NSWC/WOL TR 76-155 



APPENDIX C 

 APPROXIMATE METHOD FOR CALCULATING THE PRESSURE-TIME SIGNATURE 



Figure 3.1.1 shows the idealized form of pressure-time 

 signature used for the bladder response computations of this report. 

 For planning the 1975 series of explosion tests and for the examples 

 presented in Section 4 of this report we needed to predict for 

 arbitrary explosion geometries the variables, PMAX, 9, TPOS , PNEG, 

 DTNEG, shown in Figure 3.1.1. This Appendix tells how this was done. 



PRESSURE SIGNATURE BEFORE SURFACE CUT-OFF . The initial 

 positive portion of the signature is the direct wave from the under- 

 water explosion. The sudden negative excursion is caused by the 

 arrival of the reflected rarefaction wave from the water surface. 

 This phenomenon is known as "surface cut-off". We calculate the 

 time, TPOS, of this arrival relative to the direct arrival by 



R..-R 

 TPOS = — (CI) 



where R is the slant range from the charge, R, is the slant range 

 from the image of the charge mirrored in the water surface, and c 

 is the sound speed in the water. See Figure CI. 



We calculate the free-field portion of the signature using 

 a modification of the empirical Shockwave Similitude Equations. The 

 peak pressure PMAX and the decay constant 8 are calculated by the 

 usual similitude relations 



C-l 



