475 



THE JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA 



VOLUME 20. NUMBER 3 



Secondary Pressure Pulses Due to Gas Globe Oscillation in Underwater Explosions. 



I. Experimental Data*t 



A. B. Arons,** J. P. Slifko,*** and A. Carter 



Underwater Explosives Research Laboratory, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 



(Received January 13, 1948) 



Pressure waves emitted by the oscillating gas globe in underwater explosions of T.N.T. have 

 been recorded at depths great enough to render small the perturbation effects due to migration 

 of the bubbles under the influence of gravity. The first eight periods of oscillation have been 

 measured and the pressure-time curves analyzed in order to obtain peak pressure, positive 

 impulse, and energy flux resulting from the first two pulses. The scaling of pressure pulse 

 parameters with charge size is examined. 



I. INTRODUCTION 



1.1. The detonation of an explosive charge 

 results in the conversion of the initial solid 

 material into a globe of gaseous products at 

 exceedingly high temperature and pressure. This 

 is followed by expansion of the gas globe and 

 propagation of a strong shock wave through the 

 fluid medium surrounding the charge. 



In underwater explosions, the succeeding phe- 

 nomena are associated with the oscillation of 

 the gas globe, which expands to a maximum 

 radius and then collapses under the influence of 

 the hydrostatic pressure. A pressure pulse (called 

 the first bubble pulse) is emitted while the 

 bubble is near minimum size, and second, third, 

 etc. bubble pulses are emitted as the bubble 

 proceeds with successive oscillations. A typical 

 pressure-time curve for the entire phenomenon 

 is reproduced in Fig. 1. 



1.2. The gas globe produced in an underwater 

 explosion tends to migrate vertically upwards 

 under the influence of gravity, the migration 

 being most pronounced in the interval when the 

 bubble is near its minimum radius. In the process 

 of migration, some of the potential energy is 

 converted into kinetic energy of vertically mov- 

 ing water. This energy is not restored to the 

 bubble as it collapses, and hence the amplitude 

 of the emitted pressure wave is less than it 



This work was performed under contract NOrd 9500 

 with the Navy Department, Bureau of Ordnance. 



t Contribution from the Woods Hole Oceanographic 

 Institution No. 430. 



Present address: Department of Physics, Stevens 

 Institute of Technology, Hoboken, New Jersey. 



* Present address : Naval Ordnance Laboratory, Wash- 

 ington, D. C. 



would have been had no migration occurred. 

 This effect does not scale with charge size in 

 the same way as other parameters because of the 

 constancy of the acceleration of gravity, g. 



From the foregoing, it is evident that the 

 pressure pulse will be at a maximum under 

 conditions which make the bubble migration 

 negligible or cause the bubble to be in a "rest 

 position." It is known that for relatively small 

 charges a rest position occurs at a certain critical 

 depth below the water surface owing to the 

 balancing of gravitational eflfects by the repulsion 

 from the free surface.' However, accurate pres- 

 sure-time measurements under these circum- 

 stances are not possible because proximity to 

 the free surface causes serious interference from 

 the surface reflection of the pressure wave. 



Since migration decreases with increasing depth 

 of detonation, it is possible, by performing experi- 

 ments at sufificiently great depths, to make the 

 migration effects relatively small, or in some 

 cases completely negligible, and thus determine 

 the parameters of the bubble pulse unperturbed 

 by the effects of nearby surfaces and bubble 

 migrations. 



1.3. The object of the present investigation 

 was to obtain such unperturbed measurements 

 in order to test the scaling of bubble pulse param- 

 eters with charge size and to obtain better 

 impulse and energy flux data than have hitherto 

 been available for the stationary bubble. 



Cast T.N.T. was used in three charge sizes: 

 0.505 lb., 2.507 lb., and 12.01 lb. Measurements 



' B. Friedman, "Theory of underwater explosion bub- 

 bles," Report IMM-NYU 166, Institute for Mathematics 

 and Mechanics, New York University, September 1947. 



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