TM No. 377 



belt tc the driving pulley on the north shieve axle. This driving system 

 is so tight that there is no perceptable slippage or lag of the carriage 

 and flow meter upon starting or stopping the carriage drive. Thus, with the 

 greatly "over-powered" tow system, equilibrium speed was attained almost 

 instantly, allowing for maximum time of steady flow calibration during a 

 single trip. Also, the drive system could be run just as smoothly in the 

 reverse direction. This permitted calibration runs to be made back and 

 forth along the tank, simulating alternately positive and negative flow 

 directions , 



The most important aspect of the towing tank calibration was the 

 requirement for precise monitoring of tow speeds. It was not possible 

 to preset the tow speed closer than about 10 percent of the desired 

 value because there was no precision setting or reference dial on the 

 electro-hydraulic control valve. This had no effect, however, upon the 

 constancy of the tow speed once the control valve had been set. Thus, 

 although the preset tow speed for a particular run was only approximate, 

 the resulting speed was monitored very accurately, 



The tow rate was monitored by two independent methods. The first 

 method involved sensing the rotation of the driving shieve mounted at the 

 north end of the tank, A small alnico magnet was attached with epoxy 

 cement to the outer rim of the driving shieve. As the shieve wheel 

 rotated, the magnet repeatedly passed within 2 mm of a magnetic reed 

 switch mounted adjacent to the shieve support. This closing switch was 

 wired inseries with the remote marker input on a two-channel Sanborn 

 strip chart recorder model 320 (figure 11-20 ), which was also used for 

 recording the output of the towed wave meters. The closing of the reed 

 switch by the shieve magnet produced voltage spikes, which were recorded 

 on the same strip chart as the wave meter system outputs. One revolution 

 of the shieve represented a horizontal displacement L of the carriage of 

 hh a 6o cm. The space between two consecutive voltage peaks represented 

 both a linear distance L (equal to the shieve circumference ) and a known 

 time interval T or strip chart length proportional to the chart speed 

 used. Hence, the tow speed given by L s /T s was derived for incremental 

 points during the individual run. 



The second method of tow speed monitoring involved placement of a pair 

 of micro-switches nearly equidistant from the center of the tank and sepa- 

 rated by a measured distance of 629,9 cm. Connected to the tow carriage 

 was a small plywood cam, which struck and closed upon each micro-switch 

 button as the carriage rolled by. The first switch started a 1-second 

 sweep electronic timer, and the closing of the second switch shut it off. 

 The elapsed time for the carriage to travel the distance separating the 

 two switches was read to the nearest 0.01 second. Thus, the average speed 

 was obtained for the time of carriage travel between the two switch positions. 



For the steady flow towing tests both methods of carriage speed monitoring 

 were used. For acceleration tests the shieve rotation method was used because 

 only an average value (which is meaningless for short accelerations) can be ob- 

 tained by the consecutive micro-switching method. 



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