226 PHOTOGRAPHY OF UNDERWATER EXPLOSIONS 



detonation. The possibility of distortion by refraction in passage 

 through the shock front must be recognized in analyzing any such pic- 

 tures. This was tested by measurement of the known separations of 

 steel rods visible in Plate I, and the apparent distance found to be from 

 2 to 8 per cent greater than the actual values. (The elastic compression 

 of the rods by the pressure wave is computed to give displacements of 

 less than 0.1 inch.) The apparent bubble diameters for the 2^ inch 

 spheres were 3.7, 5.9 and 6.9 inches at times of 13, 58 and 104 micro- 

 seconds after detonation. The initial radial velocity of the bubble is 

 therefore of the order of 4,000 ft. /sec, this figure being correct only 

 as to order of magnitude because of uncertainties as to the exact times 

 and distances. 



The motion of the bubble over the greater part of its pulsations is 

 very much less rapid than in the initial phase, and can be followed in 

 sufficient detail with motion pictures. For small charges with bubble 

 periods of several hundredths of a second or less, special high speed 

 cameras are needed. The records reproduced in Plate IX and Fig. 8.1 

 for a 0.5 pound charge with a period of 28 microseconds were taken, 

 for example, with a 35 mm. Fastax camera running at about 1,800 frames 

 per second, illumination being provided by flash bulbs. For work with 

 larger charges, lower film speeds and longer exposures are possible. As 

 an example, Plate IV shows the gas products from a 56 pound TNT 

 charge fired at a depth of 100 feet. The photographs were taken from 

 40 feet with natural light by a Jerome motion picture camera running 

 at approximately 45 frames per second. The first frame at 0.18 seconds 

 after detonation shows the spherical outline of the bubble near its maxi- 

 mum expansion, and the second at 0.34 seconds the spiked protuberances 

 at the first contraction. 



Many investigations of bubble motion have been made by photog- 

 raphy, a number of which are described in Chapter 8. Comparatively 

 few results have been obtained except on a laboratory or model scale, 

 owing to obvious practical difficulties in obtaining adequate lighting 

 conditions and clear water in field work. Some very beautiful small- 

 scale photographs of both bubble motion and shock waves were obtained 

 in German investigations during World War II, but at the time of this 

 writing few technical data concerning them were available, and they 

 cannot therefore be adequately described here. 



C. Ejfeds at and near targets. Photographic techniques have proved 

 useful in understanding and analyzing the motion of water near bound- 

 aries, such as the free surface of the water and deformable plates. Some 

 examples of this kind discussed in section 10.4 show very clearly the 

 formation of cavitation in water under small negative pressures near 

 such surfaces, and a number of investigations have been carried out to 

 examine the mechanism of such cavitation. 



