normally having a laminar separation, but with a tripped boundary layer, 



and equivalent effects might therefore be anticipated. Thus, as in 



Figures 23 and 24, a cavitation suppression effect should be expected from 



the polymer fluid if the main effect of the polymer is on the boundary 



layer. Indeed, the magnitude of van der Meulen's suppression effect are 



similar to, but not as great as, those of Figure 24. 



van der Meulen further reasoned that a body not normally possessing 



a separation should not exhibit a large suppression effect. To this end, 



he adopted a body shape previously suggested for hydrodynamic cavitation 



research by Schiebe (Schiebe 1972); namely, a body having nearly the same 



minimum pressure coefficient as the hemisphere (c = -0.75) but without 



min 

 a predicted laminar separation, van der Meulen's experiments showed little 



effect of the polymer injectant on the cavitation- inception behavior of 



this body. Furthermore, his holographic schlieren observations confirmed 



the predicted absence of separation. He was also able to show that the 



position of transition estimated from these observations agreed on the 



whole quite well with the linear stability calculations. Interestingly 



in his test facility, travelling-bubble cavitation was the predominant 



form of cavitation observed on this Schiebe body. In a followup report 



(van der Meulen 1978) he summarizes his opinions about the hemisphere and 



Schiebe bodies thusly, "Since the influence of polymer additives is to 



suppress laminar boundary-layer separation on the hemispherical nose, the 



strong pressure fluctuations occurring at the position of transition and 



reattachment of the separated shear layers; and being the principal 



mechanism for cavitation inception, are eliminated and cavitation inception 



will start at much lower pressures." 



Much the same conclusions had been reached at about the same time by 



Gates (Gates 1977, Gates et al. 1978) on the mechanism of polymer effect. 



Both Gates and van der Meulen injected polymer solutions at the stagnation 



point. Gates was able to observe a significant polymer effect at extremely 



low injectant rates on both the hemispherical body (Figure 26) and the 



Schiebe body (Figure 27). In these photographs, the injectant rate is 



46 



