Blast-Wave Clearing for Detonations of High Explosives

When a blast wave impinges on the front face of a structure, rarefaction waves are formed along its free edges and propagate toward the center of the front face, and reduce the reflected overpressure: an effect known as clearing. Standards, guidelines and textbooks provide empirical equations to account for the effect of clearing. The clearing time is that instant at which the reflected overpressure decreases to the stagnation pressure. These empirical equations were used to calculate the reflected impulse for the design of a building loaded by a far-field detonation of a nuclear weapon. For such a detonation and a low-rise building, the incident shock front can be assumed to be vertical. This assumption is inappropriate for near-field detonations of high explosives because the reflected overpressure across the front face of a target varies with arrival time, standoff distance, and angle of incidence. There is little information on the effect of clearing in the literature for near-field (small-scaled distance) detonations. This paper presents the results of a computational fluid dynamics analysis of a validated numerical model that examines blast-wave clearing with an emphasis on near-field detonations. The widely used empirical equations for calculating clearing time are incorrect for near-field detonations and should not be used in design practice. Clearing in the near field is negligible and should be ignored for the purpose of design. Clearing in the far field will only be important for structural components of a width that is too narrow to be of practical importance. The physical experiments of two previous studies were simulated to gain insight into the development of the empirical equations that characterize the effects of clearing.