571 



8. Maximum Bubble Radii 



A. Free Water . The high-speed motion pictures^' of the bubble were 

 used for these measurements.* The actual size cif the bubble was calculated 

 from measiirements of its apparent diameter and of the apparent length of a 

 known scale on each print. This was chocked by calculating the size of the 

 bubble from the image size on the film and the known optical characteristics 

 of the system (see Ref. (2), Appendix l). Object sizes calculated by the 

 two methods agree to about 2%. Since the scale comparison was less 

 laborious, this method was used to find the bubble radii reported here. 



In the case of the 25 gm charges (Table XVI) the area in the picture 

 covered by the bubble was found by planimettring, and the radius Tras then 

 calculated from the area. No checks were made in this series. 



The precision of the radius measurement appears to be about 2%, This 

 degree of precision is possible, however, only up to the first maximum. 

 After this point, as the bubble collapses, its outline is obscured by the 

 streamers in the water. This makes an exact measurement of the size of 

 the bubble very difficult, and at the minimum the measurements may be 

 considerably in error. Some attempt to compensate for tne masking of the 

 bubble was made by measuring the radius of the sphere at the base of the 

 streaiasrs, assuming that the streamers themselves contained none cf tSic 

 gas. This cannot be done very well at the minimum, since th«re they form a 

 compact bl\ir completely masking the bubble. 



After the first minimum, the bubble, while nearly spherical, is 

 never as smooth as diiring the first expansion. It is thus apparent that 

 the difficulty of estimating the position of its outline becomes greater 

 In the second and succeeding cycles, and the validity of conclusions baser' 

 on the estimated radii becomes more doubtful. 



From the measured radii, smoothed radius-time curves were draw for 

 each charge photographed, in some cases through the third ralniinum. As 

 can be seen in Fig. 1, a typical curve is extremely steep right after 

 detonation and near the minima. Little reliance can be put on th«« values 

 in these regionsj however, near the first maximum, the radius can b« 

 •atimated rather accurately because of the flatness of the curve, Ac a 

 consequence, the calculations carried out were based on the max3.mu.T. radii, 

 and no attempt was made to xise the observed minjjnum radii othsr than as 

 upper limits. 



Tables XV and XVI show the results obtained for the first maximum. 

 The shot numbers refer to the same charges discussed under bubble periods 

 and the exact weight of each chstrge can be found there. From the values 

 of the measured radii were calculated values of the proportionality 

 constant In the equation 



* It should be noted that the standard proced'ire was to photograph the 

 cylindrical charge with its axis normal to the optical axis of the recording 

 system. Any exceptions to this charge orientation will be specifically 

 pointed out, 



-15- 



