92 



T V CAMERA 



MICROSCOPE 



RECONSTRUCTED 

 IMAGE 



TV MONITOR 



HOLOGRAM ON TRAVELING 

 CARRIAGE 



-BEAM DIAMETER~5cm 

 COLLIMATING LENS 



PIN HOLE 

 -MICROSCOPE OBJECTIVE 



He-Ne GAS LASER (Smw) 



FIGURE 12. 

 holograms . 



Arrangements to reconstruct and read the 



3. GENERAL EXPERIMENTAL PROCEDURES 



Before any experiments were carried out, the water 

 in each facility was de-aerated to reduce the 

 number of freestream air bubbles produced in the 

 tunnel circuit. This was of particular importance 

 in the LTWT which has no resorber. During the 

 present tests the air content in the LTWT was typ- 

 ically between 7 - 8ppm whereas in the HSWT it was 

 between 9 - lOppm (air content levels were measured 

 with a van Slyke blood gas analyzer) . At these air 

 contents there were very few macroscopic air bubbles 

 visible in the flow approaching the model in the 

 HSWT (as will be seen later) . However, in the LTWT 

 there were always many macroscopic air bubbles 

 easily visible in the approaching flow. 



In a typical cavitation test in the LTWT, the 

 tunnel velocity and polymer injection rate (if any) 

 were first adjusted to the desired values. Incep- 

 tion was then obtained by reducing as rapidly as 

 possible the tunnel static pressure until the pres- 

 ence of cavitation was visually observed on the 

 model under stroboscopic illumination. At the 

 point of inception, a schlieren photograph, a holo- 

 gram, the tunnel velocity, and the tunnel static 

 pressure were recorded simultaneously. Each test 

 had to take less than forty seconds since by that 

 time the abundant supply of cavitation bubbles gen- 

 erated at the pump would reach the test section and 

 dramatically change the freestream conditions. Af- 

 ter each test, the tunnel pressure was raised to 

 about one atmosphere and the tunnel allowed to cir- 

 culate for five minutes. This recess between each 

 test was required to let the ruby laser cool down 



and also to let the freestream bubbles go back into 

 solution or rise to the high points in the tunnel 

 circuit. 



The same general test procedure was used in the 

 HSWT except for small differences in pressure mea- 

 surement. However, desinent cavitation observa- 

 tions were also made in this facility. All holograms 

 made in the HSWT were done without the model in 

 place but at conditions of velocity and pressure at 

 which inception had been observed to occur. 



4. 



PRESENTATION AND DISCUSSION OF FULLY WETTED 

 RESULTS 



Freestream Turbulence Levels 



Observations 



The influence of gradually increasing freestream 

 turbulence level upon the viscous flow about each 

 test body is illustrated in the sequences of 

 schlieren photographs presented in Figures 13 

 through 15 . In each sequence of photographs the 

 test body is seen in silhouette and the flow is 

 from right to left. The magnification is such that 

 the surface length shown in these photographs is 



FIGURE 13. The effect of freestream turbulence level 

 on the flow past the NSRDC body (the flow is right to 

 left) at a body Reynolds number of 1.6 x 10^: (a) u'/v 

 = 0.05 percent, (b) 0.65, (c) 1.1, (d) 2.3, (e) 3.6 

 percent. 



