and enhance tip vortex decay, with little or no efficiency loss. For the 

 porous tip, care must be exercised in the perforation design to avoid local 

 cavitation, while for the mass ejection tip, the mass flows must be 

 minimized to be practical. 



Until improved analytical representation of the tip rollup process is 

 realized, progress in this area must be made through empirical means. Thus, 

 it is recommended that an experimental investigation be initiated to assess 

 the potential of the above candidate tip vortex alleviation concepts. The 

 investigative Reynolds number R should be as high as possible, using large 

 models, in order to minimize uncertainties when extending the model results 

 to full-scale. In addition, the study should be conducted in a cavitation 

 tunnel with an appropriate force dynamometer in order to provide the neces- 

 sary tip vortex cavitation inception and performance data. Due to both a 

 lack of existing data and a physical understanding, it would be prudent to 

 keep the initial experimental effort fundamental and simple, employing, say, 

 a fixed planar lifting surface. The particular concept adaptation to a 

 propeller could come at a later stage. However, the parameters which tend 

 to control the tip vortex rollup on a propeller blade should also be in- 

 corporated into the fixed planar foil: e.g., the geometric planform, espe- 

 cially the tip area, and the spanwise circulation or loading distribution. 

 A representative planform would, obviously, be ellipitical, while the load- 

 ing distribution should be similar to that of the outer portion of a typical 

 marine propeller (e.g., see Figure 12). Finally, the investigative angle- 

 of-attack range should be sufficient to evaluate the candidate concept 

 performance for off-design operation. 



In conclusion, an attempt has been made to survey the pertinent litera- 

 ture dealing with the tip vortex rollup phenomenon and, especially, its 

 alleviation. The major dissipation concepts have been briefly discussed. 

 Those few which appear adaptable for delaying tip vortex cavitation in 

 marine propellers are identified, and appropriate experimental investiga- 

 tions are recommended. The candidate concepts would appear to offer better 

 tip vortex performance than is obtainable through the present technique of 

 propeller spanwise unloading alone. 



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