10 THE SEQUENCE OF EVENTS 



The motions of the gas sphere from 300 pounds of TNT detonated 

 50 feet below the surface are sketched in Fig. 1.4. The data are ob- 

 tained from somewhat inexact analysis (see Chapter 8), and are intended 

 only to illustrate the general characteristics of the motion. 



1.5. Secondary Pressure Pulses 



The motion of the gas sphere has associated with it emission of 

 energy in the form of pressure waves advancing radially outward from 

 the bubble. If it is assumed that motion of the water around the bubble 

 is incompressible, it can be shown that the pressure in the water should 

 depend on the square of the rate of bubble expansion or contraction. 

 From Fig. 1.3, it is evident that this rate is greatest when the bubble is 

 near the point of smallest volume. One should therefore expect, both 

 from incompressive theory and the fact that only in this region is the 

 water under appreciable compression, that the pressures will be signifi- 

 cant only in a small interval about the time of maximum contraction. 

 This is the state of affairs observed; pressure pulses are emitted which 

 build up to a maximum value at times corresponding to the minimum 

 volumes and fall off again as the bubble expands. 



The form of the bubble pulses from a given charge depends con- 

 siderably on the depth of water and proximity of boundary surfaces, as 

 would be expected from the effect of these factors on the bubble motion. 

 The peak pressure in the first bubble pulse is no more than ten to twenty 

 per cent of that of the shock wave, but the duration is much greater, 

 and the areas under the two pressure-time curves are comparable. A 

 considerable amount of the energy initially present is lost at the time of 

 each pulse, both in the pulse and in turbulence resulting from rapid 

 radial and vertical motion of the gas sphere (see Fig. 1.4). As a result 

 successive pulses are progressively weaker and usually only the first 

 pulse is of practical significance. The relation between shock wave and 

 bubble pulse pressures and durations is shown in the sketch of Fig. 1.5 

 of a continuous pressure-time record at a point 60 feet from the same 

 size charge. Fig. 1.6 shows in more detail the bubble pulses from 300 

 pound TNT charges detonated at various depths in 100 feet total depth 

 of water. It will be observed that the profile of the curve becomes more 

 irregular for initial charge positions close to the surface or bottom. It 

 should also be noted that pressure waves reflected from the surface and 

 bottom give rise to interference, and the later portions of observed pres- 

 sure-time curves may be considerably different from the pressure-time 

 curve which would be observed in an infinite medium. The same inter- 

 ference phenomenon occurs in shock waves but is less effective and more 

 easily recognized because of the shorter duration and discontinuous 

 front (see Fig. 1.2). 



Because of the fact that gravity is an omnipresent factor in bubble 



