420 



From the previous investigations in uniform flow 

 some tentative explanations can be given of the 

 scale effects on cavitation as shown in Figures 

 1-3. The explanations can only be tentative since 

 the unsteady pressure distribution on the propellers 

 in the wake is not known. Propeller A in Figure 1 

 apparently had a critical radius at r/R=0.9 in 

 this blade position, which was removed by roughness 

 at the leading edge. Also, behind the model in 

 some situations no cavitation at all occurred in 

 the wake peak, which is expected to be due to a 

 lack of nuclei (as seen in Figure 21) . 



The lack of nuclei is more apparent at propeller 

 B. The critical radius is expected to be near the 

 hub, but the low encounter frequency with nuclei 

 of sufficient size makes cavitation inception more 

 or less random. The irregular collapse of the 

 cavity on propeller C is apparently due to a strong 

 change in the pressure distribution, due to a sharp 

 wake peak. The critical radius at the position of 

 Figure 3 is near r/R=0.9 but the cavity at inner 

 radii is still collapsing. This phenomenon could 

 also be seen on high speed films , where the sheet 

 cavity was seen to detach from the leading edge 

 and collapse while moving with the flow. Some 

 cavitating spots can be seen at r/R=0.8 on propeller 

 C. 



ACKNOWLEDGMENT 



Part of this program was supported by the Dutch 

 Ministry of Economic Affairs under the ICOSTE- 

 program. 



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