IM No. 3lj-2 



Thus, If the momentum transfer through the upper layers is being considered, 

 it becomes apparent that one should also examine the effects of the more 

 regular, quasi-oscillatory particle motions of the ■waves, since the waves 

 themselves are indeed a manifestation of wind stress. 



The question then arises as to what sort of orbital configurations 

 could provide a simple mechanism for the downward transfer of wind- imparted 

 horizontal momentum. One such geometry would be that of a properly tilted 

 ellipse. A particle might then acquire horizontal momentum from the wind 

 stress at the top of its orbit and transfer the momentum downward as it 

 moves in its trajectory. Stara— has also shown the probability of some such 

 mechanism on other grounds. The mechanism described in this report should 

 be capable of representation as a Reynolds stress in terms of Eulerian hydro- 

 dynamic variables. It is of interest then to consider the problem of measuring 

 these effects. 



MRMGANSETT MY MEASUREMENTS 



The following experiments were reported by the writer and can be 

 summarized as follows : 



D'uring August 1963,ineas'urements of the velocity components beneath ocean 

 waves were made utilizing two adjacent ducted current meter's mounted ortho- 

 gonally as to sense the particle velocity components in the plane normal to 

 the ■^'jave crests. The meter system is shown in figure 1. This system of 

 orthogonally mounted ducted meters is tenned OMDUM I. The cylinders are 

 about 10 cm in diameter and 20 cm long. The OMDUM I was fixed to the end 

 of a vertical steel beam supported rigidly at the end of a naval pier in 

 Harragansett Bay, R. I. Impellers, mounted within the cylinders on jewelled 

 bearings, were neutrally buoyant. The tips of the impellers contained small 

 iron slugs tAiich, upon rotation, perturbed magnetic fields of small induction 

 coils incorporated in an oscillator circuit imit attached to the sides of 

 the cylinders. The output from the amplitude modulated system reflected the 

 pulses as the impellers rotated in the fluid flow. Because of an assymetry 

 of the ms,gnetic fields of the induction coils, the sense of the rotation of 

 the impellers, i.e., the direction of flow, was presented as a unique signature 

 in the output. 



A two-channel strip cha,rt recorder was used to register the rate of flow 

 through each of the impellers. Tne ds,ta were hand-converted into a contin- 

 uous plot of the fluctuating velocities, U* and ¥*, as a function of time. 

 From this time series the sign and magnitude of the U* and ¥' were selected at 

 0.3"second inter^/als and placed on punched cards for computer processing. 



The meters were immersed about 15 cm below the trough level of the waves, 

 id-th the horizontal flow-sensing meter aimed normal to the crest line. 

 Generated over axi up-wind fetch of about 5 Km, the waves displayed clearly 



