Research Center, 1975) . The part of the nomograph which applies to 

 fetches of 1,000 mles or less is reproduced as Figure 6. 



b. Microscale Processes Involved in Wave Generation . It is well 

 known that the generation of waves on the water surface must be dom- 

 inated by pressure forces (e.g., for wave growth the pressure must be 

 higher on the backface of the wave than on the front face] , since any 

 differences between the behavior of real waves and the predictions 

 of potential flow theory are_ generally too small for detection by 

 standard measurements. If pressure forces were not the dominant factor, 

 potential flow then would not give reliable answers. Some departures 

 of real waves from "linear" potential theory can be adequately explained 

 when the nonlinear terms in the governing equations are considered. 

 If viscous shear forces played a prominent role in wave generation, the 

 waves would be rotational and differences between real waves and the 

 predictions of potential flow theory would be easy to detect. 



The first substantial success in explaining the generation of surface 

 water waves by pressure forces was achieved in 1957. Phillips (1957) 

 showed that waves could be initiated on the surface of otherwise calm 

 water by the random pressure pulses due to turbulence in the airstream. 

 Miles (1957) independently showed that if waves existed on the upper 

 surface of the water, similar waves must also exist on the lower surface 

 of the atmosphere and that under quite general conditions, the atmospheric 

 waves would extract energy from the airstream and pass it on to the water 

 waves in the form of pressure pulses. The rate at which energy and momen- 

 tum are extracted from the airstream and passed on the wave field is a 

 function of the vertical profile of the horizontal wind velocity. 

 Jeffreys (1925) proposed a similar theory in which the pressure differ- 

 ential arose from the separation of the windstream in the lee of the wave 

 crest. This theory depended on a sheltering coefficient which had to be 

 determined empirically. The sheltering theory, however, could not become 

 effective until the waves were of near maximum steepness. 



All three of the above processes are inviscid. Miles (1962) pro- 

 posed a viscous instability theory which could be effective at very short 

 fetches and high windspeed where the inviscid theories of Jeffreys (1925) 

 and r^iles (1957) could not apply. 



The generating mechanisms, as initially presented, were partially 

 idealized in the effort to simplify the presentation of complex concepts. 

 None was quantitatively correct, but together they presented an essential 

 foundation for later study of wave generation. These theories have been 

 merged and extended in many later reports by various authors. Coherent 

 developments of the theory, based on many individual contributions, are 

 presented in monographs by Phillips (1966) and Kraus (1972), and are devel- 

 oped here only to the extent necessary to consider the modeling of wave 

 generation in the laboratory. 



Later studies have shown that the mechanisms proposed by Jeffreys (1925), 

 Phillips (1957), and Miles (1957) can account for the generation of waves 



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