453 



Micro-appearances of the eroded surface such as 

 the pit shape, can also be qualified by examination 

 of roughness records and microscopic pictures of 

 the surface. 



The erosion intensity has been evaluated by 

 mean depth of penetration (MDP) [e.g., Hammitt 

 (1969) ] ,or energy absorbed by the material eroded 

 [e.g., Thiruvengadam (1966)]. In addition one of 

 the authors recently proposed a new concept of 

 erosion intensity, mean depth of deformation (MDD) 

 which functions as a bridge between surface rough- 

 ness, SR, and MDP [Kato (1975)]. Thus MDD corres- 

 ponds to SR at the initial stage and MDP at the 

 final stage of erosion. 



This paper discusses the experimental results 

 of two-dimensional aluminum foil sections (pure 

 and aluminum alloy) , various considerations of the 

 erosion mechanism in connection with the hydrodynamic 

 characteristics of the foil section along with the 

 modeling and scaling of erosion, and summarizes 

 experiments using an air injection system which the 

 authors found very effective in cavitation erosion 

 preventation. Nomenclature is shown at the end of 

 this paper. 



drastically changes with only slight changes in the 

 cavitation number. While this characteristic is 

 desirable in practical applications, it was found 

 to be undesirable in the present study since erosion 

 would occur only in a narrow range of cavitation 

 numbers which makes the experiment difficult. 



Therefore prior to starting Test Series II in 

 1977, two major improvements were made. From wind 

 tunnel tests the minimum aspect ratio necessary to 

 maintain two-dimensional flow was found to be about 

 A = 0.4 and an aspect ratio, A = 0.5, was chosen 

 for Test Series II. The smaller foil was designed 

 with a 30mm chord and a 15mm span and the larger 

 foil section was designed with a 50mm chord and 

 a 2 5mm span . 



The second improvement was to change the foil 

 section, from the NACA 16021 to the older NACA 0015, 

 which has a chordwise pressure distribution of the 

 "triangular" type. The experimental chordwise 

 pressure distribution of this foil is compared in 

 Figure 1 with the calculated pressure values. It 

 can be seen that the agreement between the experi- 

 ment and calculation is satisfactory. 



Test Condition 



2. FOIL SECTION EROSION TEST 



High Speed Cavitation Tunnel at University of Tokyo 



Erosion tests of two-dimensional foil sections 

 were made using a high speed cavitation tunnel at 

 University of Tokyo. The test sections of this 

 tunnel can be changed according to the experiment. 

 For the present test two test sections were used. 

 One was the rectangular high speed section with 

 cross section dimensions of 100mm x 10mm. Test 

 Series I was carried out using this section in 



1976. Since the side wall effect was so large that 

 the two-dimensionality of the flow was almost lost 

 near the trailing edge of the foil section, it was 

 concluded that the 10mm width was too narrow. There- 

 fore the test section was modified to a 80mm x 15mm 

 cross section prior to starting Test Series II in 



1977. The maximum velocity of the section was 

 about 50m/s. 



The second test section was the rectangular low- 

 speed section used only in Test Series II (1977) . 

 It has cross section dimensions of 120mm x 25mm 

 and a maximum velocity of 35m/s. 



Foil Section 



Two foil sections (NACA 16021 and NACA 0015) were 

 tested. The NACA 16021 foil section used in Test 

 Series I (1976) , was the same foil section used in 

 Kohl's experiment [Kohl (1968)]. Kohl made his 

 tests at an attack angle of a = 0°. Since this 

 foil section has no camber, when it is set at a = 

 0°, the inception point of cavity appears around 

 60^ chord. Thus, testing at a = 0° was not suitable 

 for cavitation erosion tests, so the authors chose 

 a test condition of a = 4°. Since its chord and 

 span are 40mm and 10mm respectively, the aspect 

 ratio A = 0.25, was so small that the spanwise 

 pattern of the cavity was not uniform. The cavity 

 closed at midspan appearing as a kind of streak 

 cavitation. Another disadvantage of using the NACA 

 16021 section is its chordwise pressure distribution 

 which is the "roof-top" type. The cavity length 



In Test Series I (NACA 16021) the following items 

 were tested: 



(1) Relationships between the mean depth of 

 deformation (MDD) , mean depth of penetration 



(MDP) , and surface roughness (SR) . 



(2) Effect of cavitation number, velocity, and 

 the water's air content on the erosion 

 intensity. 



(3) Comparison between the results obtained by 

 the soft aluminum erosion test and paint 

 test. 



(4) Influence of air injection on erosion pre- 

 vention. 



FIGURE 1. Comparison of suction side Cp for NACA 

 0015 foil section. 



