29 



Figure 2U - Photographs of Stabilized Vortex Rings 



These photographs were taken from Reference (8) . 



observed on the floats and sphere drops. On the basis of these remarks, it 

 appears that these striations should be existent in the fluid whether cavita- 

 tion has begun or not. That they have not been observed at pressures above 

 the cavitation pressure may be due to the necessity of having a "free" sur- 

 face to provide a reflecting surface for the light used in photographs or in 

 visual observations. As a corollary to these conclusions, the presence of 

 striations evidently indicates that flow separation has taken place upstream 

 from the position at which the striations are observed. 



Although separation usually occurs in the region of increasing pres- 

 sure just beyond the minimum-pressure point, the pressure in the vortex, which 

 can be lower than the minimum pressure on the body, may allow cavitation to be 

 observed first in the vortex. Since the lower pressure occurs at the outside 

 peaks in the deformed vortex, these points may be expected to cavitate first 

 or, at least, to give larger cavitation pockets than the inner peaks, result- 

 ing in striations when observed under proper lighting conditions. Since a 

 high degree of vorticlty is convected downstream from the point of separation, 

 it may be expected that a corrugated vortex sheet will be maintained until 

 the vortex sheet is dissipated. 



Returning to a consideration of the vortex ring, the deformation 

 should undergo oscillations along radii from the center of the ring, and, as 

 a matter of fact, this can be observed with ordinary smoke rings in undis- 

 turbed air. Unless the configuration is stabilized under cavitating condi- 

 tions, it should be possible to observe an oscillation of the striations in 

 directions perpendicular (radial and, therefore, also tangential) to the 

 path of the specimen. However, it is possible that nonunlformity of the 

 surface would obscure or upset such oscillations. 



