467 



0.2(-Thiruvengadam — 

 (1971) 

 o 1.5 IN, Foil 



• 3.0 IN. Foil 

 O.l' 1 



100 (F/s)200 300 



1)0 60 80 

 V (m/s) 



FIGURE 24. Comparison with 

 Thiruvengadam's data (NACA 

 16021). 



is destroyed, as a matter of course, after a long 

 exposure to cavitation. But as mentioned in Section 

 1 , it has the merits of yielding quantitative and 

 reliable erosion data, a similar appearance of the 

 full scale eroded surface, etc. 



The paint test has just the opposite merits. It 

 is a cheap and handy method. And although the 

 conditions under which the paint is removed changes 

 with very small changes in the paint composition, 

 test procedure, etc., it appears that by developing 

 standards, the paint test can be used to represent 

 relative differences between similar models. 



From this discussion of the paint test merits 

 and demerits , the paint test appears suitable for 

 daily routine tests of usual propellers. The soft 

 aluminum test is suitable for making standard com- 

 parative tests at different research laboratories 

 as well as for different types of propellers and 

 for situations where critical erosion predicitions 

 are required. 



It is valuable to make a comparison of these 

 test methods using the same foil section. After 

 testing several kinds of paint, a marking paint 

 "AOTAC" was found to be the best. Figure 23 (b) 

 shows appearances of the painted surface after 5 

 min. test. They can be compared with the theory 

 and the soft aluminum erosion test results shown 

 in the same figure. The cavitation number of 

 maximum erosion intensity is slightly different 

 between the paint test and theory. But the general 

 tendency agrees well and the paint test seems very 

 useful especially for a comparative testing. 



The position of maximum erosion intensity esti- 

 mated from the paint test also agrees well with 

 the chordwise distribution of MDD shown in Figure 

 10. 



6. AIR INJECTION SYSTEM 



Tiny air bubbles in the free stream reduce the 

 erosion intensity by the action of their damping 

 effect as mentioned in Section 4. To achieve a 

 positive damping effect an air injection system 



with air bubbles injected from holes on the foil 

 surface is sometimes adopted. This system has 

 been used very effectively to prevent erosion on 

 the inner surface of a full-scale ducted propeller 



[e.g. , Okamoto et al. (1975) and Narita et al. 



(1977)]. However the mechanism of prevention is 

 not yet fully explained, and the best injection 

 position and/or the necessary amount of air injection 

 have not been clarified. 



The authors made the air inject test using NACA 

 16021 foil sections with three air injection holes 

 of 0.5mm dia. drilled at 10^ or 37.5^ chord position 



(Figure 25). The tests were made at a = 4°, 

 V = 41.9m/s, and a = 0.438. The previous test 

 showed that the peak MDDR value falls somewhere 

 between 40 ~ 45^ chord. The injection position 

 of 10^ chord represents the injection near the 

 leading edge of the section, and that of 31.5% chord 

 represents the injection which insures effective 

 coverage of the eroded area. Air was then injected 

 at 2 , 5, and 10 cc (normal) /min. The quantity of 

 air was so small that separate air bubbles were 

 found even at the 10 cc/min, and consequently the 

 air jet column typical at high flow rate was not 

 observed. As seen in Figure 26, the injection 

 from 10^ chord gives better performance and even as 

 small a rate of the injection as 2 cc/min results 

 in drastic decrease in the erosion intensity. With 

 injection the MDDR value reduced to 1/5 of non- 

 injection level. Increasing air volume, the value 

 of MDDR decreases but the effect seems to become 

 saturated with a larger rate of air injection. 



7. CONCLUSIONS 



(1) The purpose of the present research was to 

 find the mechanism of cavitation erosion and its 

 scaling laws with special reference to the relation- 

 ship between the appearance of cavitation and the 

 erosion intensity. 



(2) Detailed observations of the cavity pattern 

 were made on a two-dimensional foil section (NACA 

 0015). Then erosion tests, using the same foil 

 section of pure aluminum and aluminum alloy, were 

 made to measure the increase of surface roughness. 

 The erosion intensity was also compared with the 

 observed cavity pattern and other hydrodynamic 



O.IC OR 0.375C 



Eroded Region 



^0' 



3^ 



Ki^ 



V- 



-X. 



"¥ 



0.5 



FIGURE 25. Location of air injection. 



