Using overpasses as shelter from tornado?
American Institute of Physics
According to the National Weather Service, doing so could
actually increase the risk of death, in part because the wind from a tornado is
thought to accelerate as it flows under the overpass, in what's known as the
wind tunnel effect.
However, few experimental studies
show exactly how this acceleration takes place or if it takes place at all. In
Physics of Fluids, by AIP Publishing, researchers from the University of Rhode
Island found no evidence of such acceleration.
"In our research, there is no
one finding that would suggest one should or should not use an overpass for
protection from a tornado as a last-resort shelter area," co-author D.M.L.
Meyer said.
The researchers examined the pressure, velocity, and force fields of tornado-strength winds surrounding an overpass. Experiments were performed in a standard wind tunnel using a scaled geometry of an overpass. A large fan was used to draw air through the tunnel at a top speed of about 130 miles per hour.
The researchers found no wind tunnel
effect in their experiments.
"However, that doesn't mean the
effect doesn't take place at all, just not in the locations we focused
on," Meyer said. "More data and analyses are needed to determine how
complex tornado-strength winds interact with the environment underneath an
overpass, and our paper provides a start."
An overpass may be a dangerous place
for shelter regardless. Wind may not decelerate, depending on the location, and
flying debris may reach areas of the overpass that appear to be protected from
the wind.
The researchers studied four
locations: immediately above the overpass, immediately below it, between the
I-beams, and the center of the travel lane under the overpass. Velocity and
dynamic pressure measurements were obtained independently at each location as
the overpass was rotated 0-90 degrees at 10-degree increments about its
vertical axis. Lift and drag forces were also measured.
Wind tunneling can be explained
using a garden hose. Water traveling through the hose flows at a constant
flowrate, because the diameter is constant throughout the hose length. Place a
thumb partially over the hose opening, and the flow constricts, accelerating
the water due to the smaller cross-section. This causes the water to spurt
further than without the thumb.
