Confusing The Wind: The Burj Khalifa, Mother Nature, and the Modern Skyscraper

Skyscrapers undergo rigorous wind tunnel testing during the initial design phase. Rowan Williams Davis & Irwin Inc. (RWDI), one of the world’s leading wind engineering consulting firm, has handled the testing for numerous projects around the world including the Burj Khalifa and Taipei 101.

These elaborate replicas go through several rounds of testing in a specialized wind tunnel. Unlike the tunnels used to test airplane wings, sporting equipment and other small projects, these boundary layer wind tunnels are designed to simulate changes in the wind speed with height and can replicate the variable wind environments in which the buildings will ultimately be constructed. Inside the tunnel, the model is rotated at all different angles and wind effects are sometimes visualized using smoke. All of this data is then fed into computer models in order to perform additional analysis. In the case of the Burj Khalifa, wind tunnel testing led to a dramatic design change: the entire building was rotated 120º to reduce wind loading. Ultimately, this process of wind testing, provides structural engineers with a nuanced understanding of wind loads.

Just a few hundred miles away from the towering Burj Khalifa stands the slightly less imposing 50-story Bahrain World Trade Center in the center of Manama. In this structure, wind is not just treated as an obstacle to be overcome, but as a source of energy to be harnessed. It’s the first skyscraper to integrate large wind turbines into its design. Three 225-kilowatt, 95-foot diameter wind turbines hang from separate walkways connecting the identical, sail-shaped towers. Together, these turbines supply about 15 percent of the towers’ electricity, the equivalent, designers say, of the energy needed to power over 300 homes. The unique shape of the buildings directs wind gusts towards the turbines, increasing wind speeds and creating an artificial wind tunnel between the two towers.14

“Usually,” says Jan Klerks, Research & Communications Manager at the Council on Tall Buildings and Urban Habitat, skyscrapers “have to fight the wind but this type of building is trying to grab the wind.”15

Around the world, other skyscraper projects are beginning to take advantage of the wind to save and even generate energy. One of them is the Pearl River Tower, a 71-story supertall set to open in 2010 in Guangzhou, China. Originally conceived as a “zero-energy skyscraper,” one that uses zero or even negative energy, the Pearl River Tower has been designed both to reduce energy consumption and generate some of its own power. The building employs a wide variety of strategies, including four integrated wind turbines, to accomplish these goals.16

“The building is actually positioned on the site to be directly facing into the prevailing wind,” says Russell Gilchrist, designer of the Pearl River Building and practice leader at Skidmore, Owings, & Merrill, the architecture firm that also designed the Burj Khalifa. “It’s slightly counterintuitive on one level to place the broad face into the wind because it attracts quite a lot of wind load. In this case, we choose to ignore that and tried to tap into the wind potential.”17

The Pearl River Tower has four integrated wind turbines placed inside separate three-by-four meter holes, or apertures, one-third and two-thirds of the way up the building. Unlike normal wind turbines, these vertical axis turbines can capture wind blowing in almost any direction.

The apertures themselves serve to increase wind speed, making the turbines more efficient. Because of the prevailing winds, pressure builds up on one side of the tower. The pressure difference between the two faces causes air currents to be sucked through the apertures. The power potential of the turbines, Gilchrist explains, is related to the cube of the wind speed blowing against the tower. As a result, faster winds through the apertures result in a sizeable increase in the energy generated by the turbines. These four integrated turbines, Gilchrist claims, will ultimately produce as much energy as sixty-four freestanding ones placed on the roof of the building.18

The apertures also serve a dual function as a pressure release valve, alleviating some of the wind loads on the building, says Gilchrist. This not only decreases structural stress but also reduced the amount of steel and concrete needed to reinforce the building.