NIST simulator tells WTC story

NIST's Fire Dynamics Simulator recreates the smoke plumes from the World Trade Center towers.

To understand the fires that helped bring down the World Trade Center towers on Sept. 11, National Institute of Standards and Technology scientists are simulating the collapse on computers.

The fires produced about a gigawatt of energy, roughly equivalent to the output of a large power plant, the NIST scientists concluded. The NIST team developed its modeling tool, the Fire Dynamics Simulator, over the last two decades in the agency's Building and Fire Research Laboratory.

Computer modeling is the only way for NIST to study why the towers collapsed because 'there is very little physical evidence left,' said Ronald G. Rehm, a fellow at the fire lab. 'Photos and models are all you have.'

Using the simulator, researchers have reconstructed the fireballs and smoke plumes generated by the crash of two hijacked airliners into the towers. By comparing the models against photos and videos, they have drawn conclusions about the energy released.

'This is probably the biggest reconstruction of a fire we'll ever do,' said Kevin McGrattan, a mathematician at the lab. NIST hopes to get funding for a more comprehensive, two-year study.

The Fire Dynamics Simulator 'is state of the art,' Rehm said, although its modeling of burning solids 'is very immature yet.' But it is the best tool of its kind.

Fire science is a relatively new engineering field. 'About 30 years ago they started using simple models in building design' with formulas that could be worked out on a calculator, McGrattan said.

By the 1990s, computational fluid dynamics on desktop computers could model airflow for weather prediction and aircraft design.

'A lot of the ideas we started with came from weather modeling' because it predicts the influence of heat on air movement, McGrattan said. 'When you look at a thunderstorm, you have a lot of the same simulations' as in a fire.

When McGrattan came to the lab 10 years ago, Rehm and fellow scientist Howard R. Baum were working on the simulator, a fairly small program at the time.

'The basic code just moved the hot air around,' McGrattan said. 'You have to do more than that. You have to describe the environment it moves in.'

Model upgrade

As computers got faster, the team expanded the model to simulate walls, floors, ceilings and furniture. They added an interface to describe these items' thermal properties. The lab made the first version of the simulator source code publicly available in February 2000.

'At that point we had something people could use,' McGrattan said. 'Compared to most commercial software, it is still not user-friendly,' but engineers and lawyers with some knowledge of computational fluid dynamics can use it.

The simulator now consists of two components. About 10,000 lines of code handle fluid dynamics computations, and another 2,000 lines make up a SmokeView visualization program added in 1999.

'I think the visualization is essential,' said computer scientist Glenn Forney, who helped develop SmokeView. 'It's very difficult to get insight by looking at raw numbers. A picture is worth a thousand words.'

Forney got the idea for SmokeView from seeing star field simulators on the Web. 'It occurred to me that stars were essentially the same as smoke particles,' he said. The software can visualize temperature gradients, oxygen concentrations, heat flux and flame sheets.

'We've designed it to run on a PC because that's what everyone has,' McGrattan said. 'At work I prefer a Unix machine, but we have to cater to our customers.'

Forney added, 'It wasn't feasible to expect people to buy a $30,000 workstation' to see visualizations.

Producing World Trade Center fire simulations on a grid of 108 cells in each direction required about 15 CPU hours on a 1-GHz PC. One of two basic computational models produces a 500-second model, and the second about 30 seconds.

'If this incident had occurred a year or so earlier, I don't know if the simulator could have done it,' he said.

Anticipating a need for such a study, the researchers have been adding enhancements, such as a multiblock model that can run simulations on widely varying spatial scales simultaneously.

To extract details about conditions inside the structures, NIST is seeking additional photographs and video images that document the initial damage and the subsequent fire growth, especially in Building 7 and the south and west faces of the two towers.

More information about the study appears on the Web at wtc.nist.gov.

About the Author

William Jackson is freelance writer and the author of the CyberEye blog.

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