479 



SECONDARY PRESSURE PULSES IN UNDERWATER EXPLOSIONS. I. 275 



The integration was made over the entire bubble 

 pulse as defined in Section 4.1, including both 

 the positive and negative phases, and the aver- 

 aged results are summarized in Table VI. 



4.6. Energy flux measured at Gl vertically 

 above the charge is systematically high and that 

 at G4 below the charge systematically low, as 

 would be expected on the basis of the migration 

 indicated in Table II. As with the peak pressure 

 (Table 111), the energy flux is higher at points 

 off the cap end (G2) and lower off the butt end 

 (03) than off the cylindrical surface (G2,3). 



First pulse energy flux measured in the hori- 

 zontal plane off the cylindrical surface (G2,3) is 

 appreciably decreased by migration losses only 

 in the most severe case — that of the 2.5-lb. 

 charge at 250 ft. The remaining scatter in this 

 column of Table VI is probably due to experi- 

 mental error. 



4.7. From the successive period measurements 

 given in Table I, it is possible to compute the 

 total energy lost by the bubble during the in- 

 tervals defining the first and second pulses : 



(B„-5„+0=5„[l-(^)] 



(4) 



where J5„ = total energy associated with the wth 

 oscillation; r„ = period on «th oscillation. 



Taking Bi as 490 cal./g. of charge and using 

 the values given in Table I, it is found that the 

 total energy lost by the bubble during the first 

 pulse is 300 cal./g. and during the second pulse 

 85 cal./g. Assuming spherical symmetry about 

 the center of the charge, the G2,3 results of 

 Table VI were converted to obtain the total 

 acoustic energy flowing through a spherical sur- 

 face having a radius equal to the value of R as 

 defined in that table. The resulting energies for 



Table V. Variation of positive impulse of first bubble 

 pulse with depth of detonation. Z = depth below surface 

 (ft.), Za = Z+33 (ft.), //W'i = reduced impulse at R = Wi/ 

 0.352, W = chaTge weight (lb. T.N.T.). 



40 

 283 

 533 



1.66* 

 1.34 

 1.15 



3.1 

 3.4 

 3.3 



the first two pulses are 120 and 15 cal./g., 

 respectively. 



It is evident that the measured acoustic radia- 

 tion accounts for only a fraction of the total 

 energy loss sustained by the bubble in each 

 case. At this time there exists no concrete evi- 

 dence pointing to the cause of the unaccounted 

 energy loss, although various speculations have 

 been advanced regarding the role of turbu- 

 lence, chemical reactions among detonation prod- 

 ucts, etc. 



V. EXPERIMENTAL TECHNIQUES 



5.1. The charges used were cylinders of cast 

 T.N.T. boostered with pressed tetryl jDellets. 

 The weight of tetryl was converted to an equiva- 

 lent weight of T.N.T. by multiplying by 1.03 as 

 suggested by the results of various period meas- 

 urements made at this laboratory. One gram was 

 added to the charge weight to account for the 

 explosive in the engineers special blasting caps 

 which were used to detonate the charges. 



5.2. Tourmaline piezoelectric gauges'' were 

 used as the pressure sensitive element. The 

 gauges were I inch in diameter and had an 

 average sensitivity of 24 micro-microcoulombs 

 per Ib./in.^ 



5.3. Eight piezoelectric gauges were mounted 

 on ^-in. steel cable in the plane of a 15-ft. 

 diameter steel ring. The ring was suspended in 

 a vertical plane and the charge was mounted at 

 its center. Pairs of gauges were placed at equal 



Table VI. Acoustic energy flux: First and second bubble 

 pulses (no correction applied for bubble migration). Dis- 

 tance of point of measurement from center of initial charge 

 position: R = Wi/0.3S2, H' = charge weight (lb. T.N.T.), 

 Z = depth of charge below surface (ft.). 



' See Section 3.3 and Table III for key to position notation. 



•Obtained at rest position for 300-g T.N.T. -charge near surface and 

 corrected for surface reflection. This value would be expected to be 

 systematically low. 



« Clifford Frondel, "Construction of tourmaline gauges 

 for piezoelectric measurement of explosion pressure waves," 

 OSRD Report No. 6256; NDRC Report No. A-378. 



3 A. B. Arons and C. W. Tait, "Design and use of 

 tourmaline gauges for piezoelectric measurement of air 

 blast," OSRD Report No. 6250; NDRC Report No. A-372. 



I 



