CAVITATION NUCLEI, BUBBLE MECHANICS AND 

 SCALING 



Nuclei and Their Measurements 



Possibly, no other subject in cavitation inception research has proved 

 to be so exacerbating as the nucleation sources of cavitation or "nuclei." 

 The many theories proposing their nature, size, and occurrence are all ex- 

 tensively reviewed in the books by Knapp et al. (1970) and Robertson and 

 Wislicenus (1969) . Significant advances have been made since that time in 

 the detection of (rather large) nuclei on a number of fronts; these are re- 

 viewed first by Morgan (1972) , and then later by Acosta and Parkin (1975) 

 and do not need to be further elaborated here except for some recent find- 

 ings and some conceptual notions. The basic idea is rather simple; there 

 must be some practical mechanism for the phase change to occur in engineer- 

 ing applications, otherwise the large tensions predicted for pure liquids 

 far removed from the critical point, would be commonly observed. As ex- 

 plained in these references, nuclei can exist as small gas-vapor bubbles 

 (perhaps with the interface contaminated with a surfactant) or a solid 

 particle. It has become common to think of solid particles as good candi- 

 dates for the nuclei because of the theoretical possibility of stabil- 

 izing a gas pocket within a solid particle. Indeed, Keller's recent work 

 makes a case for these "pore" nuclei in his own facility in a very in- 

 fluential publication (Keller 1973) . 



The measurement techniques have been both optical (single point 

 scattering, see Keller 1973) and holographic photography, (see Peterson 

 1972, Peterson et al. 1975), and acoustic and indirect via the Coulter 

 counter. The acoustic method measures only bubbles; both the Coulter 

 counter and single point light scattering techniques react to bubbles and 

 particulates. As in photomicrography, holographic photography is limited 

 in resolution but the entire cross-section of tunnel fluid can be examined 

 at leisure for the form and number of nuclei greater than about 10 micro- 

 meters diameter. (We are now discussing, of course, freestream nuclei ; it 

 is also possible for the surface of the body itself to be the site of 

 nucleation, just as it is in most boiling situations.) 



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