Motion-picture photography of the bubble motions was successfully 

 accomplished with 35~ aii^ 70-i™i Mitchell High-Speed FC Chronograph cameras . 

 The chronograph attachment was not used on either of the cameras . Although 

 higher film speeds were available, the 'JO-wm Mitchell camera was operated 

 at approximately 25 frames per second. This frame speed was found to be 

 acceptable in stopping the flow patterns for free- stream velocities up to 

 about 5 ft/sec. Figure 8 shows the photographic and pulse -generation equip- 

 ment in the observation booth of the water tunnel. Prints of these motion 

 films enabled observation of the cyclic motion of the separation point on a 

 solid surface, the subsequent roll-up of this fluid into discrete vortices, 

 and the shedding of these vortices into the downstream wake of the body. 

 For reasons of compactness and ease of film handling, the 35-™n Mitchell 

 camera replaced the 'JO-rma, Film analysis, i.e., the cyclic portrayal of 

 events behind solid bodies, proceeded in exactly the same manner. 



SOME OPERATIONAL PEOCEDURES , 



The electrical operation of the technique proceeds as follows. A 

 platinum wire of appropriate length is properly oriented in the velocity 

 field (figure 3'b ) • A suitably installed anode terminal is connected to 

 the ac or dc power source . In the case of dc power, the amplitude of the 

 voltage is adjusted to form a white and photographable sheet of bubbles . 

 The bubble diameter should be such as to minimize bubble rise due to 

 buoyancy. It is noted that excessively high voltages cause sporadic forma- 

 tion of large bubbles which rise due to their buoyancy. In the case of ac 

 excitation, the pulse frequency should be chosen in accord with an: analysis 

 similar to-that- in Reference h. The analysis is directed toward optimum 

 measuring conditions and is pertinent when the cross-derivative terms are 

 negligible in the series expansion of the longitudinal velocity. When the 

 pulse frequency has been properly chosen, the pulse width is then adjusted 

 to render the bubble lines photographable without excessive buoyancy 

 effects. 



17 



