531 



5 6 7 

 k(OT(^) 



OH 



02 



(b) 



"T 1 1 r- 



5? 



vW 



as- 



3 4 



k(cnf) 



t S 



FIGURE 2. Theoretically obtained correlation 

 of the amplification factor kC^, the phase 

 speed, Cj-, and the frequency, f, to the wave 

 number, k, in the instability of coupled shear 

 flow of the air and the water, for four values 

 of the friction velocity of the air u* of (a) 

 13.6, (b) 17.0, (c) 21.4, (d) 24.8 cm/s. 

 [Cited from Kawai (1977).] 



studies of the internal flow pattern of wind waves 

 by use of flow visualization techniques [Toba 

 et al. (1975), Okuda et al . (1976, 1977) and 

 Okuda (1977)]. 



Along the surface of individual undulations, 

 hereafter called individual waves, there is a strong 

 variation of the tangential stress exerted by the 

 wind. The stress value determined locally from the 

 distortion of hydrogen bubble lines, is several 

 times greater than the average wind stress value at 

 the windward face of the crest, and it is negligible 

 at the lee side of the crest as shown in Figure 8 

 as an example. The concentration of the shearing 

 stress results in the development of the local 

 surface wind drift forming a special region under 

 the crest where the strong vorticity is concentrated. 

 The vorticity concentration causes the forced con- 

 vection or turbulence , irrespective of whether or 

 not the air entrainment, or the breaking in a usual 

 sense occurs. As seen in Figure 9, small polystyrene 

 particles of 0.99 specific gravity placed just 

 beneath the water surface prior to the start of 

 the wind, begin to disperse into the interior by 

 the forced convection, coincidentally with the 



transition of the initial wavelets to the irregular 

 wind waves. The main stage of the growth of wind 

 waves thus seems to proceed as a strongly nonlinear 

 processes. 



4. COMPONENT WAVES AND INDIVIDUAL WAVES AS PHYSICAL 

 MODEL OF WIND WAVES 



Despite the fact that the wind waves are thus a 

 strongly nonlinear phenomenon, they have been 

 assumed as expansible to component waves, having 

 phase speeds obeying the dispersion relation of 

 free water waves, and weak wave-wave interactions 

 have been considered. 



Recently there have been some articles reporting 

 that the phase speeds of component waves do not 

 necessarily satisfy the dispersion relation, notably 

 by Ramamonjiarisoa (1974) for the one dimensional 

 case and Rikiishi (1978) for two-dimensional com- 

 ponent waves. Rikiishi developed an experimental 

 technique for the determination of the directional 

 structure of the phase speed of component waves 

 without pre-assuming the dispersion relation, and 



