FEATURE ARTICLE
High-speed Imaging of Shock Waves, Explosions and Gunshots
New digital video technology, combined with some classic imaging techniques, reveals shock waves as never before
Gary Settles
Explosions
What causes such a strong shock wave? Since a stereo system makes sound waves, can one turn the volume up to maximum and make shock waves? No, stereo speakers are only designed to vibrate in order to reproduce sound. Shock waves are made by a rapid, continuous "push," or by an object traveling at supersonic speed. Cracking a whip creates weak shock waves, because the whip tip moves faster than the speed of sound.
But the best way to generate a strong shock wave in the air is suddenly to release a lot of energy stored in a small space. Pressurized gas is an example: On release, the gas expands very quickly and pushes the atmosphere out of the way, forming a shock wave. Even popping a balloon is enough to generate a very weak shock wave from the gas released when the balloon skin ruptures. In the laboratory, shock waves are best studied in a facility known as a shock tube, where they are generated by the rupture of a thin diaphragm separating high- and low-pressure gases.
Explosives are another good way to produce shock waves. In this case, the energy is stored in an unstable chemical form—often in nitrates—and can be released in about a microsecond. Ironically, most chemical explosives contain less energy per unit mass than ordinary table butter, but fortunately the butter is too stable to explode.
The loss of life caused by an explosion is often due to fragmentation rather than the overpressure or the following wind of the shock wave itself. Shrapnel behaves like a hail of supersonic bullets, being accelerated along radial lines in all directions from the explosion center by the aerodynamic drag force exerted by the rapidly expanding gas.
But strong shock waves are also devastating to structures. In the 1995 terrorist bombing of the Murrah Federal Building in Oklahoma City, a huge truck bomb was detonated only a few meters from the building. The resulting strong shock wave and its many concomitant effects destroyed the columns supporting the north face of the building, whence it collapsed. As a result, 168 lives were lost and there were many more injuries. Both experiments and computational blast simulations now help inform building designers on how to mitigate such lethal effects and how to prevent building collapse and improve survivability.
The experiments can sometimes be dangerous and costly, however, when done at full scale. A recent trend is toward cheaper, safer, quicker simulations of blast effects using gram-range explosive charges, scale models and optical shock-wave imaging. By applying known scaling laws to small explosions in the laboratory, investigators can simulate shock-wave and fragmentation effects on planned buildings or transportation vehicles, for example, using scale models. The high-speed digital video cameras my colleagues and I use record shock position over time by schlieren or shadowgraphy, from which we can determine all post-shock fluid properties.
Even after several costly full-scale blast experiments involving real airplanes, the gas-dynamics of explosions onboard commercial aircraft remains poorly understood. Better understanding is needed if aircraft are ever to be hardened against catastrophic in-flight failure resulting from explosions, whether deliberate or accidental. Interior explosions in aircraft (as in buildings) are complicated by shock-wave reverberation from interior surfaces. In 1988, the wreckage of Pan Am Flight 103 in Lockerbie, Scotland, at first seemed to show the effects of multiple simultaneous blasts at various fuselage locations. As investigations progressed, it was realized that shock waves had traveled the length and breadth of the fuselage, sometimes reflecting and thus causing local blowouts remote from the actual terrorist bomb located in the forward cargo hold.
Optical shock-wave imaging can help explain the complicated consequences of such onboard explosions. In addition to simulations, the U.S. Transportation Security Administration recently did tests on actual air-cargo containers filled with luggage, which were blown up by planted terrorist-scale explosives. For the first time, high-speed videography captured shock-wave motion in these experiments. To do this, a retroreflective shadowgraphy method pioneered by Harold E. "Doc" Edgerton proved robust enough to function in the field despite environmental extremes and severe shock loads on the apparatus.
Retroreflective screens return to the camera lens orders of magnitude greater illumination than does the simple diffuse white screen that is often used for shadowgraphy. The screen functions like a spherical reflector, returning much of the light striking it to its point of origin. For high-speed video shadowgraphy, a retroreflective screen is a necessity for creating a bright image.
A flaw in Edgerton's original method is that the camera axis had to be slightly offset from that of the light source. This creates a confusing double image in the resulting video. A beamsplitter could be used to correct this, but with a large loss in illumination intensity. Instead, we affixed a small mirror at a 45-degree angle to the center of a filter over the camera lens and reflected the beam off of this surface before sending it to the screen. This arrangement provides perfect alignment between light source and camera axes, and there is no noticeable loss of shadowgram quality as a result of the small area of camera lens occluded by the mirror.
» Post Comment