413 



also tested in the cavitation tunnel at two Reynolds 

 numbers. No differences in cavitation pattern due 

 to variation of the Reynolds number were observed 

 in the towing tank. Notably the radial extent of 

 the cavity was unchanged, which confirmed that the 

 critical radius restricted cavitation inception 

 independent of the Reynolds number. The results 

 of propeller A at 30^ slip are shown in Figure 19. 

 In this figure the observations of the tests in 

 the cavitation tunnel are also shown. These show 

 some differences requiring further attention. The 

 cavity in the cavitation tunnel at ReN=l- 56x105 is 

 somewhat larger than in the towing tank, but the 

 difference is not significant and is probably 

 caused by a slight difference in propeller loading. 

 (The tunnel condition was taken at aK„-value 

 derived from the open water measurements. The flow 

 velocity was not measured) . Remarkable are the 

 spots of cavitation at Re„=l. 56x105 which increased 

 in number when time increased! 



At Re[^=2. 72x105 there is a sheet outside r/R=0.9, 

 the same as at Re[g=l. 56xl05 . The spots however, 

 have increased in number and they coalesce at some 

 distance from the leading edge, forming a cavity 

 until about r/R=0.8 with isolated spots until r/R=0.7, 

 which is the ideal inception radius . The increase 

 of the number of spots with time was not observed 

 in this situation, but the time to reach a stable 

 condition was much longer than at lower Reynolds 

 numbers. 



TANK 

 Re^, =0.73x10^ 



TANK 



The occurrence of cavitating spots in the laminar 

 region agrees with the observation of turbulent 

 streaks in the paint tests in the cavitation tunnel 

 at higher Reynolds numbers. Therefore, it is 

 conjectured that, in the tunnel, tiny particles 

 were deposited on the leading edge of the propeller, 

 thus creating turbulent streaks . The number of 

 these streaks may increase with time, and these 

 turbulent streaks cause spots of cavitation. 



Another possible effect is that the propeller 

 is not hydrodynamically smooth. With increasing 

 Reynolds number the boundary layer becomes thinner 

 and more sensitive to local roughness. In this 

 case the streaks would always be in the same position. 

 Not enough observations were made to verify this, 

 but the strongly reduced occurrence of turbulent 

 spots in the towing tank points to the flow as the 

 origin of the disturbances. The occurrence of 

 these streaks was also apparent in the tank when 

 the pressure was drastically lowered, as is shown 

 in Figure 20. It is of course very important to 

 recognize these cavitating spots since they indicate 

 a region of laminar boundary layer flow and a 

 possible restriction of the radial extent and the 

 volume of the cavity. 



The effect of Reynolds number on cavitation in 

 the region from the critical radius to the tip is 

 small. In nearly all cases cavitation took place 

 in this region at low Reynolds numbers . In some 

 cases no cavitation was present in this region at 

 a low Reynolds number, as shown in Figure 21. A 

 paint test is included to show the critical radius . 

 At a higher Reynolds number, cavitation was present 

 until the critical radius. The ideal inception 

 radius in this case is at r/R=0.7. A similar effect 

 was sometimes seen at propeller C and can be 

 explained by the fact that the reattachment region, 

 where inception is assumed to occur, shifts to 

 lower pressure regions with increasing Reynolds 

 number. Calculations of such an effect are given 

 by Huang and Peterson (1977). It is not certain, 

 however, that the Reynolds number is the only 

 variable since application of electrolysis also 

 caused inception at low Reynolds numbers. Apparently 

 the nuclei distribution becomes more critical with 

 lower Reynolds numbers. 



Observations with a 



NT 



0.5 



TUNNEL 

 Re^, = 1.56x10' 



,6 



TUNNEL 

 Re^i =2.72x10® 



FIGURE 19. Effect of Reynolds number on propeller A 

 at 30% slip with o = 1.5. 



Laminar boundary layer flow was seen to prevent 

 sheet cavitation at the leading edge. To see if 

 there is some threshold for inception the cavitation 

 index was drastically lowered to ffr[^=0.5. This 

 was only possible at high Reynolds numbers. In 

 Figure 22 propeller A is shown at 30? slip, a 

 condition comparable with Figure 19b, but at a low 

 cavitation index. It is clear that even in this 

 extreme condition no cavitation occurred in the 

 laminar flow region. 



A comparison of the local cavitation index with 

 the pressure coefficients as given in Figure 9 

 shows that, e.g., at r/R=0.8, the minimum pressure 

 coefficient is 0.54 while the cavitation index at 

 that radius is 0.08 to 0.012, depending on the 

 position of the blade. The cavitation index at this 

 radius is lower than the pressure coefficient over 

 most of the propeller section. When turbulent spots 

 appeared inside the critical radius these spots 

 were supercavitating, as is also shown in Figure 20. 



Bubble cavitation can be expected near midchord 



