Measurement of the Physical Properties of Blast Waves

A blast wave is formed in an ambient atmosphere when there is a rapid release of energy from a concentrated source. Examples of such sources are the detonation of an exothermic material such as trinitrotoluene (TNT); nuclear fission or fusion; the rupture of a pressurized container; a spark, or the rapid heating caused by a focused pulsed laser. The sudden release of energy causes the material of a symmetrical centered source to expand rapidly as a spherical piston. This piston produces a compression wave in the ambient gas. If the speed of the piston is fast enough, or of long enough duration, the compression wave develops into a shock wave. A shock wave is characterized by the very rapid increase, within a distance of about ten mean-free-path lengths, of all the physical properties of the ambient gas, namely, the hydrostatic pressure, density, particle velocity, temperature and entropy. Immediately behind the shock front the properties decay in an exponential fashion, and in the cases of hydrostatic pressure and density will fall below the values of the ambient atmosphere. The particle velocity also decreases until it comes to rest and begins to move in the opposite direction. A typical time-history of a physical property such as hydrostatic pressure, density, particle velocity and dynamic pressure at a fixed point in a blast wave is shown in Fig. 1. The period when the physical properties are above the ambient value is known as the positive phase, and the period when the properties are below the ambient value is the negative phase. The duration of the positive phase is slightly different for each of the physical properties. Close to the minimum of the negative phase a second