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
Optics in the Transparent World
Even a transparent phenomenon sometimes leaves telltale signs. For shock waves these signs can include moisture condensation, dust disturbance, whitecaps on water, optical distortion and shadows. Certain aquatic predators find their transparent prey by the shadows that the Sun casts on the ocean floor.
Robert Hooke discovered this effect more than three centuries ago while observing the shadow of a burning candle cast by sunlight. Above the flame he saw a plume of hot air that was not directly visible but cast a shadow because the heat changes the density of the air, which refracts light rays. What Hooke described is now called the "shadowgraph" method, and it's a simple approach that works extremely well for visualizing shock waves.
Hooke also discovered another visible trait of transparent phenomena: They can distort the features of a background pattern that is viewed through them. In this way an antique glass windowpane warps one's view of the world outside. But Hooke was ahead of his time, so this observation principle lay unused until the mid-19th century, when the German scientist August Toepler rediscovered it and used it to observe electric sparks. He saw spherical waves in the air from loud spark discharges and thought he was observing sound, but actually he was the first to see shock waves. Toepler named his optical method the Schlieren method (Schlieren means "streaks" in German). Although the technology has changed significantly, particularly for capturing large fields of view, that name for this method persists today.
In the 1880s, Ernst Mach and his colleagues used the schlieren method to observe gunshots and thereby settle an argument about what actually happens when a bullet travels faster than sound. They saw shock waves trailing from a supersonic bullet like the water waves from a speedboat. Such observations became essential to the new field of ballistics. Eventually Mach's name was linked to the non-dimensional ratio of an object's velocity V to the speed of sound: the Mach number.
Mach's motto was "seeing is understanding." He, Hooke, Toepler and other, more recent investigators all understood a principle that unites the worlds of technology and art: In order to understand a new or complicated phenomenon, one needs a physical picture of it early in its study. This is especially true of flow patterns in gases and liquids, which are usually transparent. Without at least a conceptual picture, working with fluids is like working with solid objects in the dark. Getting that picture, whether by experiment or computer simulation, invokes a special branch of the field of fluid dynamics called flow visualization. The schlieren and shadowgraph techniques used to image shock waves are vital tools for visualizing flows that have a different refractive index than the surrounding air, and therefore bend light.
Because these tools and the study of ballistics and explosions are over a century old, it may be hard to picture what could renew interest in them. In addition to the current need for counter-terrorism measures mentioned earlier, investigators also now have modern electronic high-speed cameras with which to capture transient explosive events and fast-moving shock waves. Sadly for some of us, the era of photographic film is almost over. However, with its demise, the rather painful methods of high-speed cinematography are being replaced with high-speed videography, which has an ever-improving frame rate and resolution and comparatively magnificent user-friendliness, as well as compact and robust packaging. This allows the simpler optical methods, such as shadowgraphy, to break out of the laboratory and take to the field, where one can accomplish high-speed imaging of shock waves at an unprecedented scale. Coupled with the utility of small (gram-range) explosive charges, which are used for safety and convenience in research, this technology opens new vistas in the study of shock waves and explosions.
The contribution of my lab, the Penn State Gas Dynamics Lab, to this topic has been primarily in freeing the shadowgraph and schlieren methods from the benchtop, applying them to large fields of view without the need for impractically large parabolic telescope mirrors, and even taking them outdoors. We have extended these methods to security applications such as aircraft hardening, where they had never been used before but were sorely needed. Currently we are exploring the broad range of scientific studies that can be done safely and inexpensively with small, gram-sized explosive charges and the quantitative optical measurement of shock-wave motion using modern high-speed videography. Such experimental data are important not only to elucidate the physics of explosions, fragmentation and blast damage, but also to guide and validate computational simulations of these events.
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