TM No. 377 



The experiment was made during BBELS-16 (7 September 1965). The u meter was 

 aimed at different angles of attack (©) from the on-coming wind waves (see figure 

 V-6A). With the meter placed at a depth of 0.5 meter, simultaneous records of u 

 and w were made at about 0°, 20°, 50°, and 80° from the observed wind wave 

 direction. The data (serial 120 - 123) are listed in appendix B. 



During the period of measurements (13^-0 through IU36 hours), the winds were 

 light, ranging from 3-5 111 sec - - 1 - to 4.6 m sec - -, and varied in direction from W to 

 WSW. Unfortunately, the low wind speed and the variability did not provide a very 

 consistent (i.e., stationary) wind wave field. In general, over the 56-minute 

 period of measurements, the wind waves were quite small and short-crested, having 

 a maximum height of from 25 - 30 cm and a wave length of 3 - 5 meters. 



Prior to the observations (at 1000 hours), a gentle swell was observed 

 radiating from the SSE (155° T). At lUlO the presence of swell was indeterminate. 

 After the period of observations (at 1900 hours), a slight swell was again observed 

 from the south, having a height of about 20 cm and a wave length from 20 - Uo meters. 



The auto-spectra for four of the records (serial 120 - 123) are shown in 

 figure V-17. The $u, and <5u> are depicted as broken and solid lines, respec- 

 tively. The angle 6, the time interval of measurement, the mean u component 

 (cm sec - l), wind speed V (m sec~l), and the variances O^L x and U^t 1 are shown 

 above the spectra curves. 



The w spectra q^^ exhibit a dominant low frequency peak (a) at about 100 

 mcps (6.6 sec), and a lesser peak at 400 mcps (2.5 sec). The heights of the two 

 peaks (A and B) remain at about the same value throughout the series; except in 

 serial 121, where a sharp rise is noted in peak B. (This is also reflected in the 

 ^u. spectrum. ) 



The u spectra (^^ likewise display dominant peaks at 100 mcps (A 1 ) and U00 

 mcps (B f ); however, the relative magnitudes of A' and B s are sharply contrasted 

 with A and B. In serial 120 (with ©»O e ) A' is smaller in magnitude than B'. 

 Serial 121 shews relative amplification of both B and B', probably caused by a 

 local change in the wind wave conditions (i.e., a non- stationary effect). Serial 

 122 and 123, however, display a continuous decrease in B ! , whereas A' tends to 

 increase. In serial 123 ( Qz'QO*) the ^u. curve is a crude mirror image of the 



<£u, curve in serial 120 ( &s.O° ). The low frequency peaks A and A' are attrib- 

 uted chiefly to a gentle swell. Although not always visible (the visual obser- 

 vation at lUlO hours indicated no swell), this swell was apparently present during 

 the entire sampling period. The B and B' peaks in the curves are probably 

 associated with the observed small (and rather confused and non- stationary) wind 

 waves. The spectral densities and variances OtZ 1 and QJJJ 1 at the 0.5 meter depth 

 are quite low, as would be expected with the low wind speeds (e.g., compare these 

 curves with the 0.5 meter depth spectra of serial 023 U and 024 in appendix B). 



Assume then the presence of a small swell running from about 175°T and of 

 small wind waves normal to this direction (i.e., at about 250°t). With the u meter 



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