Sandia’s one-of-a-kind multiphase shock tube began with a hallway conversation about five years ago that culminated in the creation of what engineer Justin Wagner describes as the only shock tube in the world that can look at shock wave interactions with dense particle fields.
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 understanding the physics that occurs at the first tens of microseconds of a blast can help improve computer codes that model what happens in explosions.
“Not having this correct in those codes could have implications for predicting different explosives properties,” says Justin (1515).
In that years-ago conversation, Steve Beresh (1515) and Sean Kearney (1512) asked a since-retired colleague, Melvin Baer, what he’d like to measure that he hadn’t been able to. “He started talking about some of the missing physics that were in the models that are used for predicting explosives, and Sean and I looked at each other and said, ‘We think we could do that,’” Steve recalls.
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 project was initially funded under the Laboratory Directed Research and Development program.
“We needed somebody to actually make it work so we hired Justin as a postdoc” to oversee the design and building phase, says Steve. Justin has since joined Sandia’s regular staff.
“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,” Steve says.
First fired in 2010
The machine, first fired in April 2010, is considered multiphase because it can study shock wave propagation through a mixture of gas and solid particles.
Particulates in an explosion start out tightly packed. As the explosive process continues, they disperse more and more 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. Currently, they are not thoroughly understood, and therefore existing models are equally limited, Steve and Justin say.
The team says better understanding the particle dynamics in the early part of a blast will help Sandia respond to national security challenges surrounding detonations, including improving explosives, mitigating blasts, or assessing the vulnerability of personnel, weapons, and structures.
A shock tube generates a shock wave without an explosion. “The important thing about the shock tube is it generates a planar shock wave,” Justin says. “We study the interaction of the shock wave with a dense field of particles to understand the physics relevant to explosives processes.”
The multiphase shock tube 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, Steve, Sean, and Justin say.
A better view of the physics
“We can get different things from the X-ray diagnostics, different things from the laser-based diagnostics, different things from temperature and pressure measurements, and by piecing all of that together we get a better view of the physics that are occurring in the shot,” Steve adds.
The machine’s unique diagnostic capabilities demonstrate Sandia’s ability to collaborate. The team particularly singles out the X-ray expertise offered by Enrico Quintana and Jerry Stoker’s group in Org. 1522.
“Once you get the thing built, then the diagnostics required to get useful information out of it are also difficult and expensive,” Justin says. “There’s a reason why it hasn’t been done thoroughly in the past.”
A lot of data for modeling come from explosions, but it’s difficult to isolate what happens in each part of a blast, Sean says.
“Whereas if you do an experiment like this you can delve deeper into what is really happening,” he says. “But it’s just one piece of the puzzle and they’re all important.”
The stainless steel and aluminum shock tube, about 22 feet long, is divided into a high-pressure or driver section that creates the shock wave, and a low-pressure or driven section, with a diaphragm or burst disk between the two. Pressure builds up in the cylindrical driver section and when it gets high enough, the diaphragm ruptures. Spherical particles loaded into a hopper above the low-pressure section flow into the shock tube before the diaphragm breaks, creating a dense particle curtain that’s hit by the shock wave.
Justin, Steve, Sean, and Brian Pruett (1515), along with Wayne Trott, Jaime Castaneda, and Melvin Baer, all now retired, made a presentation on the work in April 2011 to the Engineering Sciences External Review Board. The team says Elton Wright (6916) also made a sizeable contribution to the project.
Experiments and diagnostics are complicated, so team members are still gathering data to eventually incorporate into codes used at Sandia and elsewhere.
“It’s clear that we’ve learned some things that weren’t known before,” Steve says. “Those physics [inputs] are important to a code.”