TM No. 3ij-2 



A series of -wave motion observations were made 7 December I96U in order 

 to test a newly constructed orthogonally mounted ducted meter system 

 OMDUM III. This meter is a smaller version of the OMDUM I and II previously 

 described. The cylinders are about 8 cm in diameter and about ik cm in 

 length. In the OMDUM III system the drag on the meters, caused by the 

 oscillatory wave motions, caused the meters to move with the horizontal 

 motions, resulting in a damping of the amplitude of the U(t) record. The 

 variances of U(t) were usually from O.5 to 0.1 times the W(t) variances. 

 Drag is reduced since the new meter has a cross sectional area of about 

 half tlria-t of the OMDUM II. Also, more overall stability is provided by a 

 pyramidal suspension w-ith added vertical weight. 



The three series of observations were made from 1300-1^30 hours. The 

 sky was mostly clear and the air temperature 0.5°C. Sea state was 1-2, 

 with waves estimated at 3-^ m in wavelength and 20-ij-O cm wave heights. 

 From Wnr a slight swell was detected but was too ill-defined for estimates 

 of its parameters. 



The wind was light and relatively steady at 2.5 - ^+.5 m sec"-'- from the 

 ¥K¥. The U meter ws.s aimed at 285°-290° true; the wind tjave directions 

 seemed to vary from 280 - 295 • 



A summary of the statistical data, along with the meteorological con- 

 ditions, are presented in table 2. 



There was still a disproportion of the response of the U meter with 

 respect to the W meter, as shown by the variances of U and W at the various 

 depths. However, these first results were preliminary in nature, since the 

 OMDUM III system had not been calibrated for absolute speeds and the impeller 

 design had been somewhat modified. 



It is instructive, however, to examine the auto-spectra of the two 

 velocity components shown in figures 9 and 10. 



Ihe U spectra (designated by ^(f)) at the depths of 0.5^ 1.0, and 2.0 

 meters, in figure 9^ display the strong attenuation with depth at all fre- 

 quencies from to 2.5 cps. 



Two obvious peaks are displayed, the highest at about .100 to .200 cps 

 (T - 5-10 sec), and the smaller at .35 - 0^5 cps (T - 2.2 - 2.9 sec). 



The low frequency peak is caused by swell and ambient low frequency 

 oscillations from meandering motions of the tidal currents abo'at the tower. 

 There is a strong indication of low frequency oscillatory current motion 

 with periods ranging from 8 sec to 30 or kO sec . The large -^/alues of the 

 covariance function at zero lags in table 2 are caused prims,rily by these 

 lower frequency components and not by correlations of v/ind wave frequencies 

 of, say, 1-5 seconds. 



10 



