52 



answer to whether such jets actually arise on the scale of the small transient 

 cavities must await further development of experimental techniques. 



Although much thought has been given to the mechanism of collapse of 

 the small, transient cavitation bubbles, there seems to be no parallel devel- 

 opments for the collapse of large, steady-state cavities such as the one shown 

 in Figure 21 . As pointed out previously, although the mean motion appears to 

 be correctly described by free streamline theory, and attempts have been made 

 to explain the surface appearance of actual bubbles, 40 the growth and collapse 

 observed recently at the Taylor Model Basin, and in the tests of Reference 59, 

 suggest other mechanisms for the oscillation. 



In the case of the cavities studied at the Taylor Model Basin, it 

 has been suggested that the oscillations are associated with the pressure and 

 velocity fluctuations in an unstable wake. Although, on the average, such 

 cavities have been successfully described (in two dimensions) from purely 

 hydromechanical considerations, the processes in the actual cavity are still 

 not too clear. If the cavity is considered as a wake, however, in which the 

 reentrant fluid is alternately swept away from various sectors, the collapse 

 of the cavitating boundary may easily be forced by the pressure variations 

 during this shedding process. Although it is known that the shedding of ed- 

 dies in a wake gives rise to sufficiently large pressure fluctuations to in- 

 duce vibration (even at very high Reynolds numbers), it is not likely that 

 such oscillations can produce damage of the type associated with cavitation 

 collapse. Since, as mentioned above, such cavities do not appear to be made 

 up of individual, spherical cavities, the requisite pressures for damage can- 

 not be attributed to the collapse of groups of such individual cavities. Thus, 

 although damage has been observed near the end of such steady-state cavities 

 when they terminate on a solid surface, it is not clear what the origin of the 

 requisite pressure may be. In the latter case of termination on a solid sur- 

 face the conditions at the point where the end of the cavity meets the surface 

 are also not clear. At this point, the pressure would tend to rise to stagna- 

 tion pressure — which upsets the conditions of constant pressure on the solid 

 and liquid boundaries. This pressure increase may be the origin of the fluc- 

 tuations observed near the end of such cavities, with a forced condensation 

 resulting in a decrease in cavity size and subsequent readjustment, growth 

 and repetition of the cycle. Here again, it is not clear that such oscilla- 

 tions can give rise to pressures that are so much greater than the order of 

 stagnation pressure as to cause damage; as a matter of fact, judging from the 

 noise and induced vibrations, the conditions in such cavities appear to be 

 much less violent that in the case of a number of individual cavities. A pos- 

 sible source of damage in this case may be the separation of individual water 



