the light be directed at the bubbles to form an angle of about 125 deg with 

 the line of vision. For this type of illumination, the sketches: in ' ■ 

 Figure 11 show a means of increasing the amount of light properly- 

 directed at the bubble rows . 



Such an increase in the amount of light would undoubtedly allow a 

 faster shutter speed for photographic purposes . The bubble motions in the 

 higher speed flows would be satisfactorily "stopped" to permit the use of 

 this steady "sun gun" lighting for higher flow velocities. 



When higher flow velocities were achieved in the water tunnel, the 

 steady "sun gun" lighting was insufficient. Reference here is to velocities 

 in the range of 10 to 16 ft/sec. To achieve suitable lighting at these 

 velocities, strobotacs were used to slow down and "freeze" the motion of the 

 rapidly moving bubble rows . Figure 12 shows two General Radio Type I53I-A 

 Strobotac units mounted in the tujnnel test section. These units were driven 

 synchronously by a General Radio Type 1217-B Unit Pulse Generator. The 

 flash rate was accurately determined by the digital readout of Hewlett- 

 Packard Model 522B Electric Counter. The flexibility available through the 

 continuously variable flash repetition rate and the continuously variable 

 pulse frequency to the wetted wire proved to be extremely helpful in the 

 subsequent film analysis as described below • 



PHOTOGRAPHY 



The resulting bubble patterns could easily be observed visually and 

 photographically. To achieve sufficient photographic contrast between 

 bubble patterns and the dark background, a unique photographic recipe was 

 developed. The most satisfactory 35-™n still camera was a Leica Model M-2 

 with a 50-™™ Summicron lens and a dual range finder for close focusing. 

 Plus=X film rated at ASA 400 was found to give the best .results . This . : 

 film was developed in Acuf ine for the recommended time plus 25 percent . 

 The printed results were made by Polycontract "F" paper using a Wo . 9 

 filter and developed in D-72. 



16 



