Laboratory Investigations on Air-Sea Interactions 



groups of different frequency overtaking one another, or partly by 

 the generation of high frequency waves on the wave crest. To fully 

 investigate the non-linear, wave- wave interactions and to establish 

 the role of ripples in the transfer process, measurements similar 

 to those of Mollo- Christens en and additional detail measurements 

 of the velocity field below the air-water interface should be carried 

 out under the controlled conditions of a laboratory simulation. 



The incompatibilities of the Miles' mathematical model with 

 the natural wave-growth environment, as discussed in the previous 

 section, were anticipated by Miles. He stated in this 1957 paper 

 that "our model cannot be expected to have more than qualitative 

 significance for rough flow. " It would appear that a more realistic 

 model and an improved theory of energy transfer cannot be formu- 

 lated until detailed studies of the structure of air flow near the air- 

 water interface are carried out. 



4.4. Non-dimensional Pressure Coefficients 



The non-dimensional pressure coefficients a and P ob- 

 tained from the various techniques of laboratory simulation are 

 exhibited in Figs. 10 and 11 as functions of ky^ . Comparison between 

 the measured values of the in-phase pressure coefficient oi (steady- 

 state, moving- wavy- boundary; unsteady- state, wind- wave channel) 

 and the Miles' theory is shown in Fig. 10. The experimental values 

 of the out-of-phase pressure coefficient P, evaluated from wave 

 growth measurements, are shown in Fig. 11. Although there is 

 considerable scatter in the experimental data, the deviation from 

 the inviscid theory is clearly evident and is consistent with the 

 results of the wave growth measurements. Because of the limited 

 capability of the experimental facility in the steady-state, moving- 

 wavy-boundary experiment, experimental values were limited to 

 ky, = 0.1. 



The experimentally determined phase angle tp obtained from 

 the three different methods of laboratory simulation -- moving-wavy- 

 boundary, flexible boundary with progressive waves, and wind-wave 

 channel --is shown in Fig. 12 as a function of C/u . In view of 

 the uncertainties among investigators in determining u values, 

 the experimental phase angle as a function of C/Uco and their cor- 

 responding theoretical values are shown in Fig. 13. The JPL-data 

 includes negative values of C. Because the measured velocity pro- 

 files varied to some extent with U^j, and C as discussed by Kendall 

 [1970] , theoretical values of <p for the case in which Uqo =5.5 

 in. /sec and C = were calculated. 



In an attempt to detect flow separation in the region near the 

 air-water interface in the wind-wave channel experiments, pressure 

 measurements over waves of various annplitudes with constant fre- 

 quency were made. The measured phase angles for two wave 

 frequencies, 0.6 and 0.78 cps , are shown in Fig. 13. The scatter 



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