Analysis of blast parameters in the near-field for spherical free-air explosions

The report focuses on the calculation of blast parameters that are used for designing structural elements to resist blast induced loads. It addresses shortcomings encountered with the widely used relationships proposed in the Kingery-Bulmash technical manual. The parameter values in it have proven accurate for medium and large scaled distances, but serious doubts have been raised by researchers on their validity for small scaled distances, i.e. close-in detonations. As supporting experimental data in this distance range are scarce, numerical simulations have been employed in this investigation. The finite element code EUROPLEXUS is used to study the evolution of spherical blast waves and their parameter values for near and far-field explosions. Modelling of both the explosive (through the JWL equation) and of the air is made. The results from one-dimensional and three-dimensional analyses are favourably compared with those of other specialized programs and with limited actual test data. For the numerical discretisation mesh sensitivity studies are conducted and an equation is proposed that allows the use of a variable cell size in relation to the scaled distance, that ensures both accurate results and reduced computational costs. From the analysis it is revealed that the Friedlander equation cannot capture the overpressure-time behaviour at small scaled distances as it fails to take into account the effect of the expanding detonation products. At such distances it is also found that there is a big difference between the peak overpressure and positive impulse values proposed by Kingery-Bulmash and those calculated through the numerical simulations, the latter being considerably higher. As a result a new set of equations in terms of scaled distance are proposed that provide improved parameter accuracy for points close to the detonation centre.