TM No. 377 



where U is the amplitude Kg/l defined in appendix A, and -TL is the driving 

 frequency. The phase lag of the detector is related to the driving frequency 

 by the relation: 



TAtJ Acp = _T2 Tr ; 



where T c is the response time of the instrument defined in appendix A. 



Using the value for the phase lag given in table 11=3 > &4> - 90° = 

 l6. 2° + ( T* = 11.6°). Solving for the response time using equation (A-20): 



if? - j^ j (n-19) 



where T r is the period of the forced oscillation depicted in figure II-14. 

 For the mean value of -i^ (l6.£), TL = l6 milliseconds. For the upper value 

 of ^<f> (27. 8°), T = 65 milliseconds. For the lower value of ^4> (4,6°), Tn 

 is about 9 milliseconds. Thus, the estimate for T R is below 100 milliseconds, 

 which is desirable for a wave meter designed to properly detect wind wave 

 motion with a 3-5 second period. 



This manual oscillation experiment was, at best, a very crude attempt 

 to simulate oscillatory motion. Indeed, it is surprising how consistently 

 the period of oscillation was reproduced. The large scatter in the standard 

 deviation of the calculated phase angle lag is undoubtedly due to the inter- 

 polation error; i.e., the attempt to resolve small time lags with coursely 

 interpolated data. However, this crude experiment did provide a good esti- 

 mate of the response characteristics of the OMDUM II wave meter. 



The most significant result of this oscillation experiment is that the 

 instrument faithfully reproduced the quasi-simple harmonic motion with which 

 it was driven. For all its axial asymmetries, the instrument produced no 

 visible distortion of the motion while attaining a peak velocity of h-0 cm sec" 1 * 

 Moreover, there was no evidence that the wave meter grossly interfered with 

 its surrounding medium. As the instrument was oscillated in the tank, any 

 turbulence or vortex shedding produced by the interaction of the meter with 

 the fluid would most likely have intensified with time, since no advective 

 processes existed in the tank to carry away the perturbations,, However, no 

 visible time variation in the velocity function was evident, and the motion 

 appeared stationary. Any eddy disturbance caused by interaction of the 

 oscillating wave meter with the fluid was of too high a frequency or wave 

 number to interfere with the dominant motion sensed by the meter. 



Small-Scale Ducted Meters ( OMDUM III and LIMDUM i) 



The cylindrical ducted meters used in the OMDUM I and II systems were used 

 because of their availability. They proved useful as a prototype with which 

 to evaluate this new concept of wave measurement. It was fairly evident that, 

 although this system showed much potential, it could readily be improved upon. 

 Hence, it was decided to design a new system containing the main character- 

 istics of OMDUM I and II but with certain improvements. 



31A 



