for his work over the past 15 years.
So how do you study something so powerful up close? Lewellen says it’s a mixture of new technology with old school thought.
“I’m basically a theoretician, so I do a lot of old-fashioned pencil-and-paper equation work. But it’s VERY difficult to actually take measurements inside a tornado, as you can imagine, so I use computer simulations,” Lewellen explained.
“We put a lot of effort into writing computer codes that accurately simulate tornados, particularly the interaction of the tornado with the surface. I want to compare as close to the actual tornado flow as I can get,” he said.
“The codes we’ve developed have been built up now over a long period of time. So the basic equations – these large computations – give you a pretty realistic picture of what’s going in a tornado near the surface.”
What you get on-screen is a high-resolution, three-dimensional image of a tornado’s interior as it touches land. From there, Lewellen closely studies the simulation, noting the results.
“We basically solve the underlying fluid dynamic equations for a tornado over a surface,” he said.
For years, Lewellen worked alongside his father, Dr. Steven W. Lewellen, now retired from WVU, who he credits for getting him interested in tornados. Their work has been profiled in such publications as National Geographic.
So what has Lewellen learned by studying the inside of a tornado at ground level?
“I’ve learned there are some very interesting fluid dynamics going on there,” he said. “Normally, you would expect slower winds at the surface due to surface friction. Usually, you’ve got higher winds up above, lower winds down at the surface. But the geometry of a tornado is special in that it can actually produce the highest winds close to the surface. “
There is also the issue of what is in the tornado besides the air flow.
“You must also study things like dirt the tornado picks up. In a dirty tornado, most of the momentum down low is not carried by the wind – it’s actually carried by the dirt. So when it slams into a structure, that dirt-loading increases the force. But on the other hand, the dirt-loading actually changes the fluid dynamics of a tornado – and it can actually lower the wind velocities,” Lewellen said.
Why study tornadoes?
“One reason is for better predictions. If we can understand tornados better, we can predict things better,” Lewellen said. “Another reason is that in order to make structures on the ground more tornado-resistant, we need to know the underlying wind forces coming from the tornado – and this involves the winds right in the lowest layers, which is what we’re studying.
“If we can understand these things better, maybe we can build structures that will resist tornados better,” he said.
The next phase of Lewellen’s research will be to add more to the simulation mix.
“Right now, the simulations are on a flat surface. In the future, we will add topographies – hills and buildings and other things that interact with a tornado’s flow – to allow variations in the surface. By putting a simple model of a structure in there, we can better understand the forces at play,” he said.
-WVU –
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CONTACT: Scott Gillespie, WVU Research Corp.
304.293.0804; Scott.Gillespie@mail.wvu.edu
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