EXPERIMENTAL PROCEDURES 



Before the tip vortex studies commenced, the nature of the flow across the 2-D 

 test section was established. The velocity profile across the test section was de- 

 termined from pitot tube rake measurements. These measurements were correlated with 

 both tunnel impeller rpm and venturi pressure drop — taken in the converging section 

 of the tunnel just upstream of the test section. The pitot tube rake contained five 

 Prandtl-type pitot tubes and extended over one-half of the 2-D test section height; 

 vertical flow symmetry was assumed. The velocity calibration, made at atmospheric 

 pressure, showed that the highest velocity occurred nearest the wall, with monoton- 

 ically decreasing values toward the center. Since the highest and lowest velocities 

 differed by no more than 2.0 percent on the average, the linear average of the five 

 readings was taken as representative of section flow speed. Maximum speed obtained 

 during calibration was 44 ft/sec (13.41 m/sec) which corresponded to a tunnel impel- 

 ler rpm of 325. Several representative velocity profiles across the 2-D test 



1 (^ 

 section are shown in Figure 12. Since blockage effects were estimated to be small, 



(approximately 1 percent) and since interest was focused on relative results, block- 

 age corrections were not considered necessary. 



For the flow visualization experiments, a thin strip of the oil-fluorescent pig- 

 ment material was applied to the leading edge, both pressure and suction side, of the 

 parent hydrofoil. The model was installed in the tunnel and the flow was quickly 

 accelerated to a maximum velocity of 44 ft/sec (13.41 m/sec). An exposure time of 

 only a few seconds at this velocity was necessary for the mixture to migrate to a 

 steady state condition over the foil surfaces, after which the flow was quickly de- 

 celerated to zero. This procedure was followed for model angles of attack of 0.5 

 (design), 10 and, -3 degrees. Supporting efforts also included the use of tufts and 

 photography, both stills and motion picture. 



The performance of each hydrofoil was established by measuring the lift, drag, 

 and roll moment. These load measurements were made at two speeds, U = 25 and 44 ft/ 

 sec (7.62 and 13.41 m/sec) and at angles of attack 10 deg ^ a ^ -7 deg in 1-deg in- 

 crements. For these measurements the tunnel static absolute pressure was kept high, 

 about 50 in. (1.27 m) of mercury, in order to avoid model surface cavitation. The 

 force data was found to be repeatable within 2 percent for lift and 3 percent for 

 drag. 



The cavitation inception measurements for each model hydrofoil were taken at 

 the maximum tunnel speed U = 44 ft/sec (13.41 m/sec) and at an air content level of 



18 



