34 



40). Nevertheless, this is of interest in connection with the mathematical 

 theory of such cavities discussed below. Although a satisfactory quantita- 

 tive description of the processes involved in the motion and maintenance of 

 such cavities is still lacking, the tentative conclusions reached in Reference 

 40 are summarized below. 



If it is assumed that the temperature of the vapor will be the same 

 as the temperature at the surface supplying the heat of vaporization, the pres- 

 sure of the vapor will be lower than that corresponding to the temperature in 

 the interior of the liquid. On this assumption, the minimum pressure measured 

 requires a surface -temperature drop of the order of 2° to 3° C. However, that 

 this situation can actually occur has no well founded theoretical basis. For 

 systems in equilibrium, results of kinetic theory of gases indicate that the 

 molecules escaping into the vapor phase will be slowed down into a maxwellian 

 distribution of velocities corresponding to the temperature of the liquid from 

 which they came. 57 However, the violent oscillations at the end of the cavity 

 appeared to indicate a high rate of entrainment and, thus, a correspondingly 

 high rate of evaporation at the surface, so that the possibility that static 

 equilibrium is not established could not be overlooked. As it turned out, 

 temperature measurements within the cavity and in the surface of the cayity 

 failed to disclose the requisite temperature difference. Furthermore, this 

 idea does not explain the pressure rise just behind, the model. That no tem- 

 perature differences were observed at the cavity surface may be indicative of 

 highly fluctuating temperatures at frequencies beyond the response of the 

 thermocouples used. 



The conclusion finally reached was that the entire flow might be con- 

 sidered as purely a wake phenomenon in which the vapor phase plays only a 

 minor role. Here, the term "wake" is applied in the ordinary sense of a wake 

 resulting from boundary layer thickening or separation in a viscous liquid. 

 Thus, it is postulated that the motion is governed primarily by the hydrody- 

 namics of the liquid phase. Even if the pressure were constant across the 

 wake, it is clear that cavitation will first occur in the small scale eddies 

 formed at the boundary of the wake due to the high viscous shear at this 

 boundary or in the core of large vortices shed from the model as in Figure 22. 

 For the more fully developed cavities, the pressures lower than vapor pressure 

 measured near the center line might be explained on the basis of an essential- 

 ly vortical flow within the cavity. This flow is associated with the down- 

 stream motion at the surface of the cavity and the motion of the liquid reen- 

 tering at the end of the cavity in a manner similar to that described in the 

 air-water entry cavity. Because of the small mass, a vortex flow of vapor 



