scheme provides the Illumination. Using stroboscopic lighting,, let a 

 straight platinum wire^ oriented perpendicular to the wake of the body (as 

 shown in Figure l) be energized with pulsed excitation. If the strobotac 

 frequency is adjusted to be equal to. the frequency of vortex shedding^ the 

 motion of these vortices will cease. Now if the pulsed excitation frequency 

 is adjusted to freeze the bubble rows and space them about ijh in. apart 

 in the free-stream velocity field, the entire bubble pattern will become 

 stationary. The pulse frequency required to achieve a completely frozen 

 bubble pattern must be an integral multiple of the strobe frequency. The 

 spacing of l/4 in. between free-stream bubble rows is an approximate value; 

 the spacing should be such that the wake is not excessively congested by 

 needless amounts of hydrogen bubbles. (Should the strobotac (or shedding) 

 frequency be desired, it can easily be numerically determined using an EPUT 

 (events per unit time) meter to monitor the strobotac output signal.) Figure 5 

 shows a photograph of such a frozen bubble pattern. The transverse develop- 

 ment of the wake in the longitudinal direction is apparent from such a photo- 

 graph. In light of Kama's work, it is stressed that care should be used in 

 interpreting the photographic results such as those ghowli in Figure \, 



In order to specify the actual positions of vortices in the wake of 

 such a foil or flat plate body, the following procedure should be used. Let 

 a platinum wire holder configuration (as shown in Figure 3ti) be positioned 

 in and perpendicular to the wake of the body. With both wires properly 

 energized, the resulting bubble patterns should be illTimlnated using the 

 improved lighting scheme discussed in the LIGHTING section. Photograph the 

 spider-web patterns with a motion-picture camera using a film speed chosen 

 to achieve a sufficient number of frames of the cyclic phenomena taking 

 place in the time interval of the shedding period. This could be 10, 20, 

 or 30 frames per second, depending on the continuity desired between film 

 frames. Guiding values for such a film rate can easily be obtained using 

 the strobotatlc illuminating sbheme described above. Having this strip 

 of film of the cyclic translations of the bubble line intersections, use a 

 film reader to quantitatively analyze the time variant, two-dimensional 

 velocity fields. The velocity of the wake vortices is obtained by 



