rows are taken as indicative of local velocity^ large errors can occur. 

 This is due to a visually undetectable stretching or compressing of the 

 -bubble rows along their length. 



Streamline patterns for steady water flows are also obtainable with 



2 

 the hydrogen-bubble technique . As described in Clutter et al, it is 



possible to uniformly "kink" a 0,002 or 0,004-in. diameter wire by feeding 

 it through a pair of gears. A 0,001- in, diameter wire was found to be 

 inadequate in this respect because it would not retain the necessary kinked 

 configuration. When the wire is kinked and such a wire is excited by a 

 dc voltage^ electrolysis occurs along the entire length of the wire^ but 

 the hydrogen is dragged off the wire by the flow only at the downstream 

 points of the kinks. In a steady flow^ the resultant pattern is that of a 

 series of streamlines , In unsteady f low^ it is that of a series of streak- 

 lines . 



Obviously^ the larger diameter (0,00k in.) "kinked" "wire-c causes a 

 small disturbance to the flow. Therefore, the use of the kinked wire is 

 limited to a velocity range in which the influence of the wire on the flow 

 can be neglected. It is emphasized that care should be used in the employ- 

 ment of the larger kinked wire for this reason. However, for R ^ UO such 

 ° ' e 



a method does give a rapid means of visualizing the flow field by streak- 

 line traces . 



When a pulsed excitation is imparted to the kinked wire, the streak- 

 lines are changed into dashed lines. These dashed lines are then illus- 

 trative of the accelerations and the velocities which exist in the fluid. 



It was found that discrete dashes could be achieved with a voltage pulse 



2 

 having very small rise and drop times. Otherwise, dashes with blurred 



ends result, i.e., the tips and the tails of the white bubble lines are 



fuzzy. In the work carried out at DTMB it was found, however, that even 



with very short rise and dnop times (less than 15 ms), when the velocity of 



flow past the kinked wire is sufficiently large, i.e., 10 to 15 ft/sec, the 



tips and tails of dashes become blurred. This is felt to be the result of 



vortex shedding from such a wire configuration. 



