TM No. 377 



The second category of motion includes the surface gravity waves , which 

 are dominated by the relatively low frequency "swell", and the higher fre- 

 quency wind-driven waves termed "sea". These motions are characterized by 

 oscillatory flows, which are ay no means irrotationalo The flows do,, however, 

 tend to possess strongly two-dimensional geometryj i.e., they oscillate in 

 complex orbits roughly approximating circles, ellipses,, or cycloids that 

 generally lie in the vertical plane, normal to the crestline of the waves. 

 During periods of wind generation, many sets of waves of varying scales and 

 varying directions are constantly being formed and propagated in the hori= 

 zontal. All of these orbital motions peculiar to a particular wave frequency 

 or scale interact (probably in a highly non-linear fashion) with every one of 

 the other waves that happen to cross paths at a particular point. The re- 

 sults j of course, must be a highly stochastic or turbulent situation in which 

 any physical processes taking place can be analyzed only in a statistical 

 manners 



The third category of motion is that of the relatively small scale 

 structure characterized by a dimension range from 0.1 to 20 cm This sub- 

 range includes small surface gravity and capillary waves, which tend to be 

 quite regular in their structure; but, more important, these motions con- 

 stitute the true turbulent regime or "fine structure' 1 ' of the sea. This is 

 seen as the swirling: motions in breaking seas,, both in the open ocean and 

 along the ocean boundaries, These small scale motions inevitably play a 

 large part in the final disposition of wind imparted momentum and energy 

 because they possess the highest degree of quasi-random motion and so-called 

 ''eddy viscosity 1 ' 1 . 



This study concentrates upon motions attributed to swell and to wind 

 waves, for it is within this regime of motions that the sea is most 

 responsive. The first observed response to the wind upon the ocean surface 

 is the immediate generation of wind waves. Secondary effects, in terms of 

 reaction time 5 are the evolution of breaking waves (wave turbulence) and, 

 even later, the generation of gross "wind drift" currents. 



The surface motions should not be divided into strict categories, for 

 it is, after all, the interactions within the continuous spectrum of motions 

 that produce the sum of the dynamic energy transfer within the sea and 

 through its boundaries. Hence, the basic problem is that of measuring the 

 previously defined ranges of motions so as to better understand their inter= 

 actions and the energy transfer from one region of the spectrum to another. 



g§^ag§ters_j?o Be Measured — This then is the general problem. Is it 

 feasible to devise the instrumentation needed to make reasonably accurate 

 measurements of particle motions in the dynamic regime of wind waves and 

 swell? If so, can these measurements be used to provide a better under- 

 standing of the dynamics of wave generation, energy transfer, and dissipation? 

 What specific parameters should be extracted from such velocity data so as 

 to provide the most useful statistical information? In what ways can these 

 statistics be compared to the available data on the free surface observed 

 as a function of time? 



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