412 



1.0 



a PROPELLER C SUCTION SIDE SLIP =0,3 



■ „ PRESSURE SIDE .. =0G 



O PROPELLER A SUCTION SLIP = O 3 

 • ., PRESSURE SIDE 



A „ „ SUP =0.6 



FIGURE 17. Chordwise position of natural transition 

 inside the critical radius. 



of the stability o-f the laminar boundary layer 

 might give better results [Smith and Camberoni (1956) 

 Since transition occurs far from the leading edge, 

 the effect of rotation can be important. When the 

 calculation scheme of Arakeri (1973) is used it is 

 possible to take the effect of rotation into account 

 using Meyne ' s (1972) results. This was beyond the 

 scope of this paper. 



5. CAVITATION OBSERVATIONS 



The cavitation on propellers A, B, and C is sketched 

 in Figure 18 for both slip ratio's. The cavitation 



index at the blade tip in top position , 



•'NT 



(Eq. 2) 



was always 1.5. The Reynolds numbers Rejj , were 

 about 5xl05. At 30^ slip the condition is not far 

 from inception and a cavitating tip vortex is 

 present in nearly all cases. However, in some cases 

 at low Reynolds numbers, propellers A and C were ob- 

 served without any cavitation. This was not due to 

 intermittent cavitation during one test, but oc- 

 curred when tests were repeated with time-intervals 

 of some weeks. During one test the observations 

 were quite consistent, indicating that the varia- 

 tions are caused by factors which are still not 

 under enough control, e.g., air content, nuclei 

 content, turbulence. 



Correlation with Paint Test 



Of interest is the correlation of the radial extent 

 of the cavity with the observed critical radius , 

 found from the paint test. In Figure 18 the 

 observed position of the critical radius is indicated, 

 as well as the calculated ideal inception radius, 

 which is the radius where the minimum pressure on 

 the blades equals the vapor pressure. Also indicated 

 is the cavitation, observed when the leading edge 

 was roughened, as will be discussed in the next 

 section. 



On propeller A and on propeller B at 30^ slip 

 the radial extent of the cavitation is clearly 

 restricted by the observed critical radius. Some- 

 times there is a small difference between the 

 critical radius and the inception radius, which is 

 probably caused by a change in the pressure distri- 

 bution by the cavitation. 



The calculated ideal inception radii at 60% slip 

 should be considered with caustion. They are close 

 to the hub and the influence of the hub is not 

 taken into account in the calculations. For example 

 on propeller B at 60% slip the inception radius is 

 larger than calculated. In that case the critical 

 radius is smaller than the inception radius and 

 does not cause any viscous effects on cavitation. 

 The distance between the ideal inception radius 

 and the critical radius on propeller C is small, 

 so the scale effects due to the critical radius 

 will be small too. 



We can conclude that no cavitation occurred in 

 regions of laminar flow near the leading edge. The 

 radial extent of cavitation can be seriously 

 restricted by the critical radius. Since the crit- 

 ical radius is connected with laminar separation 

 this means that variation of the Reynolds number 

 does not remove this restriction until very high 

 Reynolds numbers. From Figure 17 the sectional 

 Reynolds number at r/R=0.7 has to exceed 5x106, 

 whereas a value of 3x105 is mostly considered 

 enough to avoid Reynolds effects on thrust and torque. 



Variation of Reynolds Number 



Propellers A and C were tested at a higher Reynolds 

 number in the towing tank, while propeller A was 



Re^- 73x10 



Re M = 0,51 xlO 



Re K, = 0, 66x10 



3070 SLIP 



PROP A 



FIGURE 18. 



PROP C 



WITH ROUGHNESS 



:= OBSERVED CRITICAL RADIUS 



CALCULATED IDEAL INCEPTION RADIUS 



Cavitation observations at a,,„ = 1.5. 



NT 



