457 



W 



30 



20 



10 



o.t 



0.6 0,8 



Cavitation Number 



1.0 



FIGURE 6. Standard deviation of measured cavity length 

 (NACA 0015, a = 4 deg.) . 



from Figure 8. The pressure distribution diverges 

 to a positive infinite value at the end of cavity 

 because of singularity at this point. This singu- 

 larity makes the agreement between theoretical and 

 experimental results very poor. 



The cavity shape is also compared in Figure 9. 

 The observed leading edge of the cavity is about 

 10^ chord position. Whereas, in the theory the 

 leading edge of the cavity begins at the leading 

 edge of the foil. This appears to be one of the 

 reasons why the calculated cavity thickness is 

 much thicker than the experimental thickness even 

 though the cavities have similar profiles. 



4. EROSION TEST 



Cavity Length and Position of Erosion 



The roughness increment on the foil was measured 

 for various exposure times. Spanwise roughness 

 measurements were made over the entire chord at 

 intervals corresponding to 5% the chord length. 



Two examples of the roughness distribution are 

 shown in Figure 10. Arrow marks in this figure 

 indicate the position of cavity end and the standard 

 deviation of its fluctuation. 



The figure clearly shows that the peak of erosion 

 appears slightly downstream of the cavity end, and 

 the erosion distribution agrees well with the cavity 

 fluctuation. Namely, there is an obvious peak in 

 the region of a > 0.8, but in the region of a < 0.8 

 the surface roughness distribution spreads over a 

 wider range. This result indicates that the esti- 

 mation of cavity length and its degree of fluctuation 

 are important factors in the prediction of erosion 

 intensity. 



Effect of Hydrodynamic Factors on Erosion 



Cavitation Number 



The mean increment of surface roughness, SR, and 

 its time rate of change can be determined from the 

 roughness distribtuion shown in Figure 10. It 

 corresponds to the mean depth of deformation rate 

 (MDDR) because the test was finished within the 

 incubation period. While Thiruvengadam has proposed 

 adopting the rate of energy absorbed by the eroded 

 material, which can be calculated by multiplying 

 MDP by the energy absorbing capacity of the material 

 per unit volume, the present research uses MDDR as 

 a measure of erosion intensity in order to find 

 which property is responsible for cavitation erosion. 



It is known that the erosion intensity, MDDR, 

 has a peak at the certain cavitation number. The 

 change of measured MDDR to cavitation number is 

 shown in Figure 11, where plots (a) and (b) refer 

 to the NACA 0015 foil tests while plot (c) refers 

 to the NACA 16021 foil tests. The test result of 

 Kohl and Thiruvengadam are also presented in plot 

 (d) [Kohl (1968) and Thiruvengadam (1971)]. As 

 mentioned earlier, while the same foil section 

 (NACA 16021) was tested in Test Series I, a different 

 attack angle was used. 



There are several differences in the results 

 obtained in the NACA 0015 foil tests and the NACA 



I 60 



t 40 



0,6 0,8 



Cavitation Number 



FIGURE 7. Comparison of calcu- 

 lated and observed mean cavity 

 length (NACA 0015, a = 4 deg.). 



