Also shown in the flow pattern, at approximately 15 percent chord, is a region 

 where there appears to be a strong outward spanwise flow, followed by chordwise flow. 

 This trend was not observed for the smaller a. Inboard along the trailing edge, the 

 area where the paint has failed to migrate or has accumulated represents an area 

 where the flow has separated from the foil surface (a phenomemon that was also ob- 

 served with the tufts) . 



Photographs showing the various stages of TVC development are presented in 

 Figure 15. The photographs show the foil suction side at constant a = 10 deg and 

 for R and o as indicated. Figure 15a shows the earlier stages of TVC where the 

 cavitation is primarily limited to the tip vortex wake. This stage also had the 

 tendency to be unsteady with intermittent attachment to the foil tip. For lower O, 

 Figure 15b shows the vortex attached to the foil tip at approximately midchord . 

 Here, the vortex core diameter has increased and the cavitation is steady and 

 attached. For a more advanced stage. Figure 15c shows the cavitation moving up the 

 foil leading edge with continual increase in the cavitating core diameter. The ad- 

 vanced stage of TVC is shown in Figure 15d where the vortex wake exhibits a marked 

 "twist" and the cavitating vortex has merged with the leading edge cavitation. It 

 should be noted that, in general, during development, the cavitating trajectory down 

 stream of the model does not change and that the attachment point on the foil tip 

 does not move and remains fixed at the tip midchord and just inboard on the foil 

 suction side, independent of a. 



Several of the TVC characteristics discussed above were quantified and are pre- 

 sented in Figures 16 and 17. Figure 16 presents the growth of the TVC cavitating 

 core radius as a function of cavitation index a and angle of attack a and indicates, 

 for fixed a, an approximate exponential increase in core radius as a decreases from 



2.4 to 0.3. For fixed O, the core radius increases with increasing a. Similar 



1 8 

 trends have been reported in other TVC studies. The vortex core size was estima- 

 ted visually by comparison with the foil grid of known widths and spacing. Figure 

 17 presents the TV trajectory downstream of the foil relative to the foil root. The 



trajectory was based upon the visual observation documented in Figure 15 and agrees 



19 



well with the theoretical prediction dealing with the motion of trailing vortices 



to determine vortex distributions for downwash calculations. 



24 



