29 



such a cavity behaves at a given instant of its motion in the same way as the 

 steady-state cavities mentioned above. A photograph obtained by Worthington 

 in 1908 — of the air-water entry cavity of a sphere at a later stage than that 

 of Figure 7. i.e., after the upper cavity has been detached — is shown in Fig- 

 ure 18, which is taken from Reference 55- In this case, the cavity has closed 

 at the surface and a jet is seen to be formed inside the cavity. It will be 

 observed that the jet is of suffic- 

 iently high velocity to penetrate the 

 wall of the cavity, the edge of the 

 position of the jet in the liquid be- 

 ing delineated evidently by entrapped 

 air. Since the life history of air- 

 water entry cavities is not of direct 

 interest to the present discussion, 

 details are not pertinent here. De- 

 scriptions of the motion in the entry 

 phase, with the associated splash, 

 the subsequent formation of a cavity 

 extending to the surface, first grow- 

 ing and then closing up, and finally 

 the condition in which a portion of 

 the cavity may become detached with 

 the missile continuing to fall with 

 an attached cavity, are given in Ref- 

 erences 1 4- and 56. 



An examination of the cavi- 

 tation number defined previously 



Figure 18 - Air-Water Entry Cavity 



After Surface Closure— After 



Worthington, Reference 55 



indicates that its magnitude is a function of the vapor pressure. However, 

 for steady-state cavities, it makes little difference whether the pressure 

 within the cavity is vapor pressure or some other pressure. On this basis, 

 in order to obtain cavities of very low cavitation number, Reichardt injected 

 air into the region behind a disc, thus forming cavities with internal pres- 

 sures well above vapor pressure. Sketches of these cavities were made by 

 Reichardt and are reproduced in Figure 19 from Reference ^k. In these experi- 

 ments, Reichardt used discs and cones which were about 3/8-inch in diameter. 

 Such small models were necessary to avoid distortion due" to the buoyancy of 

 the air-filled cavity. A photograph taken in a TMB water tunnel of the cavity 



