percomputer in the world, with a maximum processing speed of a thousand trillion calculations per second.

Mapping the blood stream

Think you have a pretty good idea of how your blood flows? Think again. The total length of all of the veins, arteries and capillaries in the human body is between 60,000 and 100,000 miles. To map blood flow through this complex system in real time, Brown University professor of applied mathematics George Karniadakis works with multiple laboratories and multiple computer clusters.

In a 2009 paper in the journal Philosophical Transactions of the Royal Society, Karniadakas and his team describe the flow of blood through the brain of a typical person compared with blood flow in the brain of a person with hydrocephalus, a condition in which cranial fluid builds up inside the skull. The results could help researchers better understand strokes, traumatic brain injury and other vascular brain diseases, the authors write.

Modeling swine flu

Potential pandemics like the H1N1 swine flu require a fast response on two fronts: First, researchers have to figure out how the virus is spreading. Second, they have to find drugs to stop it.

Supercomputers can help with both. During the recent H1N1 outbreak, researchers at Virginia Polytechnic Institute and State University in Blacksburg, Va., used an advanced model of disease spread called EpiSimdemics to predict the transmission of the flu. The program, which is designed to model populations up to 300 million strong, was used by the U.S. Department of Defense during the outbreak, according to a May 2009 report in IEEE Spectrum magazine.

Meanwhile, researchers at the University of Illinois at Urbana-Champagne and the University of Utah were using supercomputers to peer into the virus itself. Using the Ranger supercomputer at the TACC in Austin, Texas, the scientists unraveled the structure of swine flu. They figured out how drugs would bind to the virus and simulated the mutations that might lead to drug resistance. The results showed that the virus was not yet resistant, but would be soon, according to a report by the TeraGrid computing resources center. Such simulations can help doctors prescribe drugs that won't promote resistance.

Testing nuclear weapons

Since 1992, the United States has banned the testing of nuclear weapons. But that doesn't mean the nuclear arsenal is out of date.

The Stockpile Stewardship program uses non-nuclear lab tests and, yes, computer simulations to ensure that the country's cache of nuclear weapons are functional and safe. In 2012, IBM plans to unveil a new supercomputer, Sequoia, at Lawrence Livermore National Laboratory in California. According to IBM, Sequoia will be a 20 petaflop machine, meaning it will be capable of performing twenty thousand trillion calculations each second. Sequoia's prime directive is to create better simulations of nuclear explosions and to do away with real-world nuke testing for good.

Forecasting hurricanes

With Hurricane Ike bearing down on the Gulf Coast in 2008, forecasters turned to Ranger for clues about the storm's path. This supercomputer, with its cowboy moniker and 579 trillion calculations per second processing power, resides at the TACC in Austin, Texas. Using data directly from National Oceanographic and Atmospheric Agency airplanes, Ranger calculated likely paths for the storm. According to a TACC report, Ranger improved the five-day hurricane forecast by 15 percent.

Simulations are also useful after a storm. When Hurricane Rita hit Texas in 2005, Los Alamos National Laboratory in New Mexico lent manpower and computer power to model vulnerable electrical lines and power stations, helping officials make decisions about evacuation, power shutoff and repairs.

Predicting climate change

The challe

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