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

Gunshots

All this leads inexorably to the topic of firearms, which, after centuries of refinement, now hurl bullets with high speed and deadly accuracy. Ernst Mach was cynical about his original supersonic-bullet research, and expected to be criticized for its lack of utility because "one cannot wage war with mere photographed projectiles."

Figure 10. Color schlieren and black-and-white shadowgraph techniques...

Controversial as the topic is, we can nevertheless learn from high-speed gunshot images and perhaps use that knowledge to save lives and prevent crime. Forensic investigation of gunpowder residues, point-blank gunshot wounds, shooter hearing protection and sniper location are a few topics that can benefit from observing and understanding shock waves and related phenomena.

The gunshot images my colleagues and I have produced were taken with a massive bullet stop, permission from the Penn State campus police and all appropriate safety precautions. Previous high-speed shadow and schlieren images of gunshots were limited to small fields of view, typically a few centimeters, and could thus visualize only part of the discharge. My colleagues and I developed a set-up with a field of view of up to several square meters, which is able to reveal most or all of the process.

Figure 11. This full-scale schlieren image...

The evolving flowfield of a gunshot is rather complicated over a period of several milliseconds. The interior ballistics of firearms cannot be observed by the methods described here, so the first visible phenomenon at the muzzle is the emergence of the bullet-driven shock wave, followed immediately by the bullet itself. Then the propellant gases, the products of gunpowder combustion, exit and expand tremendously as they transfer from high pressure inside the barrel to one atmosphere outside. This rapid expansion behaves like an explosion in pushing the air out of the way and thus generating a strong spherical shock wave, or muzzle blast. The "bang" of a gunshot is almost always caused by this muzzle blast.

Figure 12. Images from high-speed videos...

If you are unlucky enough to be shot at but lucky enough to be missed, sometimes you hear instead the sound of the bullet itself. Inertia keeps supersonic bullets moving at high speed, while the muzzle blast rapidly decays in strength like the spherical shock wave from an explosion. So the bullet inexorably pulls ahead of the decaying muzzle blast, trailing oblique shock waves. These shock waves produce the sensation of a sharp "crack" as the bullet passes, followed later by the "bang" of the muzzle blast. This sequence varies with timing and the hearer's position with respect to the bullet's path, making it very difficult to determine the direction of gunfire from its perceived sounds.

Figure 13. This computer-generated simulation...

The Penn State Full-Scale Schlieren System is the largest indoor schlieren system in the world, with a field of view that is two meters by three meters. Photographs made in this system show these gunshot phenomena on a grander scale than was previously possible. Not only are the exterior ballistics of the bullet revealed, but also the interaction of the muzzle blast with the shooter. Proper ear protection is essential to prevent hearing loss. Propellant-gas interactions with the hands of the shooter, the gas-dynamic behavior of various firearms and many other related phenomena of interest to ballistics can be studied in this experimental imaging facility.

One such topic, imaged for the first time using retroreflective shadowgraphy and high-speed videography, is the effect of a suppressor or silencer in reducing the strength of a gun's muzzle blast. Suppressors are illegal in many states but can be important assets to police special forces. Their effect is believed to involve slowing and cooling the propellant gases as they leave the muzzle. However, high-speed shadowgraphy reveals another effect: The lateral expansion of the propellant gas is channeled forward into a supersonic turbulent jet, reducing the strength of the muzzle blast but also generating jet noise. In other words, some of the "bang" is converted to a "hiss," which can reduce the sound level by 10 to 20 dB or more. With high-speed flow imaging and the application of gas-dynamic principles, advances in suppressor design are possible.

Figure 14. A 1-microsecond direct-illumination photo...

Finally, direct illumination of bullet impacts with a microsecond flashlamp also produces revealing images, even though the shock waves and other gas-dynamic phenomena are not visible this way. The ballistic impact of a high-speed bullet does not usually just punch a clean hole in a target, but rather shatters brittle material and disrupts soft tissue. The images we have taken were triggered electronically by a microphone, located outside the field of view, which picked up the passage of the bullet's oblique shock wave.

Ballistics, shock waves and high-speed imaging have been and continue to be crucial to many fields. Medical and materials processing applications of shock waves are similarly fascinating to observe at high speed. Faster electronic cameras with better resolution are on the horizon, potentially yielding a million frames per second and beyond. The opportunity for ingenuity in devising and applying high-speed optical imaging systems is likewise not nearly exhausted yet, and the future holds many novel applications for such experiments.

Acknowledgments
The author wishes to thank Gary and Carol Katona for assistance with Figure 10a and E. M. Freemesser and E. F. Spencer, Jr., for assistance with Figures 10b and 12.

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