10. Surface and Other Effects 



10.1. Phenomena above the Water Surface 



The most obvious and spectacular features of underwater explosions 

 are the disturbances of the water surface above the charge. Several 

 distinct phenomena are observed which change in magnitude and ap- 

 pearance with depth, but are most prominent for charges fired near the 

 surface.^ The first effect of the explosion to arrive at the surface is the 

 underwater shock wave (assuming that the initial products of explosion 

 are completely under the surface). The shock wave arrival at dif- 

 ferent points is visible as a rapidly expanding ring of apparently dark- 

 ened water, often called the ''slick." This ring owes its contrast against 

 the background to the fact that ripples or small waves are calmed by 

 the reflected shock wave pressure and a layer of cavitated, bubbly water 

 is formed. 



The slick can be seen on a reasonably calm surface out to large dis- 

 tances compared with the size of the charge, even if the charge is quite 

 deep. Its speed depends on its position and the depth of the charge, 

 but is always greater than the speed of the shock wave, because at any 

 point the shock wave velocity is the projection of the slick velocity in 

 the direction of propagation of the shock wave (see section 10.3). The 

 slick is thus lost to view in a few hundredths of a second, if visible at all, 

 and can be resolved only by high speed photographs. 



Following the slick is the growth of the spray dome, a whitish mound 

 of broken Avater thrown up by the reflection of the shock wave at the 

 surface. The initial velocity is proportional to the pressure in the 

 incident wave and hence is largest directly over the explosion. The 

 extent to w^hich a visible upward motion can be seen around this point 

 is determined by the pressure and the effect of cavitation, and for suffi- 

 ciently deep charges the dome is not visible at all. The water thrown 

 up near the center of the explosion rises more rapidly, and continues to 

 rise for a longer time than the water further away, and as a result the 

 dome grows steeper sided as it rises. The time during which the rise 

 continues and the maximum height, reached when the opposing forces 

 of gravity and air resistance have reduced the velocity to zero, depend 

 on the initial pressure and velocity, which in turn depend on the weight 

 of charge and its depth below the surface. The analj^sis in more detail 



^ A number of writers have discussed surface phenomena from various points of 

 view. A complete list of references bearing on the subject is not attempted here, 

 but two reports by Kennard (54, 56) may be m(>ntioned here and others are cited in 

 section 10.2. The qualitative introductory discussion of this section is based largely 

 on photographic records obtained at Woods Hole, especially the results reported by 

 Slichter, Schneider, and Cole (103). 



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