To visualize flow characteristics very close to the body, i.e., in the 

 boundary layer of the body, it is necessary to insert the streaklines into 

 the boundaiy layer itself. This is done by positioning the kinked wire 

 very close to the leading edge of the body so that the streaklines begin 

 in the stagnation region and are dragged around the body in accordance with 

 the boundary- layer characteristics. Caution should be used, however, in 

 interpreting these streakline patterns. Hama has shown that very deceptive 

 streakline patterns can be obtained in shear flows. That is, streakline 

 rollup patterns can be wrongly interpreted as illustrating concentration 

 of vorticity in shear flows in which there is no wave amplification what- 

 soever. Streakline patterns about the 1-in. diameter cylinder are shown 

 in Figure I5. In this series of photographs the streaklines portray the 

 cyclic behavior of and the mainstream participation in the flow developments 

 behind the cylinder. 



The hydrogen-bubble technique can also be conveniently utilized to 

 detei-mine the position of the separation point of the flow about a body. 

 To use the technique in this manner, however, two wires operating in the 

 flow simultaneously are usually needed. Because of the separation from the 

 body of the streaklines due to boundary- layer characteristics, it becomes 

 necessary to put bubbles into the flow downstream of the separation point . 

 Thus, the streaklines: upstream of the position of separation exhibit the 

 expected velocity gradient: 



Hr) 



y.0> ° 



where u is the longitudinal velocity of the flow in the boundary layer and 

 y is the transverse coordinate perpendicular to the surface of the body. 

 The position of separation is defined as the position on the body where 



fe) 



J-O =° 



Downstream of the separation point, the flow is reversed and 



20 



