TM No. 377 



Figure V-31B displays the build-up of the horizontal energy spectra at 

 similar frequency bands. The change in secondary peak frequency is identical 

 with that observed in the vertical spectra. The spectral density is 1.6 cm^ 

 sec"-'- at 650 mcps (1.7 sec) for 0.5 m I, and attains a level of 3.5 cm^ sec - l at 

 500 mcps (2.0 sec) for 0.5 m II - only a two-fold increase. 



Thus, there is evidence of a build-up of the wind- wave spectrum bands 

 produced by the increasing winds. However, the low frequency swell (at 100 mcps) 

 shows essentially no change between the 0.5 m I and 0,5 m II observations. 



There are several small high frequency peaks above 1000 mcps (l.O sec). The 

 most notable peak occurs in the pairs of vertical velocity curves (figure V-31A.) 

 at about 1200 mcps (O.83 sec). This peak shows no change of position in the two 

 curves. This may indicate a resonant fluctuation in the flow through the meter, 

 which could be somewhat independent of the wave conditions. 



The BBELS-9 observations thus show direct evidence of energy being added by 

 the wind to the motions of the sea surface. This energy input is very frequency 

 dependent. The wind- imparted wave energy also tends to move to a lower frequency 

 range with the passage of time. 



The relationships of the variances and spectra of the three observations 

 (023, 02U, and 025) further demonstrate the ability of the 0MDUM III system to 

 respond to the two-dimensional wave motion components in the direction in which 

 the system is aimed. The u meter was pointed directly into the oncoming swell 

 from the south (whereas the wind waves were from the SE). As a result, the 

 curves for ^^ and ^^ tend to be very similar for the low frequency range from 

 to 250 mcps. The greatest differences between the u and w spectra occur in 

 the frequency range around 500 mcps (2 sec). This may very well be due to the 

 u meter being directed some U5° away from the path of the high frequency wind 

 waves, whereas the w meter would respond fully to the w component associated 

 with the wind waves. Moreover, the 0MDUM III tends to respond fully to both 

 the u and w components associated with low frequency swell. (Note that this is 

 similar to the effects discussed earlier in this chapter.) 



Wave Energy During Changing Wind Conditions - The longest and one of the 

 more interesting series of observations was BBELS-11, made on 29 and 30 March 

 1965. Wave observations were made over a 2U-hour period in order to observe 

 changes in wave motion statistics as the wind (and hence, the wave conditions) 

 changed. It was intended to use the LIMDUM I system arranged to measure w at 

 two depths with a 1.5 meter vertical separation. As explained earlier, a pen of 

 the two-channel recorder burned out shortly after measurements began; and, since 

 no spares were available, the work was continued using one recorder channel. 

 The LIMDUM I was placed at a certain depth, and each meter was recorded in 

 succession on the single usable channel. 



A total of 39 observations were made at various depths from 1622 hours on 

 29 March through 1251 hours on 30 March' (see table IV- 3). The statistical 

 analysis listings and the plotted auto-spectra for each record of w are presented 

 in appendix B (serial O3O-O69). 



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