A literature review of the frictional drag of wind on solid surfaces 

 or water indicates that the process. is not adequately understood and that 

 the usual engineering practice of expressing the wind stress on water as 

 the product of a coefficient, which is constant or a function of wind- 

 speed only, and the square of the windspeed as in equation (1) , is inade- 

 quate for agreement between calculations and natural phenomena. 



The momentum exchange between air and water, to form wind-driven 

 currents in the water, is a complex process involving both the growth 

 and decay of waves. Thus, quantitative agreement between model and pro- 

 totype experiments is not to be expected unless the wave generation and 

 decay processes are correctly simulated. 



A laboratory facility for air-water interaction studies might be use- 

 ful in obtaining a better understanding of some of the processes discussed 

 in Section II without achieving quantitative results or a quantitatively 

 correct modeling of the wave-wind current-generating mechanisms. 



2. Conclusions . 



1. Wind-wave research facilities, designed with specific research 

 objectives in mind and a clear understanding of the many difficulties in 

 modeling air-sea interaction processes in the laboratory, can be inval- 

 uable for fundamental research. 



2. The state-of-the-art in modeling of air-water interaction processes 

 in the laboratory has not advanced to a level which provides any assurance 

 that the validity of laboratory studies of wave effects on beaches or 

 manmade structures is improved for engineering applications by using wind 

 to generate or modify the laboratory waves. 



3. Mechanical wave-generation systems which can reproduce the spectra 

 and waveforms of natural wind-generated waves more accurately than the 

 mechanical wave generators now in common use are essential for the full 

 utilization of a wave tank-wind tunnel. .Thus, further development of mech- 

 anical wave-generating systems is an essential part of any plan for the 

 effective utilization of a wave tank-wind tunnel for coastal engineering 

 research. It may be possible to obtain nearly as much improvement in 

 coastal engineering studies through more effective use of wave-generating 

 systems, as through the combination of a wind tunnel with a wave tank. 



4. Any new wind tunnel-wave channels should be designed with a clear 

 view of the specific processes to be studied and a clear recognition that 

 the general purpose facilities of this type are beyond the present state- 

 of-the-art and may never be practical. 



38 



