Surface waves play an active role in transferring momentum from 

 air to water. Thus, the generation of currents by wind cannot be modeled 

 quantitatively without first modeling the generation of waves. Since 

 the two most important processes for jnomentum exchange between atmosphere 

 and sea cannot be modeled in a quantitative sense, it seems unnecessary 

 to discuss the difficulties of quantitative modeling of such secondary 

 processes as the generation of spray. 



V. SOME LANDMARK EXPERIMENTS 



Although it appears impossible to model the full process of wave gen- 

 eration for waves of significant size in a single experiment, many lab- 

 oratory studies have contributed significantly to an understanding of the 

 processes involved in wind-wave generation and the transfer of momentum 

 from air to water. 



The analytical skill of the investigator has generally been more 

 important than the size or sophistication of the laboratory facilities 

 in determining the significance of the experimental results. A few 

 significant results are briefly reviewed in this section. Significant 

 results were obtained in some of the early experiments in spite of the 

 lack of understanding of some of the phenomena discussed in Section IV. 

 Quantitative agreement between laboratory and field data, however, has 

 rarely been achieved. 



1. Significant Experimental Results . 



a. Keulegan's Experiments . Keulegan (1951) , using a wind-wave flume 

 28.5 centimeters (11.2 inches) deep, 11.3 centimeters (4.5 inches) wide, 

 and about 20 meters (65 feet) long, made several discoveries of fundamental 

 importance to all future wind-wave laboratory studies. Although these 

 discoveries have been confirmed many times, all have not yet been adequately 

 explained, and they are sometimes overlooked. 



It was discovered by accident that adding soap to water inhibited the 

 formation of waves by wind, but did not seem to interfere with the dynamics 

 of mechanically generated waves. Later investigators confirmed this dis- 

 covery and found that the same result can be obtained with synthetic deter- 

 gents in the field and in the laboratory. 



Keulegan used soap to suppress wind-wave generation, and measured the 

 stress of wind on water with and without waves, while holding other exper- 

 imental conditions nearly constant. He found that the presence of waves 

 greatly increased the stress for all winds above a critical velocity which 

 depended on the viscosity of the water. This result has been confirmed for 

 field and laboratory measurements by Van Dom (1953) and other investigators. 



By using soap to suppress wave formation and clean water to permit wave 

 formation, Keulegan also measured the velocity of the water surface with 

 and without waves. He found that for the conditions of his experiments, 

 water depths of 4 to 14.5 centimeters (1.5 to 5.7 inches) and reference 



34 



