437 



POL A ROID FILTER 



He-Ne GAS 



LASER 



==*=:t: 



STAGE WITH, 

 HOLOGRAM, 



MICROSCOPE 



FIGURE 6. Schematic diagram of reconstruction 

 set-up. 



pulse separations of 50 or 100 psec could be 

 generated. This enabled multiple imaging of moving 

 cavities on one hologram. 



Reconstruction of the holograms was made with a 

 continuous-wave He-Ne gas laser (X = 0.633 ym) . A 

 schematic diagram of the reconstruction set-up is 

 shown in Figure 6. The diameter of the laser beam 

 is enlarged by the lenses, Lj and L2 . The intensity 

 of the light can be adjusted by a polaroid filter. 

 The hologram is placed on a stage , fitted with 

 guides so that the hologram can be moved in two 

 orthogonal directions. The movement of the stage 

 is measured on vernier scales . The reconstructed 

 image is studied with a microscope with a magnifi- 

 cation between 40x and 200x. 



Flow Visualization Technique 



and cavitation inception measurements. Essentially, 

 the flow visualization and cavitation tests consisted 

 of making holograms at prescribed conditions. Prior 

 to each series of tests the model was cleaned and 

 the tunnel refilled. To adjust the air content, 

 the water was passed through the deaeration circuit 

 for a period of 1*5 h at a constant pressure in the 

 deaeration tank. All tests were made at a constant 

 air content, a, of about 5 cm^/S, (1 cm of air per 

 liter of water at STP corresponds to 1.325 ppm by 

 weight) . For each test the temperature of the 

 tunnel water was measured to obtain the dynamic 

 viscosity and the vapor pressure. The average 

 value of the water temperature was 20°C. The flow 

 visualization tests covered a velocity range of 2 

 to 30 m/s. For the cavitation tests, the velocity 

 ranged from 10 to 20 m/s. The effect of polymer 

 additives on cavitation and cavitation inception 

 was investigated by injecting a 500 ppm Polyox WSR- 

 301 solution from the nose of the models . Polymer 

 injection was provided by the Hughes Centurion-100 

 ptimp unit. The holograms were made at the instant 

 of maximum injection rate. The injection rate was 

 such that the average value of Vj^/Vq was 0.17. For 

 the cavitation inception measurements, the velocity 

 ranged from 10 to 24 m/s. Inception {or desinence) 

 was observed visually. 



A new technique had to be developed to visualize 

 the boundary layer flow about the axisymmetric 

 models. A description of the several methods in- 

 vestigated is given by Van der Meulen (1976b) . The 

 ultimate method consisted of injecting a sodium 

 chloride solution into the boundary layer from a 

 hole located at the stagnation point of the model. 

 The diameter of the hole is 0.08 ram. The sodium 

 chloride solution has a slightly different index 

 of refraction from the surrounding fluid. The 

 light emitted from the pulse laser will be deflected 

 and the deflections are recorded in the hologram. 

 Optimum conditions for flow visualization are given 

 by Van der Meulen and Raterink (1977) . In the 

 present study, the ratio of the injection velocity, 

 Vj_, to the velocity in the test section, Vq, was 

 usually between 0.1 and 0.2. The sodium chloride 

 concentration was 2 percent. At first, the fluid 

 was injected with a hypodermic syringe, but later 

 on, a plunger with a constant motion was used. 



Procedure 



The tests performed in the high speed tunnel com- 

 prised flow visualization tests, cavitation tests 



3. BOUNDARY LAYER STUDIES 



Newtonian Flow 



The holograms exhibited a distinct occurrence of 

 laminar boundary layer separation on the hemispher- 

 ical nose. The location of separation could be 

 obtained quite accurately from the holograms . At 

 this location the interference pattern usually 

 showed a V-shape. This is shown in the photograph 

 presented in Figure 7 . This photograph also shows 

 the laminar separation bubble itself and the 

 subsequent transition to turbulence and reattachment 

 of the separated shear layer. In the transition 

 region, the flow is still visualized by the sodium 

 chloride, but further downstream, where the turbu- 

 lence becomes more developed, mixing of the sodium 

 chloride prevents any further observations. The 

 determination of the length and the maximum height 

 of the laminar separation bubble from the holograms 

 was somewhat complicated by the fact that the height 

 of the bubble may show a maximum, as illustrated by 

 case A in Figure 8, or that the outer flow line 

 shows an inflexion point, as illustrated by case B 

 in Figure 8. The location of separation for the 



^'v 's ^ V\\S ' s'v^^ 



FIGURE 7. Photograph showing laminar separation bubble and subsequent transition to turbulence on SST 

 hemispherical nose. The flow is from left to right. At the position of separation the interference pattern 

 shows a "V". V = 4 m/s. 



