Blast wave

BLAST WAVE (EN: blast wave, blast air, explosive wave; DE: Explosionswelle; FR: onde explosive; ES: onda explosiva; RU: взрывная волна) is the process of the short-time disturbance of the balanced state of the medium (gaseous, liquid, or solid), which is propagating from the explosive source; it is the special case of the shock wave. Under the impact of the pressure of the gaseous products from the detonation, the particles of the medium are being involved into the movement, which is directed outward from the explosive source, and this process is transferred from the initial particles to the other particles, thus this process is propagating through the medium as the blast wave. During the passage of this blast wave, the particles of the medium experience the deformations of the compression and shear. Near the explosive source, these changes are the greatest, and they generate the significant increase of the temperature and pressure (if the gas is the environment), of the pressure (liquid), of the mechanical stresses (solid material, namely, soils, rocks, and so on). In the latter case, the particles of the medium are subjected to the mechanical destruction, and to the large non-reversible deformations.

With the transfer of the energy to the environment by the expanding products from the detonation, the pressure within the source of the explosion is decreasing, the energy, which has been transferred to the medium, is partially transferred by the blast wave, and is partially absorbed by the medium, because of the non-reversible processes, which are proceeding within this medium, while the blast wave itself influences the ever increasing volumes of the medium during the distancing from the source, and the intensity of this blast wave is decreasing. The effective width (along the direction of the propagation of the wave) of the region, which is influenced by the substantial perturbations, is limited, that is at any distance from the source, during the moment of the arrival of the front of the wave, the particles of the medium are involved into the motion, which is attenuated after the passage of the blast wave.

The character of the perturbations within the blast wave is substantially different for the cases of the gaseous, liquid, and solid media, within which this blast wave is propagating. Within the gaseous and liquid media, the leading front of the blast wave represents by itself the very narrow zone, within which the parameters of the motion are changing very abruptly, practically in the jump-like manner, from the initial values, which correspond to the non-perturbed state of the medium. Within the solid medium, the jump-like leading front of the blast wave exists only at the small distances from the explosive source; outside the limits of this region around the source, the parameters of the motion within the blast wave are changing smoothly. Within the gaseous and liquid media, there is observed only the decrease of the characteristic values of the parameters within the blast wave. At yet greater distances, the passage of the blast wave does not cause the brittle destru ction of the particles within the medium, and the wave is degenerating into the elastic wave. If the medium does not possess the brittleness (soft soils, ductile metals, and so on), then near the source the passage of the blast wave is accompanied by the elastic-plastic deformations of the particles within the medium, but outside this zone, the wave is also degenerating into the elastic wave.

As the parameters, which are characterizing the blast wave within the homogeneous and non-limited medium, they consider the speeds of the particles, pressures, mechanical stresses, temperatures, and so on, with the dependence on the time; the maximal values of the momentum, energy, and other integral characteristics of the waves for the fixed distance from the source, and also the time of the arrival for the front of the wave.

If the medium, within which there has been performed the explosion, is heterogeneous, then the process of the forming and propagation of the blast wave becomes substantially complicated. For the quantitative description of this process, there is necessary the special research in each concrete case. However, it is possible to make certain general conclusions about the character of the influence by the heterogeneity onto this process. If there exists within the medium the layer with the decreased values of the speed of the propagation of the perturbations, then the blast wave is transformed so, that its energy is transferred to the particles of the layer; as the result of this fact, the perturbation propagates through this layer with the lesser attenuation, than it would have been within the homogeneous layer, that is the layer plays the role of the waveguide. In the case of the heterogeneity, there may be observed the refraction of the rays of the blast wave, namely, their deviatio n toward the region with the decreased speed of the propagation of the perturbations. In case of the existence of the surfaces with the jump-like change of the mechanical properties of the medium, there would proceed on these surfaces the refraction and reflection of the blast wave. The quantitative description of these phenomena depends substantially on the properties of the medium, and has the important significance for the solving of the applied problems. In particular, the process of the reflection of the blast wave within the rocks and soils from the exposed surface of the massif plays the substantial role for the crushing and displacement of the part of the rock, which is being broken from the massif, and also for the destruction and ejection of the soil during the explosion for the ejection. For the determination of the parameters of the blast wave at the relatively large distances from the source, in case of the existence of the exposed surface, where the blast wave may be c onsidered as the "seismic" wave, there exist the empirical dependencies.