Mechanism and Scaling of 

 Cavitation Erosion 



Hiroharu Kato 



University of Tokyo 



Toshio Maeda 



Mitsubishi Heavy Industries Ltd. 



Atsushi Magaino 



University of Tokyo 



Tokyo , Japan 



ABSTRACT 



Recently cavitation erosion has been primarily 

 treated experimentally. However a need exists for 

 both a theoretical cavitation erosion model and 

 more quantitative erosion test methods . As a 

 contribution to the state of the art, the authors 

 have summarized their research at the University 

 of Tokyo using the soft surface erosion test method 

 (the aluminum erosion test) . 



Two test series were completed, the first using 

 the NACA 16021 foil section and the second using 

 the NACA 0015 foil section. Two-dimensional erosion 

 tests were systematically made at various velocities 

 and cavitation nimibers to obtain a correspondence 

 between the erosion and the hydrodynamic character- 

 istics of the cavitation pattern. It was found that 

 the estimation of the cavity length and its fluctua- 

 tion are important factors in the prediction of the 

 cavitation erosion. 



The results of these tests are used to illustrate 

 the effectiveness of Mean Depth of Deformation Rate, 

 MDDR, as a Cavitation Erosion Index. These test 

 results also served as a background for extending 

 the cavitation erosion scaling theory, previously 

 proposed by Kato, to include differences in the 

 cavitation number. 



After determining two empirical constants, the 

 resulting predicted MDDR Cavitation Index was shown 

 to be in good agreement with both Thiruvengadam' s 

 (1971) and the authors' test results. 



In addition to this basic research, two additional 

 studies are summarized. The first is a comparative 

 test of the aluminum erosion test arid the paint 

 test and the second is a study in the influence of 

 air injection in reducing the cavitation erosion 

 intensity. The test results obtained from the paint 

 and aluminum tests were found to be in good agreement 

 and for routine cavitation erosion checks, the paint 

 test should be adequate. It was found that small, 

 air injection rates reduced the cavitation erosion 

 intensity dramatically and large injection rates 



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did not result in substantial reduction of the 

 cavitation erosion intensity. 



1 . INTRODUCTION 



Erosion is one of the largest problems caused by 

 cavitation. Cavitation tests of model propellers 

 have been made for the purpose of predicting cavita- 

 tion erosion, especially for low-speed merchant 

 ships. However, the prediction was mainly based 

 on the observer's "feeling" of the cavitation 

 pattern on the propeller blade. Recently a new 

 testing method, i.e., paint test, was developed at 

 several laboratories [Sasajima (1972) and Lindgren 

 and Bjarne (1974)]. In this test the erosion inten- 

 sity is judged by the area of paint peeled off. 



At the University of Tokyo in the authors' 

 laboratory, erosion tests of soft aluminum test 

 pieces have been made for several years [Sato et 

 al. (1974) and Sato (1976)1. The main purpose for 

 developing the soft aluminum method are: 



(1) Development of a quantitative prediction 

 method for cavitation erosion. 



(2) Obtain a deeper insight into the mechanism 

 of cavitation erosion by the observation 

 of eroded metal surface. 



(3) Establishment of cavitation erosion scaling 

 laws. 



The test piece is usually made of pure aluminum, 

 which is easy to obtain, has stable quality, good 

 machinability , and is relatively cheap. Its 

 mechanical properties can be roughly established 

 by hardness and tensile tests. The erosion resist- 

 ance of pure aluminum is very low and its surface 

 is roughed by cavitation attack within one half 

 hour of test exposure which is similar to the testing 

 time of the paint test. The increase in roughness 

 is a first indication of erosion [e.g.. Young and 

 Johnston (1969) ] . It can be measured by a roughness 

 tester and the quantitative erosion intensity can 

 be obtained with sufficient accuracy. 



