58 



Rounded Series 



pressure. Thus, positions of abrupt changes in boundary geometry may be 

 points at which cavitation will occur in the separation zone even though the 

 ambient pressure is well above vapor pressure. Similarly, care must be exer- 

 cised in evaluating systems in which the influence of walls may lead to large 

 pressure reductions. 



Experiments in which the effect of local zones of separation on the 

 pressure distribution, before visible cavitation was observed but as the cavi- 

 tation number of the experiment was varied , have been reported by Rouse and 

 McNown. 89 Their experiments showed that below a certain cavitation number the 

 pressure distribution started changing, although no visible cavitation was ob- 

 served. They attributed this change to onset of cavitation in microscale 

 eddies in a zone of separation. Defining the cavitation number at which the 

 pressure distribution starts changing as the critical cavitation number, a^, 

 they compared this value with the minimum pressure coefficient for noncavi- 

 tating conditions. Their experiments showed that the greater the adverse 

 pressure gradients and, thus, the stronger the separation zone, the earlier 

 the change in minimum pressure coefficient occurred, Figure 34. The critical 



cavitation number for change in min- 

 imum pressure coefficient approached 

 more closely the negative of the 

 minimum pressure coefficient with 

 decreasing adverse pressure gradient. 

 Analytic methods presently 

 available for computation of pres- 

 sure distributions are evidently 

 restricted in practical use to two- 

 dimensional flows and axisymmetric 

 three-dimensional flows, although 

 in principle any flow field might 

 be determined by a proper distribu- 

 tion of mathematical singularities 

 (sources, sinks, doublets, vortices, 

 etc.)- Since these methods are all 

 based on potential-flow theory, the 

 results must be used with care for 

 bodies of small length-diameter 



Blurt 



ratios. However, in the majority 



Figure 34- Critical Cavitation Number f ca thege methods wlll be ade _ 

 for First Change in Minimum Pressure 



Coefficient and Minimum Pressure quate . Although no details can be 



Coefficient versus Caliber of seems WQrth while 

 Rounding — From Reference 09 & 



<3r 



2- Caliber 



Ogival 



l-Caliber Ogival 



Hemispherical 



C" 'T 



I 



'/t-Cal/ber Rounded 



0.2 0.4 0.6 0.6 

 Caliber 



B 



'/s-Caliber Rounded 



d 



