459 



xlO" 



3.0 



2.0 



1,0 



4.0 



3.0 



2.0 



xlO 





1,0 



0,6 0,8 1,0 1.2 



Cavitation Number 



(a) NACA0015, H2102-2, 

 Q = 4 deg., V = 35 m/s 



-o— H2102-2 ' 

 -•— H1I163-2 (5055) 

 -A— HA163-5 (6063) 



.v^S 



/ 





0.6 0.8 1.0 

 Cavitation Number 



(bl NACA0015, C = 30 mm 

 a = 4 deg., V ^ 45 m/s 



1,2 



1.0 



0.2 0.4 0.6 

 Cavitation Number 



(c) NACA 16021, H2102-2, 

 C = 40 mm, a ^ 4 deg. 



3,0 — 



2,0 



1,0 



Chord 



38,1 



Symbol Velocity 



o 48,3 K/s 

 A 53,3 M/s 

 ^ 59,1 M/s 



its, 8 M/s 

 I 53,1 H/s 



0,25 0,30 0,35 O.'iO 

 Cavitation Number 



(dl NACA 16021, 11 00F-A1 

 (Thiruvengadam, 1 971 ) 



0,15 



FIGURE 11. Summary of MDDR erosion 

 index and test results. 



16021 foil tests. First, the width of the peak of 

 NACA 16021 is narrower than the NACA 0015 peak. 

 This is caused by the difference of pressure distri- 

 bution between the two foil sections. The NACA 

 16021 's distribution is flat, resulting in a larger 

 change of cavity length with small changes in 

 cavitation number. In contrast, the NACA 0015 

 section has a triangular pressure distribution so 

 the difference between the inception cavitation 

 number and supercavitation number is large. Since 

 erosion occurs only when the cavity bubbles collapse 

 on the foil surface > it seems quite reasonable that 

 NACA 0015 has a much wider peak than the NACA 16021. 

 Here the authors would like to point out that due 

 to side wall effects the measured pressure distri- 

 bution of the NACA 16021 foil and the peak value 

 of o = 0.4 can not be obtained directly by a two- 

 dimensional calculation. 



Another difference between these two results is 

 the value of the maximum MDDR. It is much larger 

 for the NACA 16021 foil when compared with the 

 NACA 0015 foil results, even though the chord length 

 and test velocity are not that much different. The 

 main reason lies in the difference of cavity pattern. 

 With the NACA 16021 section, the cavity inception 

 is concentrated at the mid-span position and the 

 cavity was a streak type. Correspondingly, the 

 erosion pattern was a streak type, where a narrow 

 and deep eroded groove was formed along the middle 

 of foil. A picture of this groove taken by a 

 scanning electron microscope is reproduced in Figure 

 12 . Streak cavitation can induce severe erosion in 

 comparison to sheet cavitation erosion which occurred 

 in the NACA 0015 foil tests. The difference in the 

 cavity patterns seems to cause this large difference 

 in MDDR. 



