tellurium injection^ electrochemiluminesence, etc. are a few examples. The 

 merits of each visualization technique are based upon the extent of the dis- 

 turbance to the fluid flow caused either by the Injection method or by the 

 injected medium itself and upon the accuracy with which the desired flow 

 characteristics can be observed. 



In most cases, such visualization techniques as mentioned above are 

 useful for obtaining only the qualitative characteristics of a specific 

 flow. Quantitative characteristics are usually achieved by means of such 

 techniques as hot-wire anemometry or pitot-tube surveys, etc. 



Analysis of flow fields by means of dye-injection techniques to 



exhibit streaklinei patterns of the flow should be done with care as shown 



1-^ 

 by Hama. This is very Important in such unsteady flows as exist in 



boundary layer transition and in the oscillating wakes behind bodies. With 

 a view toward surmounting this ambiguity connected with the streakline 

 patterns and at the same time achieving quantitative measurements such as 

 time variant velocity profiles, the following scheme was introduced by 

 Geller, A small wire (O. 001- in. diameter), positioned in a water flow, 

 energized with a negative voltage and a positively energized terminal 

 positioned in the same flow were so arranged as to construct an electrolysis 

 of the flowing water. Because of the two-to-one ratio of the resulting 

 volumes of gas, hydrogen was chosen to exhibit the fluid motion. This hydro- 

 gen gas is produced in the form of very small photographable bubbles on 

 which the predominant force is the drag due to local fluid motion. 



This hydrogen-bubble visualization technique can be particularly use- 

 ful in propeller and hydrofoil research as performed in variable-pressure 

 water tunnels. In addition to such quantitative results as time-variant 

 velocity profiles in water flows, the bubble technique is qualitatively 

 useful for observing flows around bodies. Separation phenomena, oscillating 



Eeferences are listed on page 26 



