246 



19 



More complete figures are scarcely worth constructing until an 

 actual known case presents itself for analysis. 



CAVITATION UNDER THE SURFACE OF THE SEA 



When a charge is exploded at a suitable depth in the ocean, a dome 

 of white-appearing water is seen to rise somewhat above the surface, breaking 

 after a moment into plumes of spray. The eight frames from a motion picture 

 film. Figure 12, illustrate this phenomenon. The plumes are supposed to be 

 associated with the escape of the explosion gases. The dome, however, has 

 been ascribed to the occurrence of cavitation; a layer of water at the sur- 

 face and Just under it, after being kicked upward by the pressure wave, fails 

 to be jerked to rest again by the action of a reflected wave of equal ten- 

 sion and continues rising until stopped by gravity and air pressure. This 

 explanation will be considered briefly on the basis of the foregoing analysis. 



It is necessary first to fix upon the value to be assumed for the 

 breaking-pressure p^^. Hilliar (7) found that the dome was absent whenever, 

 according to his measurements, the maximum pressure reaching the surface was 

 under 0.5 ton or 67O pounds per square inch, and concluded that p^ was rough- 

 ly of this magnitude. It will be assumed, therefore, for the moment, that 

 p^ = -600 pounds per square inch. 



To select a specific case 

 for study, suppose that a charge of - ^"'^^ Surface 



300 pounds of TNT is detonated 50 

 feet below the surface. Then the 

 pressure wave should be reflected 

 from the surface as a wave of equal 

 tension, diverging from the mirror 

 image of the charge in the surface 

 and decreasing in intensity as it 

 progresses. Using Hilllar's data, 

 it is easy to map out the lens- 

 shaped volume within which the 

 pressure would sink momentarily 

 at least to -60O pounds per square 

 inch if there were no cavitation. 

 This volume is outlined roughly by the lower curve in Figure 13. 



Application of the criterion obtained from the analysis for the 

 propagation of a breaking-front indicates, on the contrary, that cavitation 

 would in reality be confined to a much smaller region, which is shaded In 

 Figure 13. To locate this region, it is necessary to estimate the magnitudes 



p > -600 



lOQl- 



Figure 1 3 - Diagram of Region of Reduced 



Pressure following Reflection of a 



Pressure Wave from the Surface 



