departures were fouiid in a previous survey in the open-jet test section^ 

 where the deviations from the. average were from 3 to 5 percent (see Ref- 

 erence 8), This agreement with the previous survey was comforting in that 

 the two-dimensional section introduced no new departures from the average 

 and, in fact, reduced those known to exist. 



ELECTRONIC EQUIIMEIT 



Because the load resistance (i.e., the electrical resistance across the 

 terminals of the pulse generator) can vary with each water tunnel or 

 towing tank, the electrical specifications of one pulse generator which 

 operates very well in one water turmel may be insufficient to produce 

 similar bubbles in another water facility. The proper equipment specifica- 

 tions can be obtained as follows. First, an estimate should be made to 

 determine the maximum dimension of the flow fields which are to be visually 

 studied. For instance, in the case of a hydrofoil study, the width of the 

 wake (looking peirpendicular to the chord and parallel to the span length) 

 might be the largest dimension. For a visual study of the wake looking 

 perpendicular to the span length, the span length would be the maximum 

 linear dim.ension needed for the wetted platinum wire. Once this length is 

 determined, a platinum wire of this dimension should be installed in the 

 center of a test section of the water tunnel. This wire is then energized 

 with a dc power supply using the tunnel (or tDW channel or suitably installed 

 anode) as the other terminal. Readings of voltage and current taken for 

 different flow velocities allow one to determine the load resistance of 

 the water tunnel or ^ow channel. 



From the load resistance, a desirable gize for the lengths of the 

 TOWS should be determined. It has been found that a value of 0,OhO in. is 

 an appropriately photographable width for the rows of hydrogen bubble 

 clusters. The bubble rows in Figure 7 a^e of this size and therefore the 

 above value iias so chosen. This is not a sacred number, however j it is 

 dependent upon the lighting and photographic setup in that whatever can 



13 



