Ellis 



DISCUSSION 



Frederick T. Hammitt 



Department of Nuclear Engineering 



The University of Michigan 



Ann Arbor, Michigan 



The author is to be congratulated on his very perceptive review of the 

 present situation regarding cavitation damage mechanisms as well as upon his 

 excellent photographs of bubbles in various stages and modes of collapse. Dr. 

 Ellis's extensive contribution to this area of research over the years is too well 

 known to require elaboration here. 



However, I do question his implication that large curvature, i.e., shallow 

 craters as observed by Knapp (Ellis, Ref . 17) in soft aluminum cannot be caused 

 by jets rather than shock waves. Figure Dl of this discussion shows profiles of 

 shallow craters produced in austenitic stainless steel by a water jet with a ve- 

 locity of about 4000 ft/sec (reproduced from Robinson and Hammitt* for conven- 

 ience) compared with profiles of cavitation craters produced on the same type of 

 material in a cavitating venturi using in one case water and in the other mer- 

 cury. The impact profiles are due to DeCorso.t For all these pits the diameter- 

 to-depth ratio is of the order of 100. Thus the pits are indeed relatively very 

 shallow, although this fact is obscured by the magnified depth scale used in the 

 figure. According to DeCorso the jet diameter is about 1/4 that of the resulting 

 pit, an approximate value also confirmed by Bruntont using lead as a target and 

 a somewhat higher water impact velocity (~5000 ft/sec). We estimate from 

 DeCorso's data as well as that of Engel§ that the jet velocity necessary to cre- 

 ate the cavitation pits shown is about 4000 ft/sec for water and 600 ft/sec for 

 mercury. This is not inconsistent with our experiment,* since the venturi 

 throat velocity used for the water tests was typically about 6 times that for the 

 mercury tests due to equipment limitations. 



As shown by Dr. Ellis's Fig. 11 especially, jets of a diameter very small 

 compared to that of the bubble from which they form are observed and may well 

 be an important damaging mechanism. This formation of a "microjet" from an 

 accelerating concave liquid surface is very reminiscent of the Monroe jets ob- 

 served by Bowden* in his jet impact experiments, and also recently observed in 



*M.J. Robinson and F.G. Hammitt, "Detailed Damage Characteristics in a Cavi- 

 tating Venturi," ASME Paper 66-WA/FE- 39, to be published Trans. ASME, 

 J. Basic Engr. 



tS.M. DeCorso, "Erosion Tests of Steam Turbine Blade Materials," ASTM Proc, 

 ASTM, Vol. 64, pp. 78Z-796. 



Jj.H. Brunton, "The Physics of Impact and Deformation: Single Impact," Phil. 

 Trans. Roy. Soc. A260(No. 1110):79-85 (July 28, 1966). 



§ O.G. Engel, "Pits in Metals Caused by Collision with Liquid Drops and Soft Me- 

 tal Spheres," Research Paper Z985, J. Res. Natl. Bur. Stds. 62 (No. 6):229-246 

 (June 1959). 



*F.P. Bowden, "The Formation of Microjets in Liquids Under the Influence of 

 Impact or Shock," Phil. Trans. Roy. Soc. A260(No. 1110):94-95 (July 28, 1966). 



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