436 



FIGURE 4. Computed pressure co- 

 efficient as a function of surface 

 coordinate over diameter for blunt 

 nose. 



0.2 0.4 0.6 0,8 1.0 12 1.4 1,6 

 Surface Coordinate over Diameter, s/d 



1,8 



2.0 



Holographic Method 



In the present work, in-line holography has been 

 used to study cavitation and flow phenomena about 

 the test models. The method consists of making 

 photographic records containing detailed information 

 on the cavitation and flow patterns. Holography 

 has become one of the most important areas of modern 

 optics since the invention of the laser as a new 

 light source. Holography is usually described as 

 a method for storing wavefronts on a record from 

 which the wavefronts may later be reconstructed. 

 The record, formed in photosensitive material, is 

 called a hologram. In forming holograms two sets 

 of light waves are involved: the reference waves 

 and the subject waves. In the present case of in- 

 line holography only one set of waves is used 

 basically. The undeflected light waves from this 

 set of waves act as reference waves, the light 

 waves deflected by the subject act as subject waves. 

 A schematic diagram of the applied optical system 

 is shown in Figure 5. The light source is the 

 Korad K-IQH pulsed ruby laser of the Institute of 

 Applied Physics TNO-TH. To improve the resolution 

 of the system, the red light from the ruby laser 

 is converted to ultraviolet light, with a wavelength 

 of 0.347 pm, in a KDP-crystal. The pulse duration 

 is 25 nanoseconds and the maximum energy 4 mJ in the 

 TEMqo mode. A telescopic system (L2 and L3) is used 

 to obtain a laser beam with a diameter of 30 mm. 

 A mirror reflects the beam into the test section of 

 the tunnel. In the walls of the plexiglass test 

 section, two optical glass windows are inserted. 



The location of the body in the test section is 

 such that the nose is illuminated by the laser beam 

 over a length of about 20 mm, and the body contour 

 is imaged on the hologram. A shutter is placed on 

 the first window. The camera containing the holo- 

 graphic plate is located close to the second window. 

 Agfa-Gevaert Scientia Plates 8E56 and 8E75 with a 

 resolution up to 3000 lines/mm were used as recording 

 material. The riiby laser could also be used as a 

 multiple switched laser. Two or three pulses with 



. CAMERA /HOLOGRAPHIC PLATE 



FIGURE 5. Schematic diagram of optical system for 

 making holograms of cavitation or flow phenomena in 

 test section of tunnel. 



