Researchers use shock tube for insight into physics early in blasts

Sandia National Laboratories' one-of-a-kind multiphase shock tube began with a hallway conversation that led to what engineer Justin Wagner describes as the only shock tube in the world that can look at how shock waves interact with dense particle fields.

The machine is considered multiphase because it can study shock wave propagation through a mixture of gas and solid particles.

Shock tubes have been around for decades. What makes Sandia's unique is its ability to study how densely clustered particles disperse during an explosion. That's important because better understanding of the physics during the first tens of microseconds of a blast leads to better computer models of what happens in explosions.

"Not having this correct in those codes could have implications for predicting different explosives properties," Wagner says.

Understanding how particles move and react in the early part of a blast will help Sandia respond to such national security challenges as improving explosives; mitigating blasts' or assessing the vulnerability of personnel, weapons, and structures.

The project started when Steve Beresh of Sandia's aerosciences department and Sean Kearney of the Labs' thermal and fluid experimental sciences asked a since-retired colleague what he'd like to measure that he hadn't been able to. He started talking about some of the physics missing from the models used for predicting explosives, "and Sean and I looked at each other and said, 'We think we could do that,'" Beresh says.

They came up with the idea of a multiphase shock tube that would enable researchers to study particle dispersal in dense gas-solid flows.

The machine was fired for the first time in April 2010. Experiments and diagnostics are complicated, so team members are still gathering data they eventually will incorporate into codes used at Sandia and elsewhere.

The project, initially funded under Sandia's Laboratory Directed Research and Development program, hired Wagner to oversee the machine's design and building. "When we hired Justin we had an empty room and a blank sheet of paper. Now we have a shock tube that is different from what anybody else in the world has," Beresh says.

Particles in an explosion start out tightly packed. As the explosive process continues, they disperse and quickly become widely spaced. But the physics of the densely packed particles at the start of the explosion are crucial to everything that comes later. They are not yet fully understood, and thus limit current models, Wagner and Beresh say.

"The important thing about the shock tube is it generates a planar shock wave," Wagner says. "We study the interaction of the shock wave with a dense field of particles to understand the physics relevant to explosives processes."

Sandia's machine uses such diagnostics as high-speed pressure measurements, high-speed imaging, and flash X-ray to measure gas and particle properties, and it's adding laser-based diagnostics, team members say.