the laminar flow case. Note that V /V is less than 1.0 for the turbulent flow 



max 

 because V is based on the laminar flow case, 

 max , 



Further experimental research revealed an axial velocity in the vortex core, 



which basically introduced a three dimensionality to the models. A third generation 



model incorporating this observed phenomenon also failed to correctly predict the 



observed vortices. This disagreement was not totally unexpected because the theory 



is confined to laminar flow, which renders comparison with high Reynolds number, 



turbulent flow experiments somewhat uncertain. 



The models representing the vortex sheet rollup are becoming more elaborate. 



However, they cannot, in general, predict vortices which are observed forward of the 



Q 



trailing edge. Pressures measured on the surface of a wing tip indicated the pres- 

 ence of a concentrated vortex located just above the wing tip. A detailed mapping 



9 

 of the flowfield about a wing tip was made using a small, hot, wire probe and, 



as shown in Figure 4, the results indicate that the vorticity generated in the 

 pressure-side boundary layer migrates around the tip to the suction side of the wing 

 tip. This vorticity in combination with the surrounding flow induces an outward 

 flow on the surface of the wing tip suction side. This experimental visualization 

 of the vortex rollup process indicates the importance of the detailed tip flow on 

 the resulting vortex structure. A recent numerical approach incorporating such 

 parameters as local tip geometry, viscosity, and turbulence effects has duplicated 

 these observed tip flow patterns. With the tools now available, it would appear 

 that a more realistic fourth generation tip vortex model is not far away. 



However, at this time, an analytical description of both the tip vortex rollup 



process and the prediction of TVC inception are not yet available. As a result, the 



4 11 



major efforts in this area by McCormick and Chandrashekhara, as summarized by 



12 

 Noordzij, involve formulations which are semiempirical in nature and require sup- 

 porting experimental data. These formulations do, however, emphasize the dependence 

 of the tip vortex rollup process upon the detailed flow at the wing tip and the 

 overall spanwise load distribution. Also of importance is the strong influence of 

 varying environmental conditions (i.e., changes in the size of the undisolved air 

 nuclei) upon the TVC inception data. However, assuming that the influence of such 

 environmental factors is small and that the vorticity entrained into the tip vortex 



