With respect to the inception of cavitation, no clear and 

 definite effect could be found from these tests beyond a 

 critical Reynolds number which is in contradiction to the 

 measurements by Ro W, Kermeen (Fig., 2 of part II) from which 

 the critical cavitation number not only depends on absolute 

 size but also increases when the Reynolds' number increases 

 within the whole range investigated o 



Fo:;' tests on similitude of cavitation the Weber number is 

 one of the entering parameters, the determination of which 

 requires the surface tension to be known » This quantity 

 depends on the composition of the content of the cavity which 

 is not reliably known o The method which I have tried to use 

 for determining the surface tension in the afore mentioned 

 tests is related to tip vortex cavitation., Ackeret has shown 

 that a sinusoidal shaped' surface of a cavitating vortex core 

 is in equilibrium under the action of centrifugal forces 

 and of surface tension and has established a relation between 

 wave length, wave height of the deformations and the circula- 

 tion of the vortex. Taking a hydrofoil at a certain angle 

 of incidence, the lift coefficient and, from this, the circula- 

 tion of the tip vortex are known., From photographs of the 

 cavitating tip vortex, wave length and wave height of the sur- 

 face are determinable from which, together with the circulation, 

 the S'-'-rface tension of the tunnel water against the gas con- 

 tent of the cavity follow. 



I merely want to mention ;in this connection, that apart 

 from the sinusoidal shaped deformation of the surface of a 

 cavitating vortex core (which does not depend on time) pro- 

 gressive waves are possible on the surface under the action 

 of the centrifugal force field, both in the axial and in the 

 circumferential direction, which may have some relation to 

 the excitation of singing of propellers o I am sure Mr, Eisenberg 

 will include these phenomena on the free surface of cavitat- 

 ing vortex cores in the final edition of his report o 



M^ last point refers to the forces on cavitating bodies o 

 In order to determine the lower limits of thrust coefficient 

 and efficiency of a propeller we are much interested to know 

 how lift and drag of a section behave when the cavitation 

 number decreases to zeroo There are some experimental results 

 on sections with fully developed cavitation available which 

 have been cariied out by Walchner and theoretical considera- 

 tions have been made by BetZo However, the tests are conducted 

 at small Reynolds numbers and the approximations by Betz which 

 is based on the Kirchhoff flow is restricted to the case where 

 the free stream line starts from the leading edge and does 

 not include the practically important case where the free 

 stream line starts at the point of maximum thickness of the 

 section* Such work on bodies with fully developed cavita- 

 tion which produce lift would be highly desirable o 



h 



