is equal to the momentum exchange coefficient, as is assumed for unidirec- 
tional flow, the slope, M, should be directly proportional to V,. The 
experimental results for oscillating flow did not indicate direct propor- 
tionality. Therefore, the sediment exchange coefficient for oscillating 
flow given by equation (13) is, as yet, an undetermined function of Vz. 
and U,. A discussion in Section IV indicates why this result is expected. 
III. DISTRIBUTIONS OF TURBULENT VELOCITY FLUCTUATIONS 
ils Experimental Apparatus. 
Successful measurements of turbulent velocity fluctuations in fluids 
have been made using either constant current or constant temperature 
hot-film anemometers. A constant temperature, quartz-coated hot-film 
sensor, model number 6010 made by Thermo-Systems Incorporated, Minneapolis, 
Minnesota, was used in this investigation. The sensor was connected to a 
1050 series anemometer also made by Thermo-Systems Incorporated. The 
anemometer uses a bridge and feedback system to maintain a constant resis- 
tance and therefore a constant temperature of the sensor. Any change that 
affects the heat transfer between the sensor and the environment is reflec- 
ted in the voltage output of the bridge. This output voltage is amplified 
and recorded on magnetic tape. The record of voltage fluctuations is then 
converted by use of a calibration curve to a record of velocity fluctua- 
tions. A schematic of the hot-film bridge is shown in Figure 16. The 
hot-film sensor and probe are shown in Figure 17. 
Calibration of the hot-film anemometer was done in the calibration 
tank (Fig. 18) which was divided into two chambers, a fore chamber and a 
calibration chamber. The fore chamber contained two wire screens to 
ensure a uniform velocity distribution. The two chambers were connected 
by a 1-inch-diameter nozzle located at the midpoint of the partition 
Separating the two chambers, 5.25 inches above the bottom of the tank. 
An overflow was located at the downstream end of the calibration chamber, 
4.75 inches above the nozzle. Water was supplied from the deaeration 
tank, the rate controlled by a 0.25-inch needle valve. The flow into 
the calibration chamber was a submerged jet. The probe with sensor was 
held at the downstream face of the nozzle by clamps connected to a point- 
gage assembly. The point-gage assembly was used to raise and lower the 
sensor known amounts to obtain velocity measurements across the diameter 
of the nozzle. The water collected at the overflow in a measured time 
was weighed to determine the flow rate and mean nozzle jet velocity. 
Measurements of the turbulent velocity distributions were conducted 
in the swing flume. Only minor modifications to the flume apparatus 
were necessary to accommodate the anemometer equipment. 
As shown in Figure 17, the sensor is extremely delicate and there- 
fore cannot be used in flows containing solid particles. For this reason 
the flume had to be cleaned of all the black plastic sediment used in 
44 
