Hurricane Chasers Look For Turbulent Times

December 16, 1998

Idaho Falls, Idaho-This year's hurricane season is now officially over, and it ranks as one of the deadliest and costliest in history. For those rebuilding, can something be learned from these hurricanes to reduce the devastation of future storms? How does one build a house or any structure that can withstand the torrential rain and winds of a hurricane?

Researchers at the Idaho National Engineering and Environmental Laboratory interested in answering those questions are trying to reconstruct a hurricane. While most coastal inhabitants packed up and headed for higher ground when hurricanes threatened this past season, a group of engineers and scientists bounded into several storms, hoping to collect accurate data on the structure of wind. Their hurricane chasing was part of INEEL's Partnership for Natural Disaster Reduction (PNDR) HomeSaver project.

The aim of HomeSaver is to replicate hurricanes and blow them at buildings. Projecting man-made hurricanes onto conventional full-size housing will reveal the shortcomings of current construction methods. It will also allow builders to experiment with better designs and materials. "To design a wind tunnel that you can claim to be reproducing hurricane winds, you need to know what hurricane winds are like," said Jeffrey Lacy, an INEEL structural engineer-turned-hurricane-developer.

Lacy and two other INEEL engineers, Thomas Larson and William Richins, along with scientists from Clemson University in South Carolina and Texas Tech University, lugged 5,000-pound trailers around the southeast United States through several hurricanes from Bonnie to Georges this past season. The trailers contained data-gathering equipment attached to a tower that can be erected to a height of more than 30 feet into the roof-ripping gusts of winds.

Data currently gathered by meteorologists, such as average wind speeds or peak gusts, is insufficient to reproduce hurricane-like winds. "The structure of wind is all turbulence, like any fluid," said Lacy. "You've got eddies within eddies within eddies. You've got turbulence from very large scale swirls--the hurricane itself--to very fine scale, fist-sized swirls and smaller. We're interested in the scale that affects houses and other buildings "

In addition, conventional data-gathering equipment isn't designed to withstand the worst weather. "When Hurricane Andrew came through," said Lacy, "the National Hurricane Center took down their wind-measuring equipment so it wouldn't be damaged by the storm."

The data the groups gather will be used to fabricate house-sized hurricane-force winds at the HomeSaver national test facility, planned for 2003. Recently, INEEL researchers finished constructing the Scale Windstorm Center, a 1/14-scale (in size, not power) model wind machine. More than 30 feet long, 17 feet wide and six feet high, it consists of a three-layered bank of fans and various vertical slats.

"If you turn on the rows of fans at different speeds, you get mixing layers of wind going different speeds," said Lacy. "This produces the wind speed profile we're looking for and initiates some turbulence." Rotating the vertical slats adds more lateral turbulence, he said. Eventually, the faux-hurricanes will be directed at houses and other structures built at the full-size HomeSaver facility in the Idaho desert.

Lacy joined the Clemson University team, headed by engineering professor Tim Reinhold, on the Gulf of Mexico during Hurricane Georges. Coordinating with the Texas Tech team, they distanced themselves about 40 miles apart. "A hurricane is kind of like the drain out of a bathtub," said Lacy. "It makes the hollow spot in the middle-the eye-and right at the edge of the hollow spot is where everything's going the fastest, both in the bathtub and in the hurricane."

The tower folds down for transport and is designed to withstand violent winds when it's fully extended. "It's kind of a belt and suspenders operation," said Lacy. "The whole thing's designed not to tip over at 200 mph without any anchors, and then we add the anchors for extra protection."

On the tower, at 15 feet and 30 feet, are data-gathering devices called anemometers-essentially propellers that send electronic signals to the computer at the tower's base. "They just spin with the wind in different directions," Lacy said. "Wind coming from the very side won't spin them at all. Wind coming full-on spins them the full amount. Everything else spins them a fraction."

The propellers are set up at 90-degree angles to each other in all three dimensions and sample the windspeed 15 times a second. To find where the actual wind was, Lacy will correlate the data from the different propellers. "The whole idea is to get a full 3-D picture--up, down and sideways," he said. Other devices measure barometric pressure and humidity, although not quite as often.

It is important to have accurate wind information to be able to build good hurricane-resistant structures. Other methods of wind speed measurement may result in incorrect measurements of peak gusts. For example, some meteorologists are concerned about information gained from "hot film anemometers," wind-measuring devices favored for their rapid responsiveness. Because hot film anemometers determine wind speed by how much energy is required to keep a wire at a certain temperature, they can record false high speeds if, for example, they are hit by a big wet raindrop.

An incorrect wind speed measurement can lead to a false sense of security in a structure's stability. "A storm like Georges that maybe had maximum 85-95 mph winds on shore gets recorded with a 170 mph gust," said Lacy. "And someone says 'Hey, my roof is good to 170 mph because I have a house three miles from that anemometer, so it must have seen 170 mph.' Never mind that the trees are still there."

Next hurricane season, the INEEL team will be solely responsible for one data-gathering tower and expect to hit many more storms.
The Idaho National Engineering and Environmental Laboratory is operated for the U.S. Department of Energy by Lockheed Martin Idaho Technologies Company.

Note to Editors: More information on the HomeSaver project, which includes an artist's rendering of the full-size facility, can be found at Jeff Lacy can be contacted at 208-526-2937 or Pictures of the tower, the Clemson/INEEL team, and a crushed structure are available from Mary Beckman.

DOE/Idaho National Laboratory

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