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INTRODUCTION 



In spite of a long history of effort devoted to the air-water interaction 

 problem, the basic knowledge of the mechanisms for transport processes near 

 the boundary between these two fluids has developed rather slowly. A variety 

 of theoretical and experimental studies have been reported in the literature, 

 but, because of the complexities of the physical processes involved, the 

 detailed nature of the interaction remains inadequately understood. 



Most of the experimental studies of air-water interaction have been 

 undertaken on lakes or on the ocean where the conditions of the fluids are 

 highly variable in time and space . These investigations have contributed 

 significantly to the knowledge of the atmosphere and the sea. However, their 

 usefulness in elucidating the fundamental physics of the exchange processes 

 occurring between the two fluids is limited. Therefore, more studies should 

 be carried out under controlled conditions in the laboratory to gain new 

 insights into the mechanisms of transport across the air-water boundary. 



Ursell (1956) has reviewed the fundamental laboratory experiments dealing 

 with air-water interaction that were undertaJten before 195^- Since the 

 publication of Ursell 's paper, a number of new investigations have been 

 reported which included those of Sibul (1955), Cox (1958), Fitzgerald (1963), 

 Schooley (1963) and Hanratty and coworkers (e.g., Cohen and Hanratty (1965)). 

 With the exception of Cohen and Hanratty 's work, the experiments performed 

 by these investigators were not designed specifically to verify recent 

 theoretical conclusions, or to serve as a starting point for developing 

 refined ideas about the nature of air- water interaction. With this background 

 in mind, a detailed experimental program has been initiated at NCAR and at 

 CSU to study the relationship between the turbulent flow of air and water 

 in a channel. 



Properties of the Fluid Motion 



When air moves at moderate velocities over water, a drift current develops, 

 and small waves are generated on the liquid surface. A schematic picture of 

 the development of combined air and water motion along with the growth of 

 waves in a channel is shown in Figure 1. The properties of fluid motion 

 examined in this study are indicated in this drawing. The coordinate system 

 is indicated so that x is the distance downstream, and z is the vertical 

 direction. The mean water surface is given by z = d while the surface dis- 

 placement from this level is denoted as C • The fetch F denotes the distance 

 from the leading edge of the water to a particular point somewhere downstream. 

 In terms of a two dimensional model, the velocity distribution in the water 

 is u(z), and the drift as the water surface is Uq. The air flow is given by 

 U(z'), where U^, denotes the air velocity at approximately 20 cm above the 

 mean water surface, and z' = (z - d). The wave length X and the phase 

 speed c denote properties of significant waves. 



For the purpose of this study, significant waves will refer to the 



