in the laboratory would be restricted to wavelengths of a few meters at 

 jnost. The Phillips mechanism, therefore, can be effective over a wide 

 range of frequencies in all stages of wave growth in nature, but only 

 in a small range of high frequencies in the laboratory. The range of 

 possible effectiveness is determined by the geometry of the laboratory 

 flow. If the airstream is laminar as it enters the working section of 

 the wind tunnel, only the smallest of the possible eddies will exist 

 near the entrance. If the airflow is turbulent as it enters the wind 

 tunnel, the nature of the turbulence will not be determined by the 

 surface boundary layer alone, and no basis exists for assuming similar- 

 ity of the structure of turbulence at laboratory and prototype scale or 

 the validity of equation (9) for estimating boundary shear; i.e., if 

 the Phillips wave -gene rat ion mechanism is modeled in the laboratory it 

 is necessary to model the structure of turbulence. No method for fully 

 accomplishing this modeling in wind-wave facilities has been established 

 although the importance of duplicating atmospheric turbulence has received 

 attention at some laboratories. 



The Miles (1957) invicid mechanism can be effective in laboratory and 

 field as soon as waves of sufficient height and length have been devel- 

 oped to let the phase velocity of the waves equal the component of the 

 wind velocity in the direction of wave propagation at some level above 

 the viscous sublayer. The onset of this mechanism must begin under the 

 same conditions in both laboratory and field. The magnitude of the 

 energy exchange by the Miles mechanism depends on the first and second 

 derivatives of the wind profile near the level at which windspeed and 

 wave speed are equal. This implies the necessity of modeling not only 

 the turbulent structure of the flow, but also the wind profile. The wind 

 profile changes along the fliime in response to boundary layer growth, 

 pressure gradients, and the changes in surface roughness due to wave 

 generation. However, pressure gradients do not play a significant roll 

 in determining the wind profile in the turbulent boundary layer above an 

 open water surface in the prototype. Boundary layer growth is believed 

 to be unimportant for fetches longer than a few kilometers. Controlling 

 the wind profile to approximate real prototype conditions for the length 

 of the flume will be a difficult or impossible task. 



The Jeffreys (1925) sheltering mechanism becomes effective in both 

 laboratory and field when the waves exposed to the mean wind are near 

 maximimi steepness. For short fetches with no high waves in both lab- 

 oratory and field, separation may take place from ripples short enough 

 to be governed by surface tension, and the Jeffreys mechanism will involve 

 surface tension. At longer fetches and higher waves, unrealizable in the 

 laboratory, these ripples and some waves long enough to be outside the 

 capillary range will be modulated by the longer waves and maybe sheltered 

 from the mean wind by the larger waves for a part of each wave cycle. 

 The Jeffreys mechanism will not be able to operate on these waves for a 

 part of the long-wave cycle. Thus, the Jeffreys mechanism in the lab- 

 oratory cannot be a geometrically similar model to the Jeffreys mechanism 

 in the open sea. 



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