CAVITATION, TENSILE STRENGTH, AND THE 

 SURFACE FILMS OF GAS NUCLEI 



Leslie H. Bernd 



Research and Development Center 



General Electric Company 



Schenectady , N. Y. 



Surfaces were sought that did not promote cavitation when in contact with 

 water. These surfaces were used to encapsulate sonar transducers. With these 

 transducers, appreciable tensile strengths were found to exist in water; i.e., 

 power levels higher than normal were obtained without cavitation. 



The tensile strength of water is set by the gas nuclei present in the water. 

 Using encapsulated transducers, the behavior of gas nuclei was followed by 

 measuring tensile strength. Surface films were found to form about the nuclei, 

 retarding the acquisition of tensile strength. To acquire an understanding of 

 these surface films, various waters were tested. Also, surface films were 

 created about gas nuclei from solutions of hydrocarbons and proteins in water. 



SUMMARY 



This paper summarizes some of the results of experimental investigations 

 into the basic factors that determine the tensile strength of water. The stress 

 range covered was 0-100 psi. These factors in effect determine the "cavitation 

 properties" of water, since they can set the level of inception of cavitation, and 

 to a certain extent affect its intensity and violence. 



The underlying purpose was to learn how to achieve two opposite applica- 

 tions; the prevention of tensile strength where considered undesirable, and the 

 obtaining of tensile strength where useful. In addition, the action of gas nuclei 

 and other factors related to tensile strength are in many ways something of a 

 mystery. Knowledge of these factors should be important to our basic under- 

 standing of cavitation phenomena. 



For instance tensile strength has occasionally been noted in water tunnels 

 (1,2). The inception of cavitation has occurred at higher stress levels than cus- 

 tomarily expected. Higher flow velociti'es or lower tunnel pressures than nor- 

 mal have been needed to produce cavitation about a body. The tensile strength 

 acts as if an additional static head were present. In this case, tensile strength 

 is undesirable in order to make for a uniformity of test results, and duplicate 

 "prototype" conditions. 



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