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
Shock Waves
But first consider what a shock wave is and what it is not. What does it mean to read that some political event "sent shock waves around the world"? Such "shock waves" are obviously figurative and not real. The meteorite impact that led to the demise of the dinosaurs at the end of the Cretaceous Period and the volcanic explosion of Krakatoa in 1883 really did send shock waves around the world, but such destructive events are fortunately rare.
Hollywood clearly does not understand shock waves, resulting in some ludicrous cinematic special effects. On television, Bart Simpson sent a shock wave rippling across Springfield by yelling into a row of megaphones ganged together in series. Children who try this at home will be disappointed—it doesn't actually work to produce shock waves. In movies, the hero might outrun the blast from an explosion on his motorcycle. Real motorcycles cannot begin to approach such speeds, and if they did they would not likely stay on the ground. But actual shock waves, in fact, are much more interesting than anything Hollywood has come up with so far to represent them.
A shock wave has no substance itself; rather it is an extremely thin wavefront that passes tsunami-like through solids, liquids and gases at high speeds, driven by molecular collisions at the nanoscale. It is defined as a compression wave—a sudden spike in pressure followed by a sudden drop in pressure—formed, for example, when the speed of an object (such as a bullet) is faster than the speed at which the surrounding medium (such as air) transmits sound.
Sound waves in the air, whether from a whisper or a yell, travel at the speed of sound, called a, for "acoustic" speed. This speed depends on air temperature, but a is typically about 340 meters per second in "standard" air. Shock waves, on the other hand, travel faster than a, being supersonic wave phenomena. They're also stronger and more energetic than sound waves, are highly nonlinear and cause significant jumps in temperature, pressure and density of the air over their wave thickness of only nanometers. The passage of a strong shock wave through the human body, for example, causes severe damage owing to the large instantaneous pressure change.
Normal conversation, with a sound intensity in the 60- to 70-decibel (dB) range, involves minuscule air-
pressure fluctuations of less than one millionth of an atmosphere. Painfully loud "noises," such as those from a jet engine in the 110-dB range, are actually very weak shock waves. One can see them using the optical methods described here, but they travel barely faster than sound waves, with pressure peaks of only some hundred-thousandths of an atmosphere. On the other hand, a strong shock wave in air, such as one traveling at Mach 2, produces an overpressure peak of 4.5 atmospheres—more than enough to destroy the delicate human hearing mechanism and wreak other biological havoc. However, this phenomenon can be controlled for medically beneficial purposes as well: A method called shock wave lithotripsy focuses shock-wave energy at a point inside the body to break up kidney stones without significantly damaging the surrounding tissue.
Spherical shock waves from explosions decrease quickly in strength with distance from the explosion center, rapidly leveling out to Mach 1.0, or the speed of sound. This rate of speed decrease can be extracted from a high-speed shadowgraph video. As Harald Kleine of the Australian Defence Force Academy and his colleagues outlined in their 2003 paper in the journal Shock Waves, the shape of the curve produced by graphing this speed-decrease data can be used to find an explosive's equivalent mass, as compared with the standard of trinitrotoluene (TNT).
Close to an explosion, a shock wave can travel at several times the speed of sound and reach pressures of ten or more atmospheres, producing devastating effects. Also, the "wind" that immediately follows a strong shock wave is brief but very intense. In an explosion, the fireball expands very quickly and pushes air ahead of it. As the shock wave ripples out from the explosion center, the speed of its following wind is the same as the speed of expansion of the initial fireball. A shock wave at a mere Mach 1.3 already has a stronger following wind than the fastest natural tornado-generated wind speed ever recorded. Footage of pre-1963 aboveground nuclear tests shows the shock wave smashing whole buildings, whose debris is then swept downrange by the following wind.
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