Characteristics of the Blast Wave in Air

Most of the material damage from an airburst (nuclear weapon) is caused mainly by the shock (or blast) wave which accompanies the explosion. The majority of structures will suffer some damage from air blast when the overpressure in the blast wave, i.e. the excess over atmospheric pressure (101.3 kN/m at sea level), is about 3.5 kN/m or more. The distance to which this overpressure level will extend depends on the yield of the explosion and on the height of the burst. 

A difference in air pressure acting on separate surfaces of a structure produces a force on that structure the size of these forces is dependent on the difference between ambient air pressure and the overpressure. The maximum value of overpressure, known as the peak overpressure, occurs at the front of the blast wave. 

As the blast wave travels in the air away from its source, the overpressure at the front steadily decreases, and the pressure behind the front falls off in a regular manner. After a short time, when the shock front has travelled a certain distance from the fireball, the pressure behind the front drops below that of the surrounding atmosphere and a so-called negative phase of the blast wave forms, as given in Fig. 9.1. 

For the curves marked t to t4 the pressure in the blast wave has not fallen below atmospheric pressure, but on the curve ts it is seen that at some distanee behind the shock front the overpressure is below that of the original atmosphere so that an underpressure exists. 

During the negative (rarefaction or Suction) phase a partial vacuum is produced and the air is sucked in, instead of being pushed away, as it is when the overpressure is positive. 

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The explosion hazard of a vapor cloud can be quantifled in terms of its explosive power after ignition. The explosive power of a vapor cloud can be expressed as an equivalent explosive charge (TNT or fuel-air) whose blast characteristics, that is, the distribution of blast-wave properties in the charge s vicinity, are known. 

Good descriptions of the characteristics of air blast waves appear in References 5-7. The description here is paraphrased from Reference 5. 

Atmospheric effects of large-scale TNT expins have also been studied in depth both practically and theoretically. Factors considered include pressure and impulse effects, decay characteristics and travel and duration times, all as a function of distance, and for both free-field and reflection situations (Refs 3,9,15,16, 17,24,32, 33,34,35,36,44, 53,75,76,115 116). A distinction is made between the blast area dose to the source, comprising air and the products of expln, and that farther away involving air only (Ref 53). Double-burst conditions (fireball and shock wave interaction, and torus formation) have been studied (Ref 149), as have also the dynamics of dust formation and motion (Refs 25,26 117). Performance tests were run on a naval blast valve (Ref 92), and on aircraft wing panels (Ref 4)... 

Disturbance near source of spherical blast from a polytropic expl in a surrounding medium is analyzed. A second blast wave is the result of the breakdown of continuous gas flow in the neighborhood of a singular characteristic. For all types of expls, the 2nd shock is a 2nd order effect in terms of the square root of the time from the end of detonation. Application to PETN charges in air and water is discussed... 

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