28 



and vaporization processes and the problems of energy dissipation, such solu- 

 tions, which are clearly over-simplifications, nevertheless give a clear quan- 

 titative picture of the hydrodynamics of the motion as long as the cavity 

 radius is large compared with the minimum radius. Further extensions to in- 

 clude the problems of energy dissipation associated with the compressibility of 

 of the vapor and gas mixture and of the liquid would be of great practical as 

 well as theoretical interest. For a bubble filled with a permanent gas being 

 compressed and expanded adiabatically rather than isothermally, Rayleigh's 

 case coincides with the pulsations of a gas-globe following an underwater ex- 

 plosion. 53 Although, in the latter problems, much progress has been made in 

 describing an oscillatory motion with energy dissipation, the problem of the 

 vapor condensation and formation prevents a complete analogy to gas-globe the- 

 ory and a satisfactory description of the motion of cavities on the basis of 

 this theory. 



An accurate knowledge of the processes at collapse of cavities and 

 of the associated pressures is still lacking, and is of importance in connec- 

 tion with cavitation damage. (This point is discussed somewhat further in 

 subsequent sections.) 



MECHANISM OF STEADY-STATE CAVITIES AND THEIR ANALYTICAL DESCRIPTION 



STEADY-STATE CAVITIES IN REAL LIQUIDS 



It is characteristic of some cavitating flows that the cavity ap- 

 pears as a large, smooth surface stationary in space such as that shown in 

 Figure 2. Such cavities often appear to be filled only with vapor or gas, and, 

 except for oscillations near the end of the cavity, the shape of the surface 

 does not vary with time. On the other hand, a cavitating region made up en- 

 tirely of small "transient" bubbles may exhibit the properties of a steady 

 cavity in that the average envelope of such a region does not vary with time.* 

 Another case of such "steady-state" cavities in which the average envelope 

 remained unchanged, but in which violent surface fluctuations were observed 

 without evidence of individual bubbles, was studied by the writer and Mr. H.L. 

 Pond. 40 These studies, which were an extension of work carried out by 

 H. Reichardt, 54 will be discussed herein. Finally, the cavity illustrated in 

 Figure 7 is included in this part of the discussion since, as will be shown, 



*It is not possible, at present, to predict with certainty whether cavitation on a curved surface 

 will occur in the form of a large steady state cavity or a mass of small, oscillating cavities. It 

 seems clear that the appearance of the cavitation is associated with the pressure gradients, but no 

 satisfactory criteria are available as to the initial appearance or possible transition from transient 

 cavities to a large, steady state cavity. 



