415 



Rck 



: 1.56x10° 

 :0.5 



FIGURE 22. Cavitation at very low cavitation index. 

 Propeller A at 30% slip. 



in the Depressurized Towing Tank. So when cavita- 

 tion observations in the tank are compared with 

 observations in the tunnel, we can assume that the 

 nuclei content in the tunnel is always larger than 

 that in the tank by at least a factor of 10. Perhaps 

 most important, however, is that in the tank nuclei 

 greater than 50 ym are absent. 



The nuclei content in the tank has been varied 

 using electrolysis, as described in Section 2. The 

 nuclei size distribution from the wires of 0.2 mm 

 diameter has not been measured. Exploratory 

 photographic observations showed that the bubbles 

 coming from the wires are in the range of 50 to 

 100 ym under comparable conditions. 



The influence of the wires on the propeller 

 boundary layer was checked by a paint test on 

 propeller A at 30? slip. The paint patterns with 

 and without wires were identical. So we assume that 

 the turbulence, coming from the wires, did not 

 affect the propeller boundary layer. This assumption 

 should be treated with some caution, because Gates 

 (1977) showed widely different effects of flow 

 turbulence on two headf orms , both with laminar 

 separation. 



Gates also showed that large amounts of nuclei 

 can influence the boundary layer. Notably the 

 laminar separation bubble on his hemispherical 

 headf orm was removed. To see if this was also the 

 case in our tests a paint test was carried out with 

 propeller A at 30% slip. The cavitation index was 

 just above inception, so cavitation was avoided. 

 To correct for the higher pressure in this condition 

 the current through the electrolysis wires was 

 increased to produce the same volume of gas per 

 second as in the cavitating condition. No effect 

 on the paint pattern could be observed. Especially 

 the critical radius remained unchanged. So we 

 assume that the nuclei had no disturbing effect on 

 the boundary layer. As to the effect of electrolysis 



on the cavitation pattern, three regions on the 

 suction side of the propeller blades can be 

 distinguished : 



a. At radii larger than the critical radius, 

 where, at least near the critical radius, 

 laminar separation takes place. 



b. At radii smaller than the critical radius 

 having a negative pressure peak at the leading 

 edge. 



c. At radii smaller than the critical radius 

 having a pressure distribution which is 

 nearly shockfree. 



At radii larger than the critical radius no effect 

 of electrolysis on sheet cavitation could be seen 

 in those cases where it was present. In the few 

 cases where no cavitation was present in this 

 region application of electrolysis restored inception. 

 An example of absence of cavitation, apparently due 

 to a lack of nuclei, is shown in Figure 23, where 

 blade 3 of propeller C at 60? slip showed consider- 

 able cavitation , while blade 4 was free of sheet 

 cavitation during the whole rtm (9 photographs in 

 3 different blade positions) . 



Absence of cavitation in regions of laminar 

 separation, however, is an exception in the steady 

 case. a possible explanation is that the water is 

 never completely without nuclei and sooner or later 

 a nucleus will expand in the separated region and 

 cause inception. After inception cavitation seems 

 to be more or less self-sustaining. This agrees 

 with the observation of Gates (1977) that inception 

 on a hemispherical body appeared to be insensitive 

 to freestream nuclei content as long as laminar 

 separation took place. The situation is different, 

 however, in the unsteady case, when a blade passes 

 a wake peak. Only a very restricted time is avail- 

 able for inception at every propeller revolution 

 and a high frequency of encounters with nuclei is 

 necessary to obtain inception at every revolution. 

 This can explain why the "stabilizing" effect of 

 electrolysis is more pronounced behind a ship model 

 than in the open-water tests of the current test 

 program. 



At higher Reynolds numbers absence of cavitation 

 in regions of laminar separation was not observed. 

 Apart from viscous effects this can also be caused 

 by an increase in encounter frequency of nuclei, 

 since an increase in Reynolds number of the same 

 propeller models always implied an increase in 

 propeller revolutions . 



At radii smaller than the critical radius elec- 

 trolysis surprisingly had no effect at all. No 

 cavitation was initiated in the minimum pressure 

 peak, although the pressure was far below the vapor 

 pressure. Even the cavitation pattern at very low 

 cavitation index, as shown in Figure 22, was 

 unchanged. It is not clear yet why the nuclei do 

 not expand. Possibly nuclei do not reach the 

 minimum pressure region due to a screening effect 

 as described by Johnson and Hsieh (1966). In a 

 situation as shown in Figure 23, however, nuclei 

 promoted cavitation inception and were not pushed 

 away. This is only possible when the critical 

 size of nuclei in a laminar flow region is different 

 from the critical size in the reattachment region 

 of a laminar separation bubble. 



The third region which has to be considered is 

 the region where the pressure distribution is 

 nearly shockfree and has its minimum pressure near 

 midchord. When the pressure is low in these regions 

 bubble cavitation can be expected. A situation 



