wind-wave facility. The growth of the jnechanically generated waves under 

 the influence of wind in the wind tunnel jnay then be studied, but the 

 pressure gradients resulting from growing boundary layers would still need 

 to be considered. 



Considerable progress has been made in recent years in modeling wave 

 spectra with programable wave generators to obtain laboratory wave trains 

 with statistical characteristics similar to those observed in nature. 

 The major contribution to an improved simulation of natural waves in lab- 

 oratory facilities equipped with both programable wave generators and the 

 ability to blow wind over the water appears to be due to the programable 

 wave generators . 



3. The Scales of Motion Involved in Momentum Exchange Between Wind and 

 Water. 



In the atmosphere, the boundary layer equations can be used only in 

 the lowest 100 meters. Boundary layer thickness is expected to approach 

 this value within a fetch of about 3 kilometers in neutrally stable air. 

 Wave growth may continue for fetches of more than 1,000 kilometers, 300 

 times the fetch of boundary layer growth. In the laboratory, boundary 

 layer growth generally continues for the full length of the facility. 

 Hence, a quasi-stable boundary layer condition independent of fetch 

 (similar to prototype condition) is not developed in the laboratory for 

 usable windspeed. 



In laminar airflow over calm water, only one of the wave -generating 

 mechanisms (discussed in Section III)— the viscous shear theory of Miles- 

 can be effective. This condition cannot hold over any large fetch in 

 nature unless the windspeed is extremely small and the atmospheric strat- 

 ification is extremely stable because the wind, with any significant 

 speed, is always turbulent. Laminar flow may prevail for the first few 

 meters in laboratory facilities unless turbulent flow conditions are 

 generated before the air contacts the water. The part of the flow which 

 is laminar, where wave generation is controlled by viscosity, cannot be 

 regarded as modeling prototype wave generation. 



For turbulent flow over calm water, the Phillips (1957) mechanism for 

 wave generation will be effective in both laboratory and field. Wave 

 growth by this mechanism is controlled by the local structure of turbu- 

 lence. The size of the turbulent eddies which can be effective in this 

 process is limited, to a large extent, by the thickness of the turbulent 

 boundary layer. The thickness of the turbulent boundary layer in the 

 atmosphere varies with the density stratification of the air, the windspeed, 

 and the surface roughness but is generally about 100 meters. Thus, the 

 Phillips mechanism can contribute to wave growth at all wavelengths from 

 a few centimeters to 100 meters or longer, if the windspeed is sufficiently 

 high. In the laboratory the thickness of the turbulent boundary is always 

 limited by the thickness of the airspace, which is often less than 1 meter. 

 Generally the thickness of the turbulent boundary in the wind tunnel is 

 much less than the thickness of the airspace. Thus, the Phillips mechanism 



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