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line of the cross-section of the tunnel. These wires were placed at 1.2 m 

 intervals downstream from the inlet of the tunnel . The wire itself and the 

 water serve as the two plates of a condenser, and the insulation material 

 (Nyclad) on the wire provides the dielectric medium. The capacitance between 

 the wire and the water was measured with an AC excited bridge; the unbalance 

 voltage from the bridge was linearized, aniplified and rectified so that a DC 

 output voltage was obtained which was directly proportional to the water 

 depth. The output signal was fed to an oscillograph where the gauge response 

 was continuously recorded during a run. The capacitance bridge-oscillograph 

 combination was calibrated to give a recorded amplitude linearly proportional 

 to the (varying) water depth with a flat resiKJnse to frequency (±1^) up 

 to approximately 30 cps. 



From the continuous records of the surface displacement, data were read 

 off at equal intervals of 0.025 sec. These data were used for obtaining 

 values of standard deviation a of the surface displacement, the autocor- 

 relations R(t) of the surface displacement, and the spectral density function 

 ^(f). The computations were carried out on the NCAR-CDC 360O computer. 



It was not possible to obtain the vertical velocity distribution in the 

 water. However, the surface velocity of the water u^ was measured by placing 

 a small slightly buoyant particle on the water and observing the time required 

 for it to move past fixed stations downstream. Values of the surface velocity 

 could then be calculated from the intervals of distance of travel and the 

 time of passage. 



In this study, attention was centered on the measurement of the properties 

 of water waves under conditions of steady (mean) air motion. In order to 

 attain steady conditions in the air flow, the wave development, and the set 

 up of water in the tunnel, the fan was started about 15-20 minutes before 

 the photographs, the pi tot tube measurements, and the wave amplitude data 

 were taken at a particular location in the tunnel. In cases where wave data 

 were being measured, a sample of a wave train corresponding to the passage 

 of 100-200 waves was taken for a given run. 



Samples of wave development were taken for several different conditions. 

 For the condition of water initially standing on a smooth bottom, air velo- 

 cities taken 20 cm above the water surface, were varied from to 17 mps, 

 and the depth of water was changed from 2.5 to 10 cm. The properties of 

 fluid motion in these cases were observed at distances of approximately 1.8 

 m to 12 m from the leading edge of the water. 



III. THE AIR FLOW OVER THE WATER 



Since the air is forced by the fan through the wind tunnel of approxi- 

 mately constant cross section, a pressure gradient develops in the down- 

 stream direction. The pressure in the air p^ was found to vary approximately 

 linearly with fetch through the channel. Typical values of the pressure 

 gradient 1 3P-, (cm water per cm) as measured in the last 6 m of the 

 pwg 3x 



