cylinder is 1 in. in diameter. The two remaining bodies in the figure are 

 both plates. The plate on the right in Figure 13 has dimensions 2 by l/2 in, 

 and all surfaces are flat; the other also has a 2-ino chord but a thickness 

 of l/k in. and a rounded leading edge with a flat trailing edge l/h in. 

 thick. The spanwise dimension of all the cylindrical bodies is G.66 in. 



The photographic results for the various cathode-wire configurations 

 and experimental setups are shown in Figure 1 and in Figures 1^1- through 17 . 

 In Figure 1 the flow characteristics downstream of the trailing edge of the 

 10; 1 aspect ratio foil shape are portrayed by the bubble patterns. Instan- 

 taneous velocity profiles at the location of the 0.001- in. diameter platinum 

 cathode wire are obtained in the manner previously described. For such an 

 undertaking^, an enlargement of the photograph of the bubble patterns in 

 the wake region behind the wire facilitates the velocity determination. 

 Enlargements of different photographs are shown in Figure 7 . The bright 

 area in the left side of the photographs is a light reflection off the sharp 

 trailing edge of the plate , The specification of position in such a compu- 

 tation is best done with an observable datum point "vhLch appears in the photo- 

 graph and is located in the plane of the bubbles. For unsteady flows, 

 similar to the one shown in Figure 1, the time-variant velocity profile at 

 one location in the wake is obtainable with a series of photographs encom- 

 passing the period of oscillation. An. entire wake survey can be obtained 

 with successive locations of the cathode wire downstream of the trailing 

 edge. Although this can. be a laborious process, a result of considerable 

 significance is achieved. Such a survey was perfonned^ not in a wake 

 flow, but in a very thick boundary layer^ by Hama and Nutant (Reference 3)' 

 They were among the first to use the hydrogen-bubble technique and 

 excellent photographic results were obtained in this low-speed investigation. 



Figure Ik illustrates a typical bubble pattern about the 10:1 aspect 

 ratio foil shape for a situation in which a kinked wire was positioned 

 upstream of the leading edge and energized with dc voltage . Note the dis- 

 tortions of the mainstream flow due to the presence of the body. In 

 addition, the streaklines demonstrate the involvement of the free-stream 

 flow in the wake region behind the body. 



19 



