As the above scheme for measuring two-dimensional steady or imsteady 

 velocity profiles is accomplished at specific locations in a plane, it is 

 necessary that the bubble-line intersections be sufficiently numerous to 

 enable a "continuous" determination of the velocity field. That is, the 

 separation of the streaklines from the wire rungs and the separation of the 

 time lines have to be such that the resulting quantitative data enable a 

 smooth extrapolation for the velocity value points of measured values . In 

 this way, one is then able to describe velocity profiles by a smooth 

 curve o However, a photograph with an inordinate number of bubble line 

 intersections can be tedious and difficult to interpret for quantitative 

 results . 



The velocity field may be calculated in another manner when a motion- 

 picture film strip has been taken of bubble distortions in a particular 

 velocity field. When particular fluid particles, as marked by individual 

 bubbles, are followed from frame to frame, division of the vector distance 

 between bubbles by the small known time interval between frame exposure 

 determines the magnitude and direction of velocity components throughout 

 the field . This procedure and the one previously described for determining 

 velocity profiles from a single frame are greatly expedited by the use of 

 film readers such as the Benson-Lehner Corporation Oscar Model F (GS 1026 g) 

 System. This method of velocity field determination can become extremely 

 difficult, if not impossible, when there are many similarly sized and 

 indistinguishable bubbles on successive film frames. One way to eliminate 

 this difficulty is to use the spider-web bubble patterns to enmesh the 

 desired velocity field. The bubble translations can then be traced relative 

 to some datum point observable on the film frame in a very organized fashion. 

 As previously mentioned, the adjustable grid size enables one to specify 

 the number of intersection points in the velocity field. 



The qualitative aspects of higher speed (3 to 5 ft/sec and above this 

 range) oscillating wakes may be observed without a photograph as follows. 

 .Consider the flow about a foil shape or flat plate behind which the 

 shedding vortices are moving into the wake so rapidly that physical 

 visualization of the bubble patterns is difficult when a continuous lighting 



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