33. Water speed was measured with a Nixon velocity meter, a miniature 

 propeller flow meter approximately 1 cm in diameter. Data were recorded in 

 units of Hertz from a digital readout display; the 1-sec flow meter readings 

 were internally averaged over a 10-sec interval. A linear calibration curve 

 supplied by the manufacturer was used to convert propeller cycles to flow 

 speed. The usable range of the flow meter is from 2.5 to 150 cm/sec with an 

 accuracy of +/- 1 percent, according to the manufacturer. Vertical posi- 

 tioning of the flow meter was accomplished with a point gage; both the point 

 gage and flow meter were mounted on a movable carriage riding on rails above 

 the tank which controlled stationing of the device along the width and length 

 of the tank (Figure 10). 



Flow characteristics 



34. Two characteristics of the tank flow, its steadiness through time 

 and cross-sectional asymmetry in speed at the test section (Section B) , were 

 examined prior to actual testing. Figure 11 presents the temporal variation 

 in flow speed about the respective mean at a Section B midflow elevation for 

 four flow conditions (average midflow speeds equal to 32.3, 44.0, 49.5, and 

 51.9 cm/sec). Standard deviations in flow speed over a 3.8-min time period 

 ranged from approximately 0.6 to 1.5 cm/sec if individual points were weighted 

 equally (relative deviation between 1.1 and 4.2 percent of mean flow speed). 

 As expected, averaging three consecutive speeds reduced the standard deviation 

 to approximately 0.2 to 0.7 cm/sec (relative variation between 0.4 and 1.5 

 percent of mean flow speed). Therefore, three consecutive flow speed measure- 

 ments were averaged for each data point in all calculations . 



35. Flow speed characteristics were measured along the width of Section 

 B for various depths (Figure 12). Flow speeds on the tank side opposite the 

 viewing window (left side in Figure 12) were consistently lower than those on 

 the near side. Slightly misaligned steel plate baffles (flow straighteners) 

 at the inflow end of the tank were found to be the cause of the asymmetry. 

 During testing, care was taken to place the nozzles at the center of the tank 

 where the flow speed at all depths was most symmetric. Figure 12 also 

 illustrates the decrease in flow speed in the upper layer of flow (z = 26.4 

 cm) caused by resistance from the fiber mat baffle at the tank entrance and 

 the air -water interface. 



24 



