Scale Effect — Early Results 



Departures from the rule 



a. = -c (5) 



min 



for the cavitation number at inception are called "scale effects." Indeed, 

 almost from the very start of experimentation in cavitation, discrepancies 

 from this rule were so prevalent that this became an important research 

 topic, particularly for laboratory work where the discrepancies were more 

 evident. The thrust of the earlier studies was, as it is now, to obtain a 

 more detailed microscopic picture of the formation of cavitation in the 

 hope that this would lead to a better physical model for describing the 

 process. 



Among the very most influential of these works were the beautiful 

 motion pictures of growing and collapsing bubbles travelling on the sur- 

 face of a body of revolution made by R.T. Knapp of Caltech in about 1946 

 These were sensational pictures at the time which, when coupled with the 

 brilliantly successful analysis of Plesset (see Knapp and Hollander 1948, 

 Plesset 1949) , firmly established bubble mechanics as the essential 

 scientific component of this type of cavitation. In fact, because of the 

 time-delay required for the growth of a travelling bubble to visible size, 

 it appeared reasonable to expect the scale effect to follow from this 

 mechanism. A reproduction of one of Knapp 's photographs of these travel- 

 ling bubbles is shown in Figure 1. There, then, followed in the early 

 1950' s a period of intense activity, both experimental and theoretical, de- 

 voted to settling the cause for the scale effect. To a latter day observer 

 it appears that bubble mechanics, already of vital importance in under- 

 water acoustics, became the paradigm through which experiments on cavita- 

 tion inception were to be explained. As we will see, growing travelling 

 bubbles are one form of cavitation characteristic of inception but they are 

 not the only form, nor necessarily the dominant form. However, at this 

 time in 1950 these differences were largely overlooked or unknown. 



