30 



16 



Figure 1 3 - Photograph Showing Cavitation 

 Bubbles Produced by a Shock Wave 

 in Water near a Lucite Window 



The gas globe produced by the explosion 

 is visible at the left. 



share fully in the motion. The 

 water thus becomes expanded and 

 its energy of compression is 

 converted Into the kinetic ener- 

 gy of the plate; the process of 

 expansion progresses to the 

 point where tension begins to 

 develop in the water, and cavi- 

 tation results. The water and 

 plate behave much like a spring 

 loaded with a mass. If the 

 spring is compressed and then 

 released, the motion overshoots 

 the position of equilibrium, and 

 the initial state of compression 

 thereby comes to be replaced momentarily by one of tension. A picture of 

 what appears to be cavitation due to elastic overshoot, in front of a lucite 

 window struck by the shock wave from a small charge. Is sh®wn in Figure 13. 



Under ordinary circumstances, a necessary condition for the occur- 

 rence of cavitation due to elastic overshoot appears to he that the compli- 

 ance time of the structure, or time required for it to attain a maximum 

 velocity under the action of the wave, shall be less than the diffraction 

 time: 



If this condition is not satisfied, inflow of water from regions beyond the 

 edge of the structure is likely to equalize the pressures and so to prevent 

 the occurrence of tension in the water. 



The occurrence of cavitation should be the same on the usual model 

 scale as on full scale, at least if the hydrostatic pressure is the same in 

 the two cases. For, if all linear dimensions Including those of the charge 

 are altered in a given ratio, all characteristic times will be changed in the 

 same ratio; in Equation [5a, b], for example, ^ /a and wi will be changed in 

 the ratio of the linear dimensions and x is unchanged. Thus the ratio of T„ 

 to Tj is not altered by the change of scale. 



Large hydrostatic pressure, however, may act to prevent the occur- 

 rence of cavitation. The pressure due to the Incident wave, as modified by 

 reflection and the motion of the target, is superposed upon the hydrostatic 

 pressure Pj, and, if p^ is sufficiently great, the resultant pressure may 

 never sink to the pressure at which cavitation occurs. 



