Shockwaves and High Speed Videography
New digital video technology, combined classic imaging techniques, reveals shock waves as never before.
Shock waves were discovered more than a century ago, yet there is still much we do not understand about them. Shock waves are a defined as compression waves – a type of high-speed wave which passes tsunami-like through solids, liquids and gases inducing a sudden rise, then an immediate drop in pressure.
Shock waves are created in many ways both natural and man made, they are responsible for the crash of thunder, as well as the bang of a gunshot, the boom of fireworks, or the blast from a chemical or nuclear explosion.
In an effort to better understand shock waves, a new technology has been developed which combines high-speed videography equipment with classical visualization methods to capture these transparent shock waves like never before.
Shockwaves were first observed by Robert Hooke over 3 centuries ago, but being before his time, Hooke’s findings were left unused until the mid-19th century when a German scientist called August Toepler rediscovered them.
Toepler used Hooke’s method to observe the spherical waves in the air caused by loud spark discharges; he thought he was seeing sound when he was actually seeing shockwaves. He named this technique the Schlieren method (meaning streaks) and although the technology has progresses significantly, the name remains the same.
In the 1880s, Ernst Mach and his colleagues used the Schlieren method to observe gunshots. These observations became essential to the new field of ballistics and eventually Mach’s name was linked to the non-dimensional ratio of an object’s velocity V to the speed of sound.
Without special instruments and controlled conditions in the laboratory, shockwaves are completely invisible. However, shockwaves do leave behind visible tell-tale signs such as moisture condensation, dust disturbance, whitecaps on water, optical distortion and shadows.
In this image, U.S. Navy Blue Angel F-18 fighter is flying transonically over the San Francisco Bay leaving visible sings of shockwaves. The air surrounding the plane is first compressed by the shockwaves, as the air suddenly expands, visible moisture condensation is produced. The shockwaves have also induced a wind which is strong enough to leave a small wake on the water.
A good way to generate strong shockwaves in the air is to suddenly release a lot of energy stored in a small space. This can be done by popping a balloon and releasing the pressurized gas inside. As the gas expands quickly, the surrounding air is pushed out of the way forming shockwaves.
As the balloon bursts, the skin shreds rapidly, revealing a balloon-shaped bubble of compressed air inside. Notice how a spherical shockwave is formed despite the initial non spherical shape of the balloon.
Experiments in this field can be dangerous and costly when done at full scale. Many experiments are now scaled down making them cheaper, quicker and safer.
The detonation of a small 10-milligram silver nitrate charge produces a primary and secondary spherical shockwave that are irregularly reflected by the ground. In open air shockwaves are spherical and symmetrical however, reflections of objects can make the shockwaves complicated.
A 60 percent scale-model simulation (left) investigating the luggage-container bomb that destroyed Pan Am flight 103 in 1988. And a full-size simulation (right) of Ramzi Yousef’s attempt to bring down a Philippine Airlines flight in 1994, using a nitroglycerin bomb under a passenger seat.
Explosions often cause injuries 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 due to the aerodynamic drag force exerted by the rapidly expanding gas.
These two charges are 1 gram each of triacetone triperoxide (TATP) encased in solid containers. Ignited electrically, they produce spherical shock waves each one about 1 meter in diameter. At left, the container fragments into large pieces that are hurled at near the speed of sound behind the shock wave. In the image at right, the fragments are much smaller and travel at supersonic speeds ahead of the main shock. In full-scale explosions, fragments like these are as deadly as a hail of bullets.
When it comes to firearms, the evolving shockwave of a bullet is rather complicated. First we see 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 the high pressure inside the barrel to the atmosphere outside.
These color Schlieren and black-and-white shadowgraph techniques capture similar information but emphasize complementary details. The color Schlieren photograph (left) captures the firing of a .22-caliber pistol. The air, shockwave and the transonic bullet have left the muzzle, followed by the propellant gases. Toward the right side of the image, thermal convection rises from the gun and the shooter’s hand. A frame from a high-speed shadowgraph video (right) shows the firing of a single round from an AK-47 submachine gun, with its spherical muzzle blast and supersonic bullet trailing oblique shock waves.
In this example two spherical shock waves are seen, one is centered about the gun’s muzzle (the muzzle blast) and the second around the cylinder. The supersonic bullet is visible at the far left. The cloud of gunpowder combustion that envelops the hands of the shooter leaves a trace that can be identified by forensics.
Images from high-speed videos elucidate the acoustic difference between the firing of a .45-caliber pistol (left) and the firing of the same pistol with a suppressor, or silencer, attached (right).
Without a suppressor, powder gases expand laterally out of the muzzle driving a strong muzzle-blast wave that causes a loud report. With a suppressor in place, the lateral expansion is reduced, weakening the muzzle blast and transferring the sound of a ‘bang’ to ‘hissing’ jet noise which follows the bullet. This can reduce the sound level by up to 20 dB.
This computer-generated simulation of a Schlieren image is derived from a numerical solution of the equations of motion to mimic the emergence of a supersonic bullet—here, modeled as a cylinder—from the muzzle of a gun. High-speed Schlieren imagery verifies simulations such as these, which in turn have made significant contributions to the field of fluid dynamics.
The direct illumination of bullet impacts with a microsecond flashlamp can also produces revealing images, even though the shock waves and other gas-dynamic phenomena are not visible using this technique.
This image illustrates how destructive high-speed bullets and their accompanying shock waves are towards soft tissue and cellular material. 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.
- Unavailable, please contact us for more information.