The ground impulse generated by a plane fuel-air explosion with side relief

Abstract

Detonations can be initiated in unconfined fuel-oxidizer clouds by blast waves of sufficient energy. The ground impulse generated by such detonations can cause considerable damage. An analytical theory is developed in the present paper, which makes it possible to predict this impulse, and experimental measurements which verify the theory are reported.A plane detonation wave propagating through a one-dimensional fuel-air cloud in contact with the ground and with the inert atmosphere at height h is considered. The wave is followed by an expansion wave which propagates from the inert gases into the combustion products, and an oblique shock is induced in the inert bounding the explosive. In computing the impulse only the region behind the detonation where the expansion is reflected from the ground is considered. The impulse per unit area is found to be the product of p2(h/C) and a universal impulse function Us([xi]), where p2 is the pressure behind the detonation and C is the Chapman-Jouguet velocity, and [xi] = x/l is the dimensionless distance from the detonation.Experimental values of the pressure and impulse behind a plane wave were obtained using a plastic bag, 4 x 4 x 20 ft long, filled with a stoichiometric MAPP-air mixture. The detonation was initiated using an explosive initiator at one end of the bag. Pressure transducers placed on the ground plane along the center of the bags were used to determine the variation of the pressure and impulse per unit area with time. Theory and experiment were found to be in excellent agreement.