eliminate pressure gradients. Wade and Debrule (1973) calculated the 

 amount of ejcpansion in cross section required to eliminate pressure gra- 

 dients for several conditions. Boundary layer growth near the ceiling can 

 be reduced by sucking air from the boundary layer and reinjecting the air 

 with increased momentiira (Schlichting, 1968, ch. 14; Coantic and Favre, 

 1970). 



Wind- generated waves and currents are results of processes taking 

 place in the boundary layer above the air-water interface. Therefore, 

 it may be desirable to accelerate the generation of this boundary layer 

 near the entrance of the airstream. Shemdin and Hsu (1966) , Shemdin 

 (1969a, 1969b, 1970), and Shemdin and Lai (1973) used artificial rough- 

 ness elements on the floor of the air intake to expedite the development 

 of the boundary layer near the air-water interface. Similar procedures 

 have been used by many other investigators. 



2i The Importance of Limited Fetch . 



If there is any chance of modeling the wind-wave generation process in 

 the laboratory, it is necessary to have identical values for the scaled 

 fetch for both laboratory and prototype conditions. Any of the equations 

 in Table 1 will permit an estimate of the approach of the developing wave 

 to the fully developed state. The uncertainty about the wave height 

 and period in the fully developed state may exceed a factor of two (Fig 5) . 

 Representative values might be expected for waves that have attained between 

 90 and 99 percent of the maximum wave height. This is unlikely to be 

 true when the waves have obtained less than 10 percent of maximum height, 

 i.e., less than 1 percent of maximum energy. 



Table 2 gives the wave height, wave period, and the percentage of the 

 final value achieved within fetches of 100 and 200 meters. A fetch of 

 100 meters will permit 90 percent of full-wave development for a speed of 

 10 centimeters per second. The resulting wave height is only 0.3 millimeter 

 and the corresponding period is 0.07 second. There appears to be no evidence 

 that the equations in Table 1 are valid for such low windspeeds . Tables 

 1 and 2 indicate that waves large enough for convenient use in engineer- 

 ing studies could be generated by wind alone only for the initial stages 

 of growth. There is no assurance that the resulting waveforms will be 

 typical of the waveforms encountered in the field. 



The effect of longer fetches might be simulated by using a programable 

 wave generator which can reproduce a sequence of waves with variable 

 height and period to simulate the wave conditions expected for some finite 

 fetch. D'Angremond and Van Oorschot (1969) compared wind- generated 

 waves in the laboratory and in the field and reported that wind-generated 

 laboratory waves characteristically have steeper wave fronts than wind- 

 generated waves recorded in the field. They attributed this feature 

 to the short fetches available in the laboratory. Some improvement is 

 achieved by adding mechanically generated monochromatic waves; greater 

 improvement is obtained by adding a programable wave generator to the 



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