To avoid altering the true value of the physical quantity being 

 measured, the terminals required for the electrolysis process are chosen 

 to minimize the effect of their presence on the flow. The terminal chosen 

 for the cathode is a very thin wire supported in the water flow at some 

 location where the characteristics of the velocity field are desired, and 

 the anode consists of the metal water tunnel or towing tank wall, or some 

 suitably installed metallic tennlnalo Many different materials were used 

 as cathode terminals and platinum was found to be most suitable for this 

 purpose because of its corrosion resistance. Other materials used were 

 stainless steel, copper, brass, bronze, and zinc. 



When the thin cathode wire is energized with a dc power source, a 

 continuous sheet of hydrogen bubbles is produced in the water. The rows 

 of tiny bubbles which constitute the sheet are distorted according to the 

 local characteristics of the flow field. 



Velocity profiles in two-dimensional flows are obtained by pulsing a 

 voltage to such a wire. The cyclic generation of hydrogen along the wire 

 produces patterns like those of Figure 1. Figure 1 shows an actual size 

 view of the bubble patterns in the wake of a symmetrical foil shape (chord- 

 thickness ratio is 10; l) at 0-deg angle of attack. The view is parallel 

 to the trailing edge and perpendicular to the chord of the foil. 



Figure 1 illustrates both the qualitative and quantitative aspects of 

 the hydrogen-bubble technique . In addition to the quantitative data, such 

 as the longitudinal velocity profile available at the vertical platinum 

 wire 1/2 inch downstream of the foil trailing edge, qualitative information 

 is provided on the reversal of flow at the platinum wire , This reversed 

 flow which is present at the vertical wire is noted to extend upstream of 

 the wire, past the trailing edge, and into the boundary layer of the foil 

 shape. Such a reversed flow exists because of flow separation and continues 

 as far upstream as the location of the boundary-layer separation point on 

 the foil shape , 



Figure 1 also illustrates the manner in which the platinum wire is 

 supported in the wake of the foil shape . The heaA^ wire or rod frame is 

 constructed and mounted so as to avoid errors induced by -vfibrations caused 



