112 



Authors' Reply 



EDWARD M. GATES and ALLAN J. ACOSTA 



Messrs. Chahine and Fruman have raised the 

 question of the relative importance of polymer- 

 induced changes in bubble growth versus induced 

 changes in the flow structure with regard to the 

 suppression of cavitation. Although both experi- 

 mental [Ellis and Ting (1970) ; Chahine and Fruman'- 

 (1979)] and theoretical [Street (1968); Fogler and 

 Goddard (1970) ] work demonstrate that in "no- flow" 

 situations the growth and collapse rates in polymer 

 solutions are different than those in pure water, 

 the magnitude and sense (Street predicts an in- 

 crease in bubble growth rate) of the changes are 

 open to question. On the other hand, the results 

 of Hoyt (1976), Brennen (1970), van der Meulen 

 (1976) , and the present work show drag-reducing 

 polymers have a very dramatic effect upon the flow 

 structure in jets and aixisymmetric bodies. The 

 authors believe that in the present work the influ- 

 ence of these profound flow alterations predominate 

 over any influence of modified bubble dynamics as 

 nicely shown by them as evidenced by the following 

 observations : 



First, it was obseirved in the LTWT that cavi- 

 tation inception on the non-separating Schiebe body 

 was not influenced by viscous considerations and 

 was of the travelling bubble type. In this situa- 

 tion we would expect that if the polymer effect 

 upon bubble dynamics was significant, it should be 

 well illustrated under these circumstances. How- 

 ever, we (like van der Meulen) observed no change 

 in either the cavitation index or the appearance 



of the cavitation at inception. Second, on the 

 hemisphere nose and NSRDC bodies a similarly large 

 suppression of the inception index was obtained by 

 Arakeri and Acosta (1976) through the elimination 

 of the laminar separation by a mechanical boundary 

 layer trip - a situation for which there is no 

 change of bubble dynamics. 



From these observations we infer that the in- 

 fluence of the polymer on cavitation inception is 

 dominated by changes in the flow structure rather 

 than modified bubble dynamics. However, in "non- 

 flow" sitations it must be assumed that modified 

 bubble dynamics are responsible for the observed 

 changes and the work of Messrs. Chahine and Fruman 

 is a useful addition to this area of study. 



REFERENCES 



See Reference from Chahine and Fruman discussion. 

 Street, J. R. , (1968). The Rheology of Phase 

 Growth in Elastic Liquids. Trans. Soa. RheoZ., 

 12, p. 103. 



Fogler, H. S. , and J. D. Goddard (1970). Collapse 

 of Spherical Cavities in Viscoelastic Fluids. 

 Physios of Fluids, 13, (6), pp. 1135-1141. 

 Hoyt, J. W., (1976). Effect of Polymer Additives 

 on Jet Cavitation. Journal of Fluids Engineering, 

 Trans. ASMS, March, pp. 106-112 



