17 

 Taking the smaller value, the instantaneous fluctuation may be 



p' = 3/>u' 2 



-x-^= 6^- [9] 



-FPU 2 u 2 



The instantaneous cavitation number is then 



a i = a - 6-^ [10] 



U 2 



The effect of turbulence in the boundary layer of a submerge d for m can be es- 

 timated from flat-plate results. In Reference 34 values of yu' 2 /U are given 

 which, near the plate, may be of the order of 0.15 in a region of adverse 

 pressure gradient and at Reynolds number of the order of TO 7 . For this case, 

 the instantaneous cavitation number may be 0.14 lower than the average value. 

 Thus, in a highly turbulent region, a "microscale" cavitation might occur in 

 a random fashion well before it can be observed, and suggests the possibility 

 of hearing cavitation before it is seen. 



THE ROLE OP "SURFACE-ACTIVE" MATERIALS 



The further problem of liquids contaminated with materials which 

 change the effective surface tension may be treated for liquids at rest by the 

 methods described above. 23 ' 25 " 30 Such "surface-active" materials (see, 

 e.g., Reference 35) may change the surface tension at surfaces in the interior 

 of liquids or at boundaries by large amounts for extremely small concentra- 

 tions (of the order of less than 1 percent). On the other hand, there are 

 surface-active materials which are adsorbed on the surface of materials and 

 act to increase the interfacial tension. A discussion of surface activity 

 from the point of view of the mechanism of adsorption is given in Reference 



23. 



The presence of such surface-active materials complicates the ques- 

 tion of cavitation inception in the presence of a solid boundary or of sus- 

 pended materials. It has generally been assumed that small solid particles 

 promote the formation of cavities and, on this basis, such particles or "chips" 

 have been used in boilers to prevent super-heating of water. It has further 

 been assumed that the greater the number of points on these chips, the greater 

 the ability to promote boiling. On the basis of the mechanism of cavity forma- 

 tion discussed above there is little reason to suppose that such chips should 



