EXTERNAL 



VAPOR 

 PRESSURE 



oO 



'o 



CURVED BOUNDARY 

 OF RIGID BODY. 



Figure 3. Sketch indicating growth and collapse of a vapor cavity. 



directed toward each of the aspects mentioned. For our purpose, here, it will be 

 sufficient to select a few examples of theory, experiment, and computation best suited 

 to illustrate the way in which cavitation acts as a source of sound. Those interested 

 in more general aspects of the various phenomena touched upon will find extensive 

 discussions in bibliographical material relating to cavitation [16]. 



Calculation of the sound pressure: acoustic theory. — In the previous section, 

 the sound pressure generated by a gas bubble was related to the fluctuating environ- 

 mental pressure encountered by the bubble. It is of interest to ask whether, in the 

 case of a transient vapor cavity, a corresponding relation can be given. 



One of the characteristics of the behavior of transient vapor cavities is the 

 violence of the collapse. In that part of the motion, the inward radial velocity of the 

 fluid in the immediate vicinity of the cavity may exceed the velocity of sound in the 

 liquid and pressures comparable to the modulus of compressibility of the liquid may 

 be developed. The description of the motion in the final stages of collapse therefore 

 requires consideration of the compression of the liquid. But during the period of initial 

 growth and most of the period of collapse, the incompressive theory yields a correct 

 description of the motion of the cavity, and the sound pressure (at sufficiently great 

 distance r ^> R) is given accurately by Eq. (4) in Section 2. 



If the temporal variation of the environmental pressure encountered by the 

 cavity is known, the resulting motion, or at least the incompressive stages thereof, 

 may be computed from Eq. (12). The roles of surface tension and of any minute 

 quantity of gas which may be contained in the cavity are important only when the 

 cavity is very small. For the purpose of the present discussion it is sufficient to con- 

 sider that their combined effect is simply to forestall the inception of growth of the 



248 



