Some Effects of Several 

 Freestream Factors on Cavitation 

 Inception of Axisymmetric Bodies 



Edward M. Gates 

 University of Alberta 

 Edmonton, Canada 



Allan J. Acosta 



California Institute of Technology 



Pasadena, California 



ABSTRACT 



Some of the effects of freestream turbulence and a 

 dilute polymer solution on the fully wetted flow 

 and the subsequent cavitation inception has been 

 investigated for three different bodies. Two of 

 these bodies possess a laminar separation and one 

 does not. In the fully wetted investigation the 

 flow on one of the bodies was found to be insensi- 

 tive to the present disturbances whereas the other 

 two were found by comparison to be very sensitive. 

 Although there is a pronounced "suppression" of 

 inception by the polymer, it seems clear that the 

 effects observed are due primarily to the change 

 in the real fluid features of the flow past the 

 bodies themselves and not to an intrinsic cavita- 

 tion process. There appeared to be no special poly- 

 mer effect, insofar as cavitation is concerned, on 

 the body not having a laminar separation, confirm- 

 ing the results of van der Meulen. Due to practical 

 limitations the effects of turbulence per se on in- 

 ception could not be separately evaluated. 



The inception index on all bodies was found to 

 be greatly dependent on the distribution of nuclei 

 within the water tunnel. For those cases in which 

 a turbulent transition was established well upstream, 

 travelling bubbles were a common form of cavitation 

 observed on all test bodies. The number of these 

 cavitation events were so few, however, that in one 

 test facility having a resorber, it was just as 

 likely for an attached cavity to form as it was to 

 observe a travelling bubble. In both cases the 

 inception index was far below the customary minimum 

 pressure coefficient reference value. Nuclei counts 

 made with the aid of holograms reveal significantly 

 fewer microbubbles within the flow of this test 

 facility than in those not having a resorber. 



1 . INTRODUCTION 



Our understanding of the details of the process of 

 cavitation inception (and thus our ability to scale 



laboratory results to prototype conditions) is far 

 from complete [e.g. Acosta and Parkin (1975), Morgan 

 and Peterson (1977) ] . This lack of understanding 

 is well illustrated by our ability to do no more 

 than indicate reasons which are believed to be 

 responsible for the large variations in the results 

 of the ITTC comparative test series [Lindgren and 

 Johnsson (1966), Johnsson (1969)]. These results, 

 some of which are presented in Figures 1 and 2, did, 

 however, prompt a considerable amount of effort to 

 investigate more systematically the factors influ- 

 encing cavitation inception. In particular there 

 are three areas in which there have been significant 

 developments: (i) the influence of viscous effects 

 on inception, (ii) the discovery that in some situ- 

 ations the presence of drag-reducing polymers in the 

 water cause a suppression of the inception index, 

 and (iii) the development of equipment to accurately 

 measure freestream nuclei populations. 



0.8 

 z 

 o 



H 



0- 0.7 



O 



h- 

 < 



» WITHOUT RESORBER FACILITY 

 • • WITH RESORBER FACILITY 



\TOKYO- (JAPAN) 



PENN STATE (DTMB-BODY A) 



.>j:N^ 



SAFH-(U S.A.) 



~ "DTMB 36" BODY B 

 (USA.) 



DTMB 36" BODY B -(U.S A ) 

 PENN STATE (SAFH 60DY)- 



NPL 2- 



(ENGLANDI , 



■ 10 20 30 40 



TUNNEL VELOCITY -U , ft/sec 



FIGURE 1. Results of the comparative inception test 

 on a modified ellipsoidal headform sponsored by the 

 International Towing Tank Conference, Lindgren and 

 Johnsson (1966) . 



86 



