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formed behind a disc 1 1/2 inch in 

 diameter when air is injected is 

 shown in Figure 20. The distortion 

 of the cavity due to buoyancy is 

 marked. It will be observed that 

 the surface of the cavity is quite 

 smooth and transparent. Cavities 

 which are maintained by vaporization, 

 on the other hand, and look essen- 

 tially smooth to the eye, reveal a 

 very distorted, almost opaque sur- 

 face that is oscillating rapidly.* 

 In Figure 21a is shown a photograph 

 taken in a TMB water tunnel of a 

 cavity formed behind a disc taken 

 with an exposure time of about 2 

 seconds. The same cavity, with an 

 exposure time of 1/10,000 second, is 

 shown in Figure 21b. The latt,er 

 photographs were taken during the 

 studies reported in Reference kO. 

 It appears from the results so far 

 available that there are essential differences between the air-filled and 

 vapor cavities in the details of the mass flows within the cavity surface. 

 But there is little difference in the average resultant shape of the cavity 

 surface . 



The studies of Reference 40 revealed that some care must be exer- 

 cised in the definition of the flow configuration that is to be considered a 

 steady-state cavity flow for cavitation about blunt obstacles. On such forms, 

 cavitation may occur in the vortices induced in the zone of separation of flow 

 or along the boundary of this zone before it occurs at the point of minimum 

 pressure itself (as pointed out previously). Thus, at high cavitation numbers, 

 cavitation may be observed in the cores of vortices which are shed periodi- 

 cally from the model, and which appear to form a well-defined cavity when 

 viewed by the eye or seen in a photographic exposure of long duration. This 

 situation is illustrated in Figure 22. 



Figure 19 - Cavities Formed by the 

 Emission of Air Behind a Disc- 

 After Reichardt, Reference 5^ 



♦There is a difference of experience on this point among various investigators. Whereas the writer's 

 experience with experiments in the TMB tunnels is that the bubble surface is clear only when the cavity- 

 contains large amounts of air, others have reported that when it is clear it contains only water vapor. 

 The writer has also observed clear cavities in the California Institute of Technology tunnel which, 

 in the opinion of Dr. E.T. Knapp, the Director of the CIT laboratory, were completely vapor filled. 

 The reasons for this anomaly are not clear. 



