Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

University Creates a Tempest in a Tank to Study Hurricanes

By Catherine A. Cardno, Ph.D.

The University of Miami's massive wind-wave tank is capable of mimicking Category 5 hurricanes.

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The new wind-wave tank at the University of Miami is the first in the world capable of replicating category 5 hurricane conditions. Its size—23 m long, 6 m wide, and 2 m high—makes it possible to study wind, wave, and structure interactions in three-dimensions. Gort Photography

August 11, 2015—Category 5 is the highest rating a hurricane can be given on the Saffir-Simpson Hurricane Wind Scale. And with winds of 157 mph or more, such a storm will cause catastrophic damage. According to the Miami-based National Hurricane Center, a high percentage of framed houses will be destroyed and the destruction will include complete roof failure and wall collapse. Entire regions will be rendered uninhabitable, and falling trees and power poles will cause power outages that will last for weeks and possibly months.

In comparison, one of the best known storms in the United States—Hurricane Katrina—only briefly strengthened to a Category 5 hurricane over the Gulf of Mexico before dropping to a Category 3 when it made landfall on August 29, 2005. That storm still resulted in hundreds of deaths and catastrophic damage, estimated at $75 billion in the New Orleans area and along the Mississippi coast, according to the National Hurricane Center. A decade later the city of New Orleans is still in the process of recovering. (Read " Special Report: Defending New Orleans" by Robert L. Reid in Civil Engineering , November 2013, pages 48-67 and 83.)

As sea levels rise and flooding threats increase in magnitude, frequency, and duration, preparing for ever-larger storms to make landfall is crucial for life safety and the resiliency of the built environment. (Read "Reports Help Planners Create Flood-Resistant Communities" by Catherine A. Cardno, Ph.D., on Civil Engineering online.) Now, for the first time, a wind-wave tank capable of replicating Category 5 conditions in a laboratory has been built and is successfully replicating the conditions of such large storms. Located on Virginia Key, a barrier island just off the coast of mainland Miami, the lab is housed within the campus of the University of Miami's Rosenstiel School of Marine and Atmospheric Science.

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The lab that houses the hurricane tank is structurally isolated within the university’s Marine Technology and Life Sciences Seawater research complex. Because of its location on a barrier island off the coast of mainland Miami, the complex has been designed for storm surges and to meet hurricane codes. Gort Photography

"Previous studies of wind-wave interaction in hurricanes have pointed to the need to really understand what happens in these extreme conditions," said Brian K. Haus, Ph.D., a professor of ocean sciences at the Rosenstiel School and the director of the lab. Haus wrote in response to questions posed by Civil Engineering online. "This facility is the first in the world that will be able to simulate extreme category 5 conditions," Haus noted. "Furthermore the width of the facility will enable true three-dimensional studies of wind-wave-structure interactions."

The lab is located within the campus's Marine Technology and Life Sciences Seawater (MTLSS) research complex, a building designed to withstand significant challenges from its environment. "The entire building is elevated approximately 4 m because it is in a storm-surge zone and must also meet Miami-Dade county hurricane codes, which are the toughest in the United States," Haus said. Approximately one-third of the building is devoted to the lab, an open-plan, two-story space that contains the 23 m long, 6 m wide, and 2 m high tank as well as two offices, two work rooms, a storage room, staging areas, and mechanical and electrical rooms.

"The building [itself] is really two structures because the SUrge STructure Atmosphere INteraction (SUSTAIN) lab that houses the wind-wave tank is physically separated from the rest of the building—no connecting structural components—to prevent transmission of vibrations," said Haus.

The tank—which comprises two sections, one acrylic and one concrete—is elevated an additional 2.5 m above the floor of the already elevated building. An 18 m long by 6 m wide by 2 m tall acrylic test section, which can be filled up to halfway with water, is fitted into a steel structural grid that is used to rigidly mount instrumentation. Waves are created by a mechanical wavemaker with 12 piston-type paddles that is housed in a 5 m long concrete basin attached to the acrylic test section.

A sloping "beach" reaching 1 m in height has been created from aluminum parabolic sections and is located at one end of the acrylic test section, acting as both a mounting surface for test structures and as a way to dissipate wave energy.

The "wind" for the hurricane tests is created by a 1,460 hp axial fan that blows air through a series of ducts located above the test section. "The wind in the test section can exceed 65 m/s, which, when scaled by the boundary layer scaling [method] usually used, is [equivalent to] well over 100 m/s," Haus noted. Operating the tank is such a power draw that a diesel generator is used to reduce the peak demands on the grid when the tank is operating, according to the lab's website.

For each test, the tank is filled to the desired level with seawater drawn from existing intakes that supply the campus laboratories. The water passes through a number of filters and a settling tank before it can be used, starting with a 1 in. screen and ending with a 10 micron screen. "After use, if nothing is added to the water, the water has its thermal energy extracted, is filtered again, and is deep injected into a brine layer," Haus explained. "If something is added (seeding material, dye, et cetera), the water is treated in the sanitary sewer system," he noted. "No water is returned to the sea once it has been pumped from the settling tanks."

The thermal energy extracted from the water is used within the building, reducing the structure's overall reliance on the energy grid, Haus noted. "Many other energy-saving features are enabled by the intelligent building controls, such as on-demand heating/cooling and lighting," Haus said. "[The] building was designed to meet LEED [Leadership in Energy and Environmental Design] silver standards, which was challenging given its mission, its location in a storm surge zone, and Miami-Dade wind codes, which imposed many other design constraints."

The tank was officially launched at the end of May, and its first few months of testing have gone well, with only minor modifications of the system required. "Initial testing in extreme winds (>60 m/s) revealed that the exit ductwork needed to be strengthened and that some modifications were needed to improve how exiting sea-spray was handled," Haus said. "These have been completed and testing is commencing."

Thus far, the structural tests for the tank have included foundations fixed to a rigid platform, but future collaborations and testing of the built environment could expand the types of foundations tested in the tank. "There are many questions related to what happens when the atmosphere, ocean, natural and built structures, and organisms come together in extreme conditions," Haus explained.

"We have a unique and unprecedented opportunity to address these in a quantitative way [and] this will allow us to address many longstanding questions, including loading on structures, the role of sea-spray, and hurricane intensification," Haus said. "However, I hope that a decade from now we will be working on interdisciplinary research that I cannot envision at this time."

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