Shock and Awe

As they reported in the journal Annual Review of Fluid Mechanics, Settles and his colleagues at the Penn State Gas Dynamics Lab are using high-tech photography to actually look at the shockwaves produced by different kinds of explosions — from terrorist bombs to improvised explosive devices and firearms. Since shockwaves are transparent, you need more than just an ordinary camera to see them.

"We visualize the motion of shockwaves first by using special optical techniques — the Schlieren and shadowgraph techniques — to make the shockwaves in transparent air visible, and secondly by having a very high speed camera so that we can slow the motion down enough to actually see what's going on," he says.

The shadowgraph and Schlieren techniques are not new — they were developed centuries ago by scientists Robert Hooke and August Toepler. If you've ever driven on a stretch of empty road on a hot day, you may be familiar with what Hooke first observed — the shadow of air rippled by heat waves. Heat actually changes the density of air causing it to refract light rays. Because this refraction can actually cast a shadow if you shine intense light on it, it can be photographed. shockwaves also produce this same kind of shadow. The Schlieren (meaning "streaks" in German) method, relies on similar principles. Scientist Toepler rediscovered Hooke's earlier observations and used them to visualize shockwaves by looking at a regular background pattern through the disturbance they caused in the air. This would make the pattern appear distorted and reveal the shape of the shockwave.

image: Penn State Gas Dynamics Lab

In the past, only film cameras existed for photographing the shockwave shadow patterns. Now high-speed digital cameras simplify the process by eliminating the need to painstakingly develop each frame of film.

His team has also been able to expand the range of these technologies by using them to visualize much larger shockwaves than ever before. "Usually [these technologies] have been confined to the laboratory, but we've figured out ways to look at very large fields of view so we can see shockwave motion over a very large size, and we can even take it outdoors and see outdoor explosions and firearms," Settles explains.

So why would you want to visualize shockwaves in the first place?

In order to be able to predict the behavior of things such as explosives, you really have to understand what's making them tick. As Settles explains, "The more we can learn about the behavior of the terrorist explosives and the devices they make, the better able we will be to protect against them, to develop blast resistant materials and detection devices, and to negate the effects of these explosives."

According to Settles, the properties of shockwaves are pretty well understood. What isn't clear is what happens when they start interacting and bouncing around, like in the complicated series of reverberations that goes on once shockwaves are unleashed inside a building or aircraft. Those interactions are so complicated that the best way to really understand them is by watching what happens in real-time.

One of the ways Settles and his team have done this is by recreating past events using scale models, like the case of Pan Am flight 103. Describing the effects of the explosion, Settles says, "Of course it destroyed the airliner and everyone on board was killed. What happened was the shockwaves from the explosion traveled the length and the breadth of the fuselage and actually caused damage at locations remote from the original explosion because they reflected off of surfaces and generated a high pressure. These are very complicated phenomena and it's extremely hard to understand what's going on unless you can see it, and that's what we do we do, we visualize the motion of the shockwaves."

Such reconstructions allow them to look at exactly what happened and also to predict the effects of future explosions. In theory, shockwaves could even be traced backwards to detect sniper locations.

Aside from such military applications, there are many other ways to use this technology. Settles has applied the technique to improve aircraft aerodynamics and even understand the sniffing abilities of dogs. "[Shockwaves] are used in mining, there are medical uses of shockwaves like breaking up stones inside the body, and explosive welding uses shockwaves," Settles explains. "So there are many, many applications for this technology."

With faster cameras on the horizon, our understanding of the forces behind shockwaves is likely to keep improving. With the work of teams like Settles', that means the future is wide open for new applications of this technology.