TM NOo 377 



BBELS 5 and 7, in chapter IV) were made using OMDUM II in the configuration 

 shown in figure 11-10. The purpose of the experiments was to evaluate the 

 instrument response to motions occurring in the XZ plane or in the plane of 

 the angle 0„ Results were very encouraging, in that they seemed to produce 

 a very realistic record of the wave oscillations. 



However, the question of instrument response to variations of motion in 

 the XZ plane had not been probed sufficiently,, Data obtained from these 

 preliminary experiments were therefore considered to be of questionable 

 validity until further calibration experiments allowed proper interpretation. 



What is the deviation from the cosine law for motion in XZ plane? Figure 

 11-10 reveals a symmetry in the XZ plane j i.e., the vector rotating in the 

 XZ plane experiences the same cross sectional area for a particular value 

 of +©. Thus, R(O) and R(-©) should be identical and close enough to 1.0 so 

 that the OMDUM II system can approximate the oscillatory motions of the waves. 



The actual distortion of the wave motions in the XZ plane was further ex- 

 amined during the calibration of the OMDUM III system. Fortunately,, the data 

 obtained in BBELS 5 and 7 proved to be valuable for the overall studies „ 



The modification of the ducted meters, involving replacement of the 

 iron slugs with heavier magnets, required that the response time of the 

 new impeller configuration should be examined. Despite the modifications, 

 it was still hoped that the response time of the impellers would be less 

 than 0.1 second (or have an equivalent frequency response of more than 

 10 cps). 



A simple experiment was performed by oscillating the OMDUM II system 

 vertically in an acoustic test tank at NUWS. Specifically, the orthogonally 

 coupled meter system was suspended with a heavy shock cord to a depth of 

 about 30 em in the center of the round acoustical test basin, which was 

 about h meters deep. A waterproof accelerometer was fixed to the side of 

 the vertical cylinder (w meter). The accelerometer (Model k^h-9 ASA 

 a-20-2700, manufactured by Statham Laboratories, Los Angeles, California) 

 was an internal strain gauge type, utilising a balanced bridge system with 

 a 3^ volt DC input and a 0=3 volt DC output for moderate acceleration ranges «, 



The OMDUM II system (weighing about 3 kgm) was oscillated vertically, 

 producing a quasi-simple harmonic motion. The amplitude and period of the 

 oscillation were about 8 cm and 0.7 second, respectively. 



The w channel output was recorded simultaneously with the accelerometer 

 voltage. A sample of the two-channel record is shown in figure II-13. The 

 accelerometer signal produced a well-defined sinusoidal trace. There was 

 a slight DC drift and some evidence of 6o~cycle pickup, but this did not 

 affect the gross signal. The OMDUM II output voltage pulses (upper trace) 

 oscillated at a frequency similar to that of the accelerometer. 



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