SHOCK WAVE MEASUREMENTS 24S 



very good account of the experimental observations.^ The predicted 

 peak pressures are of the order of 10 per cent higher than experimental, 

 but the exponents of W^'^/R agree rather well, the poorest agreement 

 being in the case of tetryl, for which the data were scantier and covered 

 a smaller range of W^^^/R. The comparison of absolute values of im- 

 pulse is somewhat meaningless unless some decision is made as to when 

 and how the shock wave ends (see section 4.8), but the agreement of 

 observed and calculated values is as good as can be expected in view of 

 this difficulty. The energy-flux parameters are somewhat less am- 

 biguous and, although absolute theoretical values are systematically 

 high, the weight-distance exponent is in good agreement. In nearly 

 all cases, comparison of the Kirkwood-Bethe theory with experiment 

 shows that the former leads to pressures and initial time constants of 

 decay which are somewhat too large, the discrepancy being larger for 

 time constants. 



The variations with distance of all shock wave parameters are pre- 

 dicted rather well, and it can be concluded that the theory agrees with 

 experiment at least as well as could be expected. 



The upper limits of pressures to which the similarity curves discussed 

 are fitted do not exceed 25,000 Ib./in.^ corresponding to a distance of 

 about 7 charge radii. At closer distances, the effects of dissipation at 

 the shock front and the true form of the detonation wave in the ex- 

 plosive are more important. Rather limited data at higher pressures 

 reflect this difference by showing that the rate of decay with distance 

 becomes greater close to the charge. More extensive measurements in 

 this region would be of great interest in determining the conditions be- 

 fore the shock wave has progressed very far, and in particular for esti- 

 mating the total shock wave energy release (see section 4.8). The data 

 so far available give only meager or rather inaccurate values of pressure 

 and duration, although they do show the expected increase in rate of 

 dissipation and rate of pressure decay (decreased time constant). 



At the other extreme of low pressures, a number of complicating 

 phenomena are present, which are associated with the long paths in- 

 volved. Ultimately, in an ideal fluid of infinite extent, an ordinary 

 acoustic wave of much lower pressure and longer duration than the 

 shock wave near the charge would be expected from theory. Water is, 

 however, not an ideal fluid, seemingly small viscosity effects are no 

 longer negligible, differences in temperature with depth cause refraction 

 of bending of sound rays, and the surface and bottom of the sea intro- 

 duce reflected waves which interfere with the direct wave to a point of 

 observation. A more detailed discussion of these complicating phe- 



^ The agreement is not immediately obvious from the constants and exponents 

 listed in Table 7.3, as numerical values of the constants are sensitive to small changes 

 in the exponent. 



