The following is a brief description of the equipment utilized in the present 

 study. For the 2-D test section velocity survey, to be discussed later, the pres- 

 sures from both the rake pitot tubes and the tunnel venturi taps were sensed by 

 Validyne variable reluctance pressure transducers. Both the pressure gauge and 

 strain gauge force dynamometer analog signals were conditioned with Endevco carrier 

 demodulators and Dana-Ectron dc amplifiers. These conditioned signals then were 

 processed by a Vidar integrating digital voltmeter and printer. 



The test section velocity was also determined by the use of a single pitot- 

 static tube and a mercury manometer. Mercury manometers were used to measure the 

 tunnel static pressure — the pressure of the air above the water in the tunnel. The 

 tunnel water temperature, the tunnel air content, and the atmospheric pressure were 

 determined from meters permanently installed at the tunnel. 



FLOW VISUALIZATION 



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

 tation is predicated upon some basic knowledge of the viscous rollup process. Ac- 

 cordingly, the initial experimental efforts were devoted to flow visualization stud- 

 ies. Three flow visualization techniques were employed. First, tufts were used to 

 define the gross flow (i.e., outside the foil boundary layer) over the model. The 

 tufts, consisting of a rather dense matrix of approximately 1/2-in. (0.0127-m) long 

 segments of yarn, were cemented to the foil surface. When subjected to flow, the 

 tuft orientation was sketched in relation to the grid marked on the foil surface. 

 Second, paint flow studies were conducted to determine the flow characteristics in 

 the model boundary layer — on the foil surface. This technique involved the applica- 

 tion of a viscous mixture, equal parts by weight, of 90 SAE motor oil and fluorescent 

 pigments to the leading edge of the model. When subjected to flow, this mixture mi- 

 grated over the foil surfaces, leaving a detailed and permanent pattern of the flow. 

 These patterns were studied and documented with photographs. Third, photography 

 (both stills and movies) was used to capture the various stages of tip vortex cavi- 

 tation development, particularly the tip vortex attachment point on the " \^ . 

 Selective results of the flow visualization work will be presented late 



17 



