CONCEPTS FOR DELAYING TIP VORTEX CAVITATION 



The results of the literature survey identified approximately 20 concepts for 

 alleviating tip vortices. Since a majority of the concepts were directly related to 

 work in the aircraft industry, few were suited for marine applications where such 

 factors as structural suitability, reliability, and operational environment had to 

 be considered. Additionally, the problem for the two industries are somewhat differ- 

 ent in that the aircraft industry seeks to dissipate the energy of the entire tip 

 vortex wake, while the marine industry generally seeks only to increase the tip vor- 

 tex core pressure to alleviate cavitation. However, the major limiting factor when 

 considering marine application is that the device should not be a source of any ad- 

 ditional local cavitation and should not introduce prohibitive performance penal- 

 ties. Those concepts which appeared to meet the above considerations were recom- 

 mended for experimental evaluation and are included in the current research. A 

 general discussion of these devices follows — a more physical description will be 

 given later. 



THE BULBOUS AND ROUGHENED TIPS 



4 

 These two concepts exploit the hypothesis that the thickness of the foil tip 



viscous boundary layer plays an important role in the occurrence of tip vortex cav- 

 itation. For the bulbous tip, (defined as any selective increase in the foil tip 

 thickness) , the increased surface area results in a thicker tip viscous boundary 

 layer which increases the viscous mass flow entering the vortex core. This, in 

 turn, accelerates the dissipation of the vortex core energy. In addition, the bulb 



acts in a manner similar to an endplate and retards the tip vortex rollup process. 



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 The bulbous tip concept has been previously applied to both model and full-scale 



marine lifting surfaces and although the results are promising, the bulb must be 



carefully designed to minimize both local surface cavitation and efficiency loss. 



Similarly, the artifically roughened tip increases the thickness of the viscous 



boundary layer flow entering the tip vortex core, thus destabilizing the vortex core 



energy. Although relatively little attention has been given to this idea, an ear- 



4 

 lier qualitative study did report that a roughened surface on the pressure side of 



the foil can substantially reduce the tip vortex cavitation inception index. 



Although no supporting performance data were reported, the concept warranted further 



pursuit. 



