about 40 percent of saturation at atmospheric pressure. This air content level was 

 the minimum obtainable and generally limited the tunnel working pressure, for visual- 

 ly calling cavitation inception, to a minimum absolute pressure of 12 in. (0.305 m) 

 of mercury. This tunnel pressure limitation also limited the investigative range of 

 angle of attack to -6.deg jl ot <^ 4 deg. Procedurally, for a set a and U, the tunnel 

 pressure was lowered to the point where tip vortex cavitation was observed — usually 

 in the vortex wake. Then, the tunnel pressure was increased arbitrarily until the 

 cavitation disappeared and this pressure was recorded. This procedure was repeated 

 twice and the average of the three pressures was taken to be the inception pressure 

 for this condition. The tip vortex cavitation was well-defined visually with the 

 aid of appropriate back lighting. The model angle of attack then was changed in 1/2- 

 deg increments, and the inception procedure was repeated. Due to the unsteady nature 

 of the cavitation at inception, desinence and inception are equivalent. 



RESULTS AND DISCUSSION 

 The results of the tip vortex study are presented in two sections. The first 

 section presents the results of the basic aspects of the tip vortex study. This 

 work was performed on the parent or unaltered hydrofoil and included flow visual- 

 ization, forces and TVC inception. The second section, which was based upon the 

 results of the first, presents the results of the various TVC delay concepts and 

 includes primarily force and TVC inception data. Also included in this section is 

 a brief dicussion of the rationale which lead to the specific design of the partic- 

 ular concept. 



TIP VORTEX CHARACTERISTICS— PARENT FOIL FLOW VISUALIZATION 



As discussed earlier, the use of the various concepts to delay tip vortex cav- 

 itation is predicated upon some basic knowledge of the viscous rollup process. For 

 example, both the active and passive mass injection schemes require precise position- 

 ing of the nozzle to insure injection directly into the vortex core, and for rough- 

 ness, the extent of the spanwise rollup is essential in determining the specific 

 area to be treated. Accordingly, the initial experimental efforts were devoted to 

 flow visualization studies. Three flow visualization techniques were employed — 

 tufts, paint flow, and photography. 



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