642 



THE DETERMINATICN OF PEAK PRESSURE OF AN UNDERWATER EXPLOSION 

 FROM A STUDY OF THE INITIAL DOME VELOCITY 



I. INTRODUCTION 



When an explosive charge is detonated beneath the surface of water, 

 provided the depth of the explosive is not too great, the resulting 

 shock wave hitting and being reflected from the surface causes a mass 

 of water to become detached and leave the surface. This detached mass 

 of water is projected normal to the surface with an initial velocity 

 exactly twice that of the particle velocity of the shock wave itself. 

 This doubling of the particle velocity at the surface is due to the 

 reflection from the free water-air interface of a rarefaction wave of 

 the same magnitude but opposite in sign to the incident compression 

 wave. The particle velocity in a shock wave is given by the expression^'* 



U - 



^^ U 



and the Initial velocity of the projected spray by twice that, or 



Vo - -^ (1) 



where u is the particle velocity, V^ the initial velocity of the rising 

 spray dome, Pj, the peak pressure of the shock wave, /=>^ the initial denaity 

 of the medium, and U the propagation velocity of the shock wave. 



Although the surface effects resulting from an underwater explosion 

 have been intensively studied for various reasons during the war,^' the 

 velocity of the rising spray dome has not been seriously proposed 

 heretofore as an accurate method for the determination of the peak pressure 

 of the shock wave except by earlier workers, among them, Hilllar,3) The 

 Underwater Explosives Research Laboratory became interested in this method 

 because the Increasing dependence upon the piezoelectric gauge made 

 desirable the development of an independent method for determining absolute 

 peak pressures as a check on the piezoelectric results. 



Introducing a finite force T necessary to rupture water at its 

 surface so that a spray dome may be formed, the equation^-) 



* All such numbers refer to the List of References at the end of this report. 



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