n 



25 



The plate will then swing away from the water; see Figure 8. It 

 may swing freely until it Is arrested by the combined action of the elastic 

 or plastic stresses In the plate and of any difference in pressure that may 

 exist between its two sides. The kinetic energy to be absorbed In this pro- 

 cess will be only that of the plate Itself; and the swing time will be that 

 of the plate without water-loading. 



The motion of the water surface during this time must also be con- 

 sidered. According to the results of analysis, the velocity of the surface 

 should decrease, but it should not entirely disappear, because of diffraction 

 effects; see the discussion In the Appendix. Furthermore, if a considerable 

 part of the shock wave arrives after the departure of the plate, this will 

 cause further acceleration of the water. It is possible, therefore, that the 

 plate may be overtaken by spray projected from the water surface, and it will 

 certainly be overtaken eventually by the water surface itself; the motion of 

 the plate may thus be prolonged, with a corresponding increase in the plastic 

 work ( 3 ) . 



If the plate Is held at its edges, the outer parts of the plate 

 must be Jerked to rest by the support almost immediately, while the central 

 part continues moving. Such motion has been observed in 10-inch diaphragms 

 at the Taylor Model Basin. Cavitation occurring over the outer parts of the 

 diaphragm must, therefore, be short-lived; here the water must overtake the 

 plate almost Immediately. 



As an alternative to th*? 

 simple process Just described, the 

 cavitation might begin in the wa- 

 ter itself, in the form of bubbles, 

 so that for a time there would con- 

 tinue to be a layer of unbroken wa- 

 ter next to the plate, as in Figure 

 10a. Or, as a special case, it 

 might begin at the plate and pro- 

 ceed at once to spread out into the 

 water, as in Figure 10b. This pos- 

 sibility has been explored in gen- 

 eral terms (12), and Its practical 

 application has been discussed in 

 TMB Report 511 (13) and Independ- 

 ently by Klrkwood (9). 



If the cavitation process 

 is of this character, the motion of 



/ 





Figure 10a - With Bulk Cav- 

 itation at Some Distance in 

 Front of the Plate 



Figure 10b - With Bulk 



Cavitation Extending 

 Outward from the Plate 



Figure 10 - Illustration of Deflection 

 of a Plate 



The broken curve represents the front of the 

 reflected shock wave. 



