567 



6. Second sSi Third Bubble Periods in Free Water 



No piezoelectric records of the bubble pulses after the first were 

 obtained; however, it was found possible to get a period record from the 

 motion pictures taken of the bubble in free water. The values were 

 obtained from radius-time curves which are probably not as precise as 

 piezoelectric records. Comparisons between piezoelectric and photographic 

 records of the first period show differences of 1-2/.; the later periods 

 are more difficult to measure and somewhat greater discrepancies would be 

 expected. 



The results for second period measurements are shown in Table XI. 

 The period constant for torpex-2 is about 5% higher than that for TNT, and 

 those for tetryl and pentolite about 1-25S higher. This order is essentially 

 the same as for the first period. The ratios T2/T1 were also calculated 

 and in Table XII are compared with values obtained from other sources. It 

 might be noted that the aluminized explosives show a smaller value of this 

 ratio in each series. 



Table XIII gives the values obtained for the third bubble periods 

 from measurements on the photographs. The bubblo period constants cal- 

 culated from these values show greater scatter. This would be anticipated 

 because of the difficulty of discerning the outline of the bubble after the 

 first maximum, 



7. First Bubble Periods Close to a Free Surface 



As 'part of a study of the interaction of bubbles with the surface, the 

 explosions of a number of 25 gm tetryl charges were photographed very close 

 to the surface!) . Charge depths ranged from 15 in. to 5 ft, which distances 

 correspond approximately to 0,75 and 3 bubble radii. Thus in the shallowest 

 case, the bubble aLLways vented; at intermediate depths, the behavior was 

 not exactly reproducible, presumably because of uncontrolled variations in 

 the surface or in the depth. At 21 in. — a distance only very slightly 

 greater than ^^enting depth — the bubble appeared to drag in air from the 

 atmosphere as it collapsed. This resulted in an increase in the apparent 

 minimum size and a damping of the oscillation. As a result, no piezoelectric 

 record of the bubble pulse was obtained in these cases. 



The periods were measured on the high-speed photographs by counting 

 the number of 1000 cycle timing marks between the frame showing detonation 

 and that showing the minimum. Such an estimation of the period is based 

 on a subjective judgment as to the times of detonation and of the minimim 

 relative to the frames and the timing marks. The error Incurred may easily 

 amount to as much as a millisecond, and is probably more at the 21 in. 

 depth where the minimum is indefinite. The values of the periods so obtained 

 are shown In Table XIV, the values for loose and pressed tetryl being shown 

 eeparately. 



To obtain period constants comparable to the vsd-ues obtained in free 

 water, corrections for the effect of the free surface of the ocean and for 

 the bottom must be applied. This was done by the method Indicated in the 



-11- 



