Blast Wave


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blast wave

[′blast ‚wāv]
(physics)
An air wave set in motion by an explosion.

Blast Wave

 

motion of a medium caused by an explosion. Under the influence of the high pressure of the gases formed during the explosion, the initially unperturbed medium undergoes abrupt compression and takes on high velocity. The state of motion is transmitted from one layer of the medium to another, so that the region enveloped by the blast wave expands. On the front of the expanding region, the medium passes abruptly from the initial, unperturbed state to a state of motion, with higher pressure, density, and temperature. The abrupt change in the state of the medium (the shock wave) propagates with supersonic speed.

A blast wave is characterized by a change with time in the pressure, density, and velocity of the medium at various points in space or by the distribution of these quantities in space at fixed moments of time.

One of the important parameters that determines the mechanical action of a blast wave is the maximum pressure created by the wave. During explosions in gaseous and liquid mediums, the maximum pressure is attained at the moment of compression of the medium in the shock wave. Another important parameter is the time interval of the action of the blast wave. The maximum pressure decreases and the time of action increases with increasing distance from the place of the explosion. (See Figure 1.)

During the propagation of a blast wave in solid mediums, the shock front dissipates comparatively quickly, and the blast wave is converted into a number of consecutive, rapidly decaying oscillations, which are propagated with the velocity of elastic waves.

Blast waves have the property of similarity. In accordance with this property, the distances at which the blast waves for explosions of chemical charges of identical shape but different masses have the same maximum pressure value are related as the cube root of the charge masses. The time interval of the action of the blast wave varies in the same way. For example, if the distances and time interval given in Figure 1 increased by a factor of 10, then such a blast wave will correspond to the wave produced by the explosion of 1 ton rather than 1 kg of trinitrotoluene (TNT).

A blast wave tends toward the rapid loss of the characteristics determined by the nature of the explosion; therefore its subsequent motion is mainly determined only by the amount of energy transmitted to the surrounding medium. Because of this circumstance, blast waves produced in the same medium by explosions of different types prove to be basically similar; this makes possible the introduction of the so-called TNT equivalent for explosion characteristics.

A propagating blast wave expends a considerable part of its mechanical energy in heating the medium near the center of the explosion. For example, at a distance of 10 km, a blast wave in air caused by a 1,000-ton chemical explosive charge contains about 10 percent of its original explosive energy; in a nuclear explosion of the same energy, the wave has half as much energy (5 percent) owing to greater losses in heating the air. The maximum pressure increase in the wave for the abovementioned values for distance and explosion energy is

Figure 1. Change with time of the pressure of a blast wave in air at distances of 1 m, 2.7 m, and 11m from the center of an explosion of a spherical 1 — kg charge of TNT

measured in hundreds of newtons per sq m (thousandths of a kilogram-force per sq cm). At great distances the blast wave is a sound wave (or an elastic wave in a solid medium).

Sound waves in the atmosphere (or elastic waves in the earth’s crust) caused by explosions of sufficiently great energy can be recorded by special devices (microbarographs, seismographs, and so on) at great distances. For example, for explosions with energies on the order of 1013 joules (several thousand tons of TNT), waves are recorded at distances of several thousand kilometers; for explosive energies of -1016 joules (several million tons of TNT), waves are recorded at almost any point on earth. At such great distances a blast wave is a long sequence of extremely low-frequency oscillations of atmospheric pressure (or ground tremors in the case of underground explosions). (See Figure 2.)

Figure 2. Recording of atmospheric-pressure oscillations in an air wave at a distance of 11,500 km from an explosion with an energy of 1016 joules. The wave traverses such a distance in about 10 hours.

REFERENCES

Raschet tochechnogo vzryva s uchetom protivodavleniia. Moscow, 1957.
Sedov, L. I. Melody podobiia i razmernosti v mekhanike, 4th ed. Moscow, 1957.
Liakhov, G. M., and G. I. Pokrovskii. Vzryvnye volny v gruntakh. Moscow, 1962.
Gubkin, K. E. “Rasprostranenie vzryvnykh voln.” In the collection Mekhanika v SSSR za 50 let, vol. 2. Moscow, 1970.

K. E. GUBKIN

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