were achieved. The result of such a process was a wire installed as in ^ 

 Figure 1. However, such a configuration required very little lateral drag 

 force to cause breakage of the wire - usually at the solder joint . Suf- 

 ficient lateral drag force for breakage was produced at water velocities as 

 low as 1 to 3 ft/sec. Obviously;, the susceptibility to breakage also depends 

 on the initial wire tension. 



To surmount this difficulty, a new wire- support mechanism was devised 

 by Mr. John Coon of the Model Basin, This new manner of positioning wires 

 utilized two rods of elliptical cross section (major axis l/h in., minor 

 axis l/l6 in . ) , One such rod was mounted through the top and the other 

 through the bottom of the closed- jet test section. The plexlglas test section 

 was modified to enable positioning the platinum wire at any location in the 

 streamline direction. Because of the elliptic section, rod vibration due to 

 vortex shedding was radically reduced and the Increased rigidity of the 

 supports was apparent . A disadvantage of such a support system was that the 

 wire could not be easily stretched into a straight configuration. When 

 this was attempted, breakage usually occurred. The non-taut wire was then 

 used and, although the wire assumed the form of a catenary, an extremely 

 high water velocity could be obtained before breakage occurred. In fact, 

 tunnel velocities as high as l6 ft/sec were achieved. Figure 5 shows the 

 results of such a high- velocity flow situation. 



For the above-mentioned velocity profile determination, it is not of 

 critical importance that the cathode wire be straight. The significant 

 features are bubble-line separation in the transverse direction or loca- 

 tions of particular bubble-line intersections in successive film frames, 

 pulse frequency and motion-filjn rate, and the scale factor encountered in 

 the photography. For the streamwise specification of the location of the 

 computed velocity profile, it is convenient to adjust the catenary- shaped 

 wire so that the vertically sloped section occupies the center of the 

 wake. This provides for initially straight and transverse bubble lines 

 in the wake region at the streamwise location of the wire . 



Additional difficulties in using the hydrogen-bubble technique are 

 caused by vortex shedding behind the thin cathode wire . This vortex 



22 



