TM No. 377 



By examining the wave meter output in terms of the indicated velocity 

 with respect to the acceleration, one can determine any lag in the OMDUM II 

 response, using the accelerometer trace as a comparison,. This is valid 

 because: (l) the time response of the accelerometer with its high impedence 

 output is less than a millisecond and may thus be used as an absolute refer- 

 ence; and (2) the accelerometer trace, being the virtual derivative of the 

 velocity function, can represent the relative velocity of the oscillated 

 meter with a phase shift of 90° • Thus, the phase lag of the wave meter 

 trace can be determined with respect to the absolute velocity of the 

 system. 



The voltage pulse data in the upper trace of figure 11-13 was con- 

 verted to velocity (the method used is described in detail in chapter III). 

 For purposes of comparison, the data from both instruments were interpolated 

 at 0.1-second intervals and plotted on the same time base. Figure II-14 is 

 a segment of the processed record,, Clearly, the accelerometer record 

 (lower curve) appears to be approximately $0 out of phase with the OMDUM II 

 velocity record (upper curve). 



Tne interpolated time of zero crossings was used to indicate one mean 

 pnase difference of tne two traces. From tnese two traces, tne various 

 statistical parameters listed in table II-3 were derived. The mean phase 

 angle of the lag in the vertical velocity output with respect to the acceler- 

 ometer trace is about 106° + 12°. 



TABLE II - 3. Results of OMDUM II Oscillation Test 



OMDUM II Accelerometer 



Number of consecutive 

 oscillations 12 12 



Time interval (sec) 9.40 9.^0 



Mean period (sec) 0.773 0.776 



Standard deviation (sec) 0.029 0.017 



Velocity amplitude (cm sec"-'-) h0 



Mean phase angle A$> between OMDUM II and accelerometer outputs 

 determined from (a) zero crossings* A<P = 106.8 , with a stan- 

 dard deviation 0Z<t> ° f 12.1°; and (b) maximum and minimum values: 

 <^<J> = 106. 2°, with a standard deviation 02$ of 11. 6°. 



A discussion of the theoretical phase angle relationship of a simple har- 

 monic oscillator and its driving function is given in appendix A. If the 

 OMDUM II wave meter is driven by an oscillatory wave motion u(t), then 



UM - U c Cos-Qt; (11-18) 



31 



