TM No. 377 



The ££u. spectra generally display more energy at frequencies above 1000 

 mcps (see 002 through 008). The spectra of ^ a and <^u, in serial 001 and 002 

 appear to demonstrate the selectivity of swell and sea. The energy of ^ w is 

 lumped in the range from 100 to 700 mcps, whereas the C^^ energy is highly 

 damped in this frequency range. This gap in the ^l energy could be caused 

 partially by the swinging of the suspended meter in response to the dominant 

 frequency of the waves. Such a resonant oscillation could highly damp the impeller 

 response to the u velocity fluctuation by tending to follow (give with) the back and 

 forth motions. However, it was suggested earlier in this chapter that the difference 

 in the variances of the horizontal and vertical motion components could be caused by 

 the directional characteristics of the 0MDUM systems. 



All spectra show strong attenuation of energy with depth, especially at fre- 

 quencies above 600 - 700 mcps. This is discussed later in this chapter. 



The auto- spectra of BBELS-8 and 9 (serial 020 through 025) were made using the 

 OMDUM III system, which was kept more stable in the water by a more elaborate guy 

 system (than in BBELS-5: see table V-l). These records show similarly shaped auto- 

 spectra that are unlike the previous (001-019) records. Also, the variances are 

 more nearly comparable in magnitude (see table V-l). The curves of $u. in BBELS 

 8 and 9 appear in profile very similar to ^^ , except for a constant flat (fre- 

 quency independent) filtering over the whole range. Note also that both ^»*. and 

 ^«j attenuate about the same with increasing frequency. It appears from BBELS-8 

 and 9 that the u and w meters were sensing and recording similar types of motions, 

 although the energy per unit bandwidth was consistently higher for C^^ than for 3^. 



The BBELS-12 observations (7 April 1965) permit a comparison of the spectra 

 associated with motions at the surface and near the bottom. Serial 071 and 072 

 were obtained with OMDUM III at 0.2 meter below the wave troughs. For 071 (1235- 

 1238) the wind was from the east at k.k m sec" 1 . For 072 (12U3-IUU8) the wind 

 was from the ESE at ^.k m sec -1 . During this period the waves were small, with 

 an estimated height of about 20 cm. The variances, corresponding to the low sea 

 state, are also small (see table IV- 3). The ^u. , as usual, falls much below 



Z&vu . The low values of w (0.1 cm sec" 1 ) for both observations indicate good 

 vertical positioning of the w meter. 



In the afternoon, two deep measurements were made using OMDUM III at 1.0 

 and 5.0 meters above the bottom. The spectra of these deep measurements 

 (P. 075 and O76 in appendix B) show no indication of the surface wave motions. 

 Wo spectra of the w component is shown, since no vertical motion was detected at 

 either level. This indicates that no motions attained the threshold velocity of 

 OMDUM III - about 7-9 cm sec -1 . 



The variance Ou. 2 * was 3.13 cm^ sec - 2 at 5.0 meters off the bottom and 8.U 

 cm sec~2 at 1.0 meter off the bottom, indicating a higher degree of turbulence 

 near the bottom. The value of u at 5.0 meters was +17.5 cm sec -1 ; u at 1.0 meter 

 was +12.5 cm sec -1 . One would expect that if turbulence were being generated 

 near the bottom, a mean vertical shear would exist in the horizontal flow. It 



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