238 



n 



of water due to the Incident wave, or the same as the displacement of the 

 water surface when the plate is absent. 



The effect of cavitation, on the other hand, will vary somewhat, 

 according to the point at which it occurs. There are two possibilities: 



1 . The plate may break loose from the v;ater, or 



2. cavitation may occur first in the water itself. 



1. The plate may break loose from the water; see Figure 7- 

 The pressure at which this occurs may be either the cavity pressure 

 Pj or some lower pressure p^'. In either case, the surface of the liquid 

 then becomes a free surface at which the pressure is constant and equal to 

 p^, and the remainder of the incident wave is reflected from this free sur- 

 face. The plate, meantime, will continue moving forward until it is arrest- 

 ed by other forces. The pressure p^, atmospheric or otherwise, acting on 



Figure 6 - Diagram representing Plane 



Waves of Pressure p in Water, 



falling upon a Large Thin Plate 



This plate is backed by gas at the pressure p . 

 A reflected wave of pressure p" travels 

 back into the water. 



Figure 7 - Diagram illustrating the 



Case in which Cavitation occurs 



at a Thin Plate 



Here the cavitation takes the form of a definite 

 cavity in which the pressure is p . 



the opposite face of the plate, may be assumed to exceed the pressure p^ in 

 the cavity behind it; the difference, p^ - p^, will suffice eventually to ar- 

 rest the motion of the plate and to cause its return to contact with the wa- 

 ter. There may also be other forces of elastic or plastic origin. It may 

 happen, however, as suggested by Professor G.I. Taylor (5), that spray pro- 

 jected from the water surface will tend for a time to support the outward 

 motion of the plate. When the returning plate strikes the water, an impact 

 wave of pressure will be produced in the water as the plate comes exponen- 

 tially to rest. 



If the incident wave is of exponential form, explicit formulas are 

 easily obtained. This case is discussed at length by Taylor (5), but a few 

 details may be given here. 



