460 



I ,F 



tf 





»^ 





Flow 

 Direction 



L.E, 



1 2 3 'I 



^Eroded Region 





-iOmm 



T.E, 



r"#*^?^;« '*<*•? .ITS 





FIGURE 12. Scanning electron 

 microscope photographs of eroded 

 surface (NACA 16021, H2102-2, 

 C = 40 iran, a = 4 deg. , V = 41.7 

 m/s, a = 0.450) . 



.> 



25"C 



Imh 



T.E, 



SCALE 



Referring to Figure 11 (b) in the 3 test series 

 where only the material of the foil was changed, 

 the position of maximum MDDR changes. This seems 

 irrational because the flow condition is not changed 

 by the material. The reason of this shift is the 

 occurrence of the foil's bent trailing edge. On a 

 full scale propeller, cavitation erosion is some- 

 times accompanied by a bent trailing edge . The 

 same thing happened in the present test. The foil 

 section made of pure aluminum is much weaker than 

 those made from an aluminum alloy, and it was bent 

 more at the trailing edge causing the shift of 

 peak MDDR to the larger cavitation number. 



An example of a bent trailing edge is shown by 

 the profile view in Figure 13. The amount of bend 

 is large at the corner of the trailing edge, which 

 exaggerates considerably the shape shown in this 

 figure. The bent trailing edge was observed on 

 every NACA 0015 foil sections when the erosion 

 occurred. On the contrary, it hardly appeared on 



NACA 16021 foil section because of its thicker 

 trailing edge. 



Velocity 



It is well known that the erosion intensity is af- 

 fected very much by the mean velocity since Knapp's 

 suggestion of 6th power law [Knapp et al. (1970)]. 

 The effect of velocity on the peak value of MDDR 

 is shown in Figure 14. Usually the exponent obtained 

 experimentally, has a large spread falling somewhat 

 between 3 and 9. In the present tests with the 

 NACA 16021 foil the exponent, n, was 9 and for the 

 NACA 0015 foil tests the exponent, n, was 6. 



Chord Length 



The chord length of a foil also has a large ef- 

 fect on the erosion intensity. This is very 



