539 



PHOTOGRAPHIC MEASUREMENTS OF THE ATTRACTION 



OF AN UNDERWATER EXPLOSION 3U83LE TO A 

 BOX MODEL TARGET 



A. R. Bryant 



April 1946 



StroDoscopic photographs at i miUisecoPd intervals have been taken of the bubble produced 

 when 1 oz. charges of polar Anvpon-ngel i jnite were fired at various distances from the target plate of 

 the R.R.L. BOX Model. The photographs show that the buoble was strongly attracted to the target. 

 For a charge distance of about 194 inches the bubble made contact with the target plate at the time 

 of its first minimum radius. Graphs of the radius and displacement of the bubble when near its 

 minimum ^re given as functions of the time for various charge distances 



The displ aCrmont of the bubble At the minimum was in reasonable agreem>"nt with values calculated 

 on the assumption that the flat target plate and rigid surrounding flange may be treated as ?: rigid disc 

 of equal aroa. This agreement is regarded as Indicating that the theory provides e good estimate 

 of the linc.r momentum f.ssoclated with the moving bubble, except perhaps during ^ short time near the 

 minimum. The observed minimum radii were considerably larger than the predicted v=.1ues, end the 

 maximum velociti,-s correspondingly lower than the predicted values, throughout the range of charge 

 distances considered. 



Int ro due ti en. 



In early measurements of the deflection of the R.R.L. Box Model target plate it was apparent 

 that the explosion bubble was contributing appreciably to the damage (l). The following experiments 

 were designed to determine photographically the movement of the bubble towards the target in order to 

 assist in an analysis of the damage and to confirm the theory of reference (2), with regard to the 

 bubble's displacement. 



Expt-.ri'nental '■:ethod. 



The R.R.L. Sox Model was rigidly held with its target plate vertical at a depth of 5 feet in 

 R.R.L. tank, and 1 cz. charges of polar Ammon-gel ignite were fired at distances ranging from 30 to 18 

 inches from the centre of the target plate. The method of mounting the box model may be assumed to 

 hold the box absolut-'ly rigid. Photographs at one millisecond intervals were taken of the bubble by 

 a camera situated abcut lO feet from and at the same depth as the charge. The Arditron equipment 

 described in reference (3) was used to provide the stroboscopic light flash, and the bubble appeared 

 as a silhouette against a bright background. A simple rod framework attached to the rigid flange of 

 box model served to hold the charge in position and to provide reference marks in the photographs. 

 A plan and elevation of the camera and target positions are shown in Figure 1. Target plates of 

 Doth -i inch and -i inch thickness were used. 



In each shot a group of about 20 pictures was taken near the time of the first bubble minimun. 

 In a few cases a further group were taken near the expected time of the second minimum, but the 

 pictures were cf no interest as the Bubble was then in all cases an irregular mass of gas clinging to 

 the surface of the target plate and with little observable tendency to expand or contract. 



