LABORATORY INVESTIGATIONS ON 

 AIR-^SEA INTERACTIONS 



E. Y. Hsu and H. Y. Yu 



Stanford University 

 Stanford^ California 



I. INTRODUCTION 



Since the comprehensive review on wind wave generation by 

 Ursell [1956] , there have been renewed, intensive studies, theoreti- 

 cal as well as experimental, on the subject. Although significant 

 contributions have been made by many investigators, the final goal 

 of achieving a basic understanding of the fundamental mechanism of 

 energy transfer between a turbulent air stream and the ocean has not 

 been realized. A unified, comprehensive theory of wind wave gener- 

 ation must provide adequate explanation of the energy transfer 

 between the two media at all stages of wave growth from capillary 

 waves to sea swell. In the absence of such a unified theory, a 

 convenient classification of various flow regimes in wind- wave 

 generation may be made by use of the ratio of water wave celerity 

 C and the air shear velocity u* at the interface. When C « u , 

 the dominant mechanism of energy transfer between air and water 

 is the "viscous mechanism," characterized by the critical layer 



cai laye 

 [1962]. 



being within the laminar sublayer and treated by Miles 

 When C ~ 10 u , the critical layer is outside the laminar sublayer 

 and the dominant mechanism of energy transfer is the "inviscid 

 mechanism" (Miles [1957, 1967] and Benjamin [ 1959] ) with transfer 

 arising from the normal pressure acting on the interface and the neces 

 sary phase angle between the pressure distribution and the progres- 

 sive wave. As pointed out by Longuet-Higgins [ 1969] , neither of the 

 above theories accounts for two well-established features of wave 

 generation: (1) the existence of some wave energy in a frequency 

 range corresponding to waves traveling faster than the mean free- 

 stream velocity and (2) the damping of a swell by an adverse wind. 



The experimental investigations of Sutherland [ 1967] , Hires 

 [ 1968], and Chang [ 1968] and many others are limited to the viscous 

 range. Because of the high Reynolds number in a typical wind blowing 

 over the ocean surface, the viscous mechanism can be safely neg- 

 lected as irrelevant to full-scale wave energy transfer. Hence, 

 Miles' inviscid model has received most attention and been widely 

 employed in comparisons with experimental data obtained in full 

 scale (ocean) and laboratory simulations. 



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