THE INFLUENCE OF THE TRAJECTORIES 



OF GAS NUCLEI ON CAVITATION 



INCEPTION 



Virgil E. Johnson, Jr., and Tsuying Hsieh 



Hydronautics, Incorporated 



Laurel, Maryland 



INTRODUCTION 



As discussed in Ref . 1, it is understood that cavitation is the result of the 

 unstable growth of gas volumes caused by the reduction of the pressure sur- 

 rounding these "nuclei." The nuclei may be of three types: (a) quasi- spherical 

 entrained gas bubbles, (b) gas volumes stabilized in the crevices of entrained 

 solid particles, or (c) gas volumes stabilized in the crevices of the boundary 

 material. This report is directed to the first of these nuclei — the entrained gas 

 bubble. As pointed out in Ref. 2, this type of nucleus is probably the important 

 one in most practical cases. 



As pointed out in Ref. 1 spherical gas nuclei may be characterized by defi- 

 nite conditions of instability. For a given ambient radius a pressure exists be- 

 low vapor pressure for which a bubble will grow in an unstable manner. In most 

 practical cases, the times to which such nuclei are subjected to the critical 

 pressure are adequately long (greater than a few microseconds) so that condi- 

 tions for instability are adequately defined by the results of simple static equi- 

 librium theory. Consequently, for this type of nucleus, the critical cavitation 

 inception pressure may be readily determined. If the minimum pressure in the 

 flow field is also prescribed, the conditions for cavitation inception should also 

 be prescribed. 



In flow where the boundary layer is not separated the minimum pressure can 

 be determined, and thus if the nuclei sizes present in the flow are known, the 

 condition for inception should be determinable. However, experience has indi- 

 cated that scale effects exist even in water tunnels where nuclei are of such a 

 size that critical pressure variations should not be expected. It is speculated 

 that in such cases the forces acting on bubbles entrained in the free stream may 

 tend to move the larger bubbles away from the body and thus leave only smaller 

 bubbles with more negative critical pressures to be active in the inception proc- 

 ess. The forces acting in this screening process are such that the screen will 

 be graded according to the relative size of the body, the speed of the flow, and 

 the oncoming nuclei sizes present in the flow. 



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