flow patterns in the wakes of these bodies^ and the time and space relation- 

 ships for these phenomena are examples of the quantitative .value of the 

 technique . 



Unfortunately, the bubble technique is not without disadvantages, e.g., 

 certain velocity limitations. Included below is a discussion of the velo- 

 city limitations and the application of the hydrogen-bubble visualization 

 technique to two-dimensional unsteady flows. A scheme is put forth through . 

 which a quantitative analysis of the longitudinal and transverse aspects 

 of an unsteady water flow is achieved . 



The following is a description of the hydrogen-bubble visualization 

 technique, its diversified capabilities, and its establishment at the 

 David Taylor Model Basin. The study presented here was carried out under 

 the General Hydromechanics Research Program, S-ROO9-OIOI, Task OIO3 . 



USES AND LIMITATIONS OF THE HYDROGEN-BUBBLE TECHNIQUE 



Basically, the hydrogen-bubble flow-visualization technique consists 

 of an electrolysis process created by the excitation of cathode and anode 

 terminals wetted by flowing water. The resulting gas formed at the 

 cathode terminal is visible hydrogen gas which may be produced in the form 

 of very small bubbles. Analysis of the forces on a buoyant sphere in a 

 steady slow-speed (Stokes flow) water flow shows that the buoyancy to 

 drag ratio satisfies 



b/d = g d^/lS^; U 



If the bubble size is sufficiently small, say a few thousandths of an 

 inch, the buoyancy force is very small compared to the drag force . Con- 

 sequently, the motion of the bubbles is dictated by the local water velo- 

 city. This predominancy of drag OA^er buoyancy is verified by the negligible 

 rise rate of the small hubbies . Through this predominance of drag over 

 buoyancy, water velocity profiles may be accurately obtained in two-dimensional^ 

 low-speed flows. 



