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Author's Reply 



G. KUIPER 



Both the hemisphere and the ITTC body are 

 known to exhibit laminar separation in the range 

 of Reynolds numbers (estimated at about 2 x 10^) 

 used in the experiments of Tamiya et al. as was 

 already shown by Arakeri and Acosta (1973) . They 

 now point to an apparent discrepancy between the 

 results as described in my paper and their obser- 

 vations: on the propellers nuclei were foimd to 

 generate sheet cavitation in the very few cases 

 where it was not yet present, and the nuclei never 

 changed the appearance of the cavity. 



First of all, the cavitation patterns, both 

 with and without electrolysis, on the headforms of 

 the discussers show a remarkable resemblance to 

 various patterns shown on the ITTC bodies at other 

 facilities [Lindgren and Johnsson (1966) and also 

 reproduced by Gates and Acosta in their paper on 

 this program] illustrating that the nuclei content 

 was at least one of the factors causing the varia- 

 tion in type of cavitation observed at different 

 facilities . 



From the observations of the discussers it can 

 be concluded that the nuclei , generated by elec- 

 trolysis, removed the laminar separation bubble in 

 the same manner as shown very clearly by Gates and 

 Acosta in their symposium paper. This phenomenon 

 was found when there were many large free stream 

 bubbles in the flow, as can also be observed in the 

 pictures of the discussers. In our case, however, 

 we verified with a paint test that electrolysis did 

 not remove the laminar separation bubble by veri- 

 fying that the critical radius was unchanged. 



The observations of the discussers show that 

 an overdose of nuclei can change the situation 

 considerably. Gates and Acosta assume that the 

 free stream bubbles do trip the boundary layer. 

 Another possibility, however, is that the dynamic 

 behavior of the bubbles near the minim.um pressure 

 region changes the pressure distribution on the 

 body, specifically by decreasing the low pressure 

 peak. This would also remove laminar separation, 

 leaving the boundary layer laminar over a longer 

 distance. In fact the nuclei do not only affect 

 cavitation inception but they change the free 

 stream conditions , making a correct comparison of 

 the inception phenomena impossible. 



Rutgersson, in his discussion, gives an illus- 

 tration of a possible effect of nuclei and roughness 

 on bubble cavitation. With the pictures alone, 



only some assumptions can be made as to what hap- 

 pened on this propeller, but I will make an attempt 

 to give an explanation. 



Although the Reynolds number was rather high 

 it looks like the boundary layer within r/R =0.8 

 is laminar over a large portion of the chord, while 

 the minimum pressure region is near midchord 

 (Figure 2). An increase of the nuclei content 

 leads to occasional cavitating spots, starting at 

 the low pressure region (Figure 3) . On the painted 

 blade, however, the boundary layer seems to be 

 turbulent and bubble cavitation starts there, near 

 the minimum pressure region (Figure 4) . 



If my tentative description is correct there 

 is a difference between the discussers' case and 

 Figure 29 (and also Figure 24) from my paper, since 

 there the boundary layer in the region of low pres- 

 sure was turbulent, and still no bubble cavitation 

 occurred. Only when cavitation, generated by rough- 

 ness , at the leading edge took place , a separate 

 region of bubble cavitation also appeared. 



Whatever may be the case, it must be kept in 

 mind that these descriptions of phenomena do not 

 explain them, because it is not clear to me why 

 there should be any interaction between the bound- 

 ary layer and the free stream nuclei and which 

 parameters would control this . I think more sys- 

 tematic research is necessary to be able to 

 simulate bubble cavitation on model propellers 

 in a reproducible way. 



I agree with the suggestion of the author that 

 the increased amount of bubble cavitation, as shown 

 many times by roughened propeller models, may well 

 be representative for full-scale cases. Bubble 

 cavitation seems to be inhibited on scale models 

 very easily. When bvibble cavitation does occur on 

 scale models the situation is so bad that invari- 

 ably erosion problems do occur on full-scale. 

 Ironically a better simulation of bubble cavitation 

 may not make the interpretation easier. 



In general both discussions have made it clear 

 again that it is impossible to make general state- 

 ments about the effect of nuclei or roughness. To 

 make any interpretation and to avoid confusion the 

 test conditions must be given as complete as pos- 

 sible. Finally, I thank the discussers for their 

 discussions and for their kind attention to my 

 paper. 



