multiplying their wave length by the frequency of the strobotac lighting 

 which "freezes" their motiono When the velocity of the vortices in the wake 

 is subtracted from this determined velocity field, one Is then able to 

 specify the actual centers of rotation of the wake vortices to within the 

 accuracy permitted by the clarity of the photographs . Such a result enables 

 quantitative analysis of the entire wake flow field, provided of course, 

 that the transverse dimension of the wake permits the bubble line inter- 

 sections to be observable on the screen of the film analyzer. Excessive 

 bubble line congestion can cause this analysis to be tedious, if not impossible, 

 In addition to wake width dimension, which is dependent on body sizes, water 

 speeds, etc., the experimenter may be able to eliminate bubble line congestion 

 by altering the pulsed excitation frequency and/or the spacing between the 

 the platinum wire rungs of the ladder wire conf igioration . 



TEST FACILITY 



To achieve a two-dimensional water flow, the test section of the 12- 

 inch variable-pressure water tunnel at the David Taylor Model Basin (Ref- 

 erence 7) was modified in the following manner, A plexiglas circular tube 

 (Figure 6) was installed in the existing open- jet test section to form an 

 axl symmetrical closed- jet test section. Into this plexiglas tube were 

 installed the straight, parallel plexiglas liners that are shown in Fig- 

 ure 6. The perpendicular distance between the liners is G .GG in. The 

 flow which precedes this test section was made to change smoothly from the 

 circular upstream timnel shape to the straight-sided plexiglas section by 

 means of two alamlnum traxisition pieces installed in the entrance nozzle. 



A pitot survey of the longitudinal velocity distribution revealed that 

 departures fromi a total average velocity (averaged over the whole test 

 section) were less than 3 percent . Transverse velocity components were not 

 measured. Departures from the average test section velocity occurred pri- 

 marily in two places. The first occurred near the wall of the test section 

 as could be expected due to the presence of the boundary. The other occurred 

 due to a slowly moving "slug" of water centered in the test section. S±mi.lar 



12 



