Chapter 27 



FORMATION AND DISSOLUTION OF AIR BUBBLES 



Am MAY BE ENTRAPPED mechanically at the ocean 

 surface and dispersed in the form of bubbles; a 

 familiar example is the appearance of white caps on a 

 rough sea. A great deal of air is also trapped along the 

 waterline of any vessel under way. Proof that such 

 entrained air is capable of producing acoustic wakes 

 comes from experiments on the wakes of sailing ves- 

 sels. Probably the most copious source of bubbles in 

 wakes, however, is propeller cavitation at high 

 speeds. 



27.1 FORMATION OF BUBBLES BY 

 CAVITATION 



When a cavity is created in water containing dis- 

 solved air, gas enters the cavity by diffusion, and 

 when the cavity collapses, this gas remains behind as 

 a bubble. The process of underwater formation of 

 bubbles, therefore, involves two quite different 

 phenomena: (1) the mechanics of cavitation, and 

 (2) the thermodynamics of diffusion and solution of 

 gases in liquids. 



27.1.] Mechanics of Propeller 

 Cavitation 



The phenomenon of propeller ca^dtation has long 

 been known to engineers. According to hydrody- 

 namical theory, cavities in liquids originate when 

 certain patterns of flow produce regions of negative 

 pressure near propellers. Such regions are set up in 

 the vortices formed near the propeller tips, provided 

 that the tip speed exceeds a certain critical limit, and 

 also on the back side of the propeller blade. Hence, 

 it is customary to speak of tip vortex cavitation and 

 blade cavitation. These theoretical deductions have 

 been verified experimentally by taking high-speed 

 photographs of propellers rimning imder water, 

 shown in Figures 1, 2, and 3. 



By driving a propeller in an experimental chamber 

 and observing it through a window, the process of 



cavitation can be followed visually imder strobo- 

 scopic illumination. When the speed of the propeller 

 is gradually increased, bubbles are seen first to form 

 at the propeller tips, from which they spiral back- 

 ward in a long stream. Then bubbles begin to cover 

 the part of the blade closest to the tips, forming a 

 sheet on the blade. This phenomenon is sometimes 

 described as laminar cavitation, in order to distin- 

 guish it from the formation of larger bubbles on the 

 blade face nearer to the hub, called burbling cavita- 

 tion, which starts at still higher speeds. Physically, 

 there is no sharp distinction between laminar and 

 burbling cavitation, and it would be more appro- 

 priate to classify them together as blade cavitation. 

 While persistent cavities are particularly likely to 

 be formed in the tip vortices and on the propeller 

 blades, cavitation also may be produced around 

 sharp projections on the ship's hull, especially during 

 periods of sharp acceleration of the ship. For in- 

 stance, white foamy spots have been observed visu- 

 ally from a launch on the superstructure of a sub- 

 merged submarine that passed at shallow depth. The 

 appearance of the white spots did not suggest the re- 

 lease of a stream of entrapped air; hence, the spots 

 were tentatively attributed to cavitation occurring 

 on the superstructure.' This result cannot be re- 

 garded as general, since the submarine had not been 

 submerged for a long enough time to justify assimi- 

 ing that all surface air entrained during the dive 

 had been dislodged by the time of the observation. 



27.1.2 Growing and Shrinking of 

 Bubbles 



After a cavity has been formed in sea water which 

 is saturated with air at an external pressure of 1 

 atmosphere, gas begins to diffuse into the vacuum 

 from the surrounding liquid. Since the diffusion con- 

 stants for oxygen and nitrogen are nearly equal, the 

 gas collecting in the cavity must have the same com- 

 position as that dissolved in the sea water. This com- 



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