SECONDARY PRESSURE WAVES 



S91 



least forty per cent of the calculated total energy of the first bubble 

 pulsation. 



The measured impulse is plotted against charge depth for the three 

 gauge positions in Fig. 9.10, and shows only the increases with depth to 

 be expected from the inclusion of surface and bottom reflections. When 

 corrections are made for these, the points of Fig. 9.11 are obtained, 

 which indicates values of the order of one-half the theoretical curve 

 (fromEq. (9.25)). 



The experimental results do not provide any support for the pre- 

 dicted increase in effectiveness of the bubble pulse as a result of stabili- 

 zation of the bubble against migration. The failure to realize this state 



1 Impulse and energy values for the shock wave obtained by integration to 2.0 msec, after shock 

 front, for the bubble pulse by integration over times of pressure in excess of hydrostatic. 



Table 9.2. Comparison of shock wave and bubble pulse pressures 60 feet from 

 300 pound TNT charges fired in approximately 100 feet of water. 



is to be attributed to the failure of the bottom to act as the rigid, undis- 

 placed surface assumed in theoretical analysis, as discussed in section 

 8.11. At other depths than that for the anomalous pulse, the peak 

 pressure and energy of even the first bubble pulse are both very much 

 inferior to the corresponding experimental values for the shock wave. 

 The positive impulse is comparable with usually quoted values for the 

 shock wave, but the danger, emphasized in section 9.3, of drawing hasty 

 conclusions from such a comparison must be remembered. The relative 

 magnitudes of shock wave and bubble pulse parameters for the 300 

 pound charges used in this investigation are summarized in Table 9.2, 

 which includes shock wave data for 2 charges fired on the same type of 

 bottom as in the bubble measurements. It may be added that the 

 measured energy flux in the first and second pulses, except for the 

 anomalous pulse already discussed, are of the order of 5 and 1 per cent 

 of the shock wave energy flux. 



