Shock Diffraction in the Vicinity of a Structure
NAVAL ORDNANCE LAB WHITE OAK MD WHITE OAK United States
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A study of the shock-diffraction pattern in the vicinity of a structure was conducted to determine the loading of a structure in the vicinity of another structure and to determine at what distance the diffracted shock recovered to its free-field condition. Fourteen Wiancko inductance-type pressure-time gages were mounted at ground level, and 5 feet above the ground, in an array to the rear and side of the 3.1t structure. This structure was 12 feet long, 6 feet wide and 6 feet high, and was located 2,200 feet from ground zero. It was oriented with its long side nearly, but not quite, parallel with the shock front. Pressure applied to the gage varied its inductance which, in turn, varied the frequency of a Hartley oscillator, of which the gage formed the inductive element. The frequency-modulated signal was transmitted by wire, and recorded on magnetic-tape recorders. In playback, the frequency-modulated signals were converted to amplitude-modulated signals and presented as pressure-time traces. Even though the shock loading on the 3.1t structure was different for Shots 9 and 10, the diffraction effects disappeared at about the same distance from the structure on both shots. Diffraction effects to both the side and rear were visible, but almost gone, at 4S, where S is a characteristic dimension of the structure front-face height or half width, whichever is less the effects had disappeared at 8S, and it is estimated that the blast wave again was identical with the free-field condition at 6S. Although the diffraction effect in front of the structure was not measured, it seems reasonable to expect that such an effect extended to the same distance as did the effects observed to the side and rear.