Laboratory Investigations on Air-Sea Interactions 



stream and was monitored by two identical pressure sensors, one 

 at the interface and the other in the free stream, through a Pace 

 P-90 differential pressure transducer. The whole system was 

 allowed to follow the wave motion so that the lower pressure sensor 

 was kept a fixed distance from the instantaneous air- water interface 

 and inside the critical layer. The unwanted pressure signal caused 

 by the motion of the system was determined by calibration tests and 

 removed in the final data reduction. 



3.3. Measurement of Wave Growth 



A series of experiments was run to measure wave growth 

 rate in the Stanford wind-wave channel. Small- amplitude, deep- 

 water waves with frequencies varying from 0.9 to 1.4 cps were used 

 with the maximum wind speed ranging from 1 2 to 44 fps (fan speed of 

 100-300 rpm). Time records of wave profiles were obtained with 

 capacitance-wire sensors at seven locations spaced at 10 ft intervcils 

 along the centerline of the test section. Air velocity distributions 

 were taken at six intermediate locations with a conventional pitot- 

 static probe. 



Although the mechanic ally- generated waves were initially of 

 small amplitude and closely sinusoidal, they become steep and some- 

 what non-sinusoidcd with increasing fetch in response to the wind 

 action. The true wave profile could be viewed as a superposition of 

 a mean wave and a spectrum of ripples. Therefore, a phase averag- 

 ing procedure was adopted to determine the mean wave profile at 

 each fetch and fan speed. The mean wave profile at each phase angle 

 was the result of averaging 35 waves in the time series. The streana 

 function fitting technique introduced by Dean [ 1965] and outlined for 

 this application by Bole and Hsu [ 1967] was used for evaluating the 

 kinetic and potential energy of each mean wave profile. Finally, the 

 total wave energy at each location of the test section was adjusted 

 for wave energy dissipation due to viscous action. The dissipation 

 was determined experimentally for conditions without wind. 



Along- with the mean wave profile, the ripple variance of the 

 water surface about the mean wave profile at each phase angle of the 

 wave and the mean ripple variance and standard deviation for all the 

 phase angles were calculated. The ripple variance is, of course, 

 proportioncil to the potential energy contained in the ripple. 



IV. RESULTS AND DISCUSSION 



4. 1 . Water Surface Roughness (Unsteady-State, True Air- Water 

 Interface) 



When mechanically generated waves were subjected to wind 

 action, ripples were always present and were superposed on the 

 waves. Thus, the water surface can no longer be regarded as smooth 



17 



