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Sandia’s 3-D code enhances accident simulations

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When the space shuttle Columbia disintegrated on re-entry in 2002, sophisticated computer models were key to determining what happened.

Sandia National Laboratories researchers Steve Plimpton, left, and Michael Gallis look at a projection of a model of the Russian MIR space station, which fell out of orbit several years ago and disintegrated, with parts crashing into the Indian Ocean. Using Sandia's 3-D code SPARTA, the calculation is simulating an instance of the process of de-orbiting. (Randy Montoya/Sandia Labs)

Sandia National Laboratories researchers Steve Plimpton, left, and Michael Gallis look at a projection of a model of the Russian MIR space station, which fell out of orbit several years ago and disintegrated, with parts crashing into the Indian Ocean. Using Sandia’s 3-D code SPARTA, the calculation is simulating an instance of the process of de-orbiting. (Randy Montoya/Sandia Labs)

A piece of foam flew off at launch and hit a tile, damaging the leading edge of the shuttle wing and exposing the underlying structure. Temperatures soared to thousands of degrees as Columbia plunged toward Earth at 27 times the speed of sound, said Sandia fluid science and engineering researcher Michael Gallis, who used NASA codes and Icarus – a Sandia Direct Simulation Monte Carlo, or DSMC code – for accident simulations that proved critical to investigators.

The investigation helped researchers realize a more sophisticated code would be even more valuable in future inquiries at the same altitude and speed and for broader applications. Now Gallis and computational scientist Steve Plimpton have created a parallel three-dimensional DSMC code called SPARTA. In July, Sandia released it as open source, available to anyone here. In addition, Gallis presented results of work using SPARTA at an invited keynote lecture in China at the 29th annual International Conference on Rarefied Gas Dynamics.

3-D codes like SPARTA represent physical reality more accurately than 2-D codes such as Icarus. Better simulations mean designers can account for many more details in new spacecraft or satellites. However, there’s a computational price for greater realism. “A 3-D simulation is like a series of 2-D ones, sometimes making it thousands of times more demanding,” Gallis said.

DSMC codes simulate molecules moving and bouncing off each other and objects, just as they do in real gas flows. Underlying statistics determine when and how molecular collisions occur, enabling predictions of energy transfer and chemical reactions. The DSMC approach typically is used to model low-pressure gases. Physical problems where gas is at low pressure are less common than problems with gas at higher pressures.

“Monte Carlo” refers to the randomized way in which collision parameters are chosen for pairs of particles, based on statistical principles. The order in which molecules collide is random, but not the rate or outcome of a large number of collisions, which can be described by well-known mathematical models.

More than 20 years ago, Sandia researcher Tim Bartel and Plimpton developed Icarus, still considered a workhorse for DSMC applications. Gallis and Plimpton began working on SPARTA about two and a half years ago – doing some of their brainstorming while walking on the Santa Fe Plaza during breaks at an international DSMC workshop Sandia hosts every two years.

“Michael is very, very good with DSMC physics and Steve Plimpton is very, very good at formulating problems so that large parallel machines can solve them. That combination has given us a parallel code with very sound physics that runs quickly,” manager Dan Rader said.

Sandia is largely interested in DSMC for two research areas where gas molecules are relatively far apart: re-entry vehicles, including the effects of flight through the outer reaches of the atmosphere, and micro-electrical-mechanical systems (MEMS) that have features at the micron and submicron scale. Examples of MEMS include chemical flow and pressure sensors, accelerometers and transducers.

The chaotic nature of turbulence makes it difficult to investigate, but Gallis said being able to use DSMC to study fluid mechanics at a more fundamental level may help to better understand the mechanisms of turbulence.

The next frontier is to take advantage of DSMC to model and study flow physics – energy exchange and chemical reactions in colliding molecules – at a more fundamental level than possible with continuum codes, Gallis said.

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