TM No„ 377 



The suspended wave meter may be considered as a simple pendulum having a 

 period represented by: 



' a 



(v- 5 U) 



where T^is the period of swing, L is the length from the point of pivot, and 

 g is the acceleration of gravity. For the system mounted on BBELS, the value 

 of L ranges from 19 meters, when the instrument is just beneath the water surface, 

 to 2k meters when the instrument is 5 meters down. This gives T]_q = 8.75 

 seconds, and T2U = 9«83 seconds. These periods are about twice the period of the 

 wind waves usually observed at BBELS. Theoretically, this would tend to dampen 

 the reaction of the suspended meter to the U-5 second wave motions. These periods 

 are, of course, for a simple pendulum suspended at one point. For the OMDUM 

 observations the wave meter was suspended by no less than four guys. For the 

 later observations (BBELS-lU, 15, and 16) a vertical anchored guy was also used 

 (see table V-l). The use of multiple guys should provide strong, if not critical, 

 damping to the system. It should also tend to increase the natural period of 

 swing of the system. 



The manner by which the OMDUM or LIMDUM systems are suspended should, ideally, 

 provide w = 0. The records (see table IV-3) show that w generally lies within 

 ±2-3 cm sec-1, but occasional values occur as high as ±5-8 cm sec-1. The most 

 obvious explanation for a non-zero mean (of w) is that the axis of the vertical 

 sensor may not be vertical due to deflection of the meters and suspension by the 

 drag force of the current. If this is the case, then strong horizontal currents 

 should correlate with a large negative value of w. This is because z is positive 

 upward, and the meter tilting away from the current would produce UJ K. O (mean 

 flow downward). For the BBELS-11 observations, the LIMDUM I system was used to 

 detect w. The instrument was suspended from the single main guy. Under these 

 conditions, the tidal current should have tended to tilt LIMDUM away from the 

 horizontal mean flow, and thus provide a mean value w less than zero. Of the 

 thirty-six values of w from BBELS-11, 25 show w < and 11 show w > 0, indicat- 

 ing that some mean tilt occurred in response to the current. 



The OMDUM III system was, of course, supported by an array of cable guys, 

 giving a certain amount of horizontal stability. It was of interest to see if 

 large absolute values of u caused tilting of the system sufficient to produce a 

 detectable increase in values of w. A correlogram was plotted of the values of 

 u versus w obtained from the tables in appendix B, but there was no significant 

 relationship. Both large and small values of w corresponded to large values of 

 u. Also, out of 36 records examined, in 17 cases w ^ 0, and in 19 cases w y 0. 

 This indicates no significant trend toward preferential tilting by the mean 

 current component u. Further, it indicates that the pyramidal guy system, when 

 carefully adjusted, stabilized OMDUM from mean horizontal drag forces. One may 

 therefore conclude that, in general, the values of w obtained were only slightly 

 associated with the guy system maintaining the meter at a slight tilt. 



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