TM Wo. 377 



Off ~angle calibrations demonstrated a functional variation of instrument 

 response with angle of attack of steady flow., In the orthogonal meters , a 

 method was worked out (using the relative responses of the two meters) to 

 correct for off -angle response aberrations in the measurement of the true flow 

 vector lying in the plane formed by the two orthogonal meter axes. 



The response time of the ducted meter (used in OMDUM III and LIMDUM i) was 

 found experimentally to be about 60-70 milliseconds „ Further tests showed that 

 the OMDUM II system (slightly larger impeller) clearly resolved a simple harmonic 

 vertical oscillation having a 0.7-second period. Hence, the system can faith- 

 fully respond to frequencies well above the 0.5 to 0.2 cps associated with wind 

 waves . Moreover, the spectra of motions were analyzed from d.c. to 2.5 cps. 

 Aside from the low moment of inertia of the impellers, the fast response is due 

 to the geometric character of the enshrouded impeller. The flow which cannot 

 diverge around the impeller, imparts dynamic pressure on the blades very 

 efficiently and quasi~instantaneously. 



The ducted meters were used in two ways; (l) mounted side-by-side and 

 orthogonally to measure both the horizontal and the vertical velocity component 

 simultaneously, as with OMDUM IIIj and (2) mounted rigidly and linearly on a 

 vertical rod to measure either the horizontal or the vertical velocity component 

 simultaneously at two different depths up to three meters apart, as with LIMDUM I. 

 The couplings were simple, and the change could be made from one mode to another 

 in several minutes. 



The OMDUM I system was mounted on a rigid vertical pipe in Warragansett Bay. 

 Both OMDUM III and LIMDUM I were suspended by a pyramidal and counterweighted 

 system of guys from the BBELS. This guy system provided minimum horizontal swing 

 and complete vertical, damping. The guy device could be quickly lowered or 

 raised, permitting rapid observations at different depths. 



For gross measurements of velocity and auto~spectra, the method of suspension 

 did not seem critical. However, the measurement of the co-variance properties 

 of the motion is dependent heavily upon stable mounting of sensors. 



Shortcom ings of In strumentation -■= The raw wave meter data produced on two- 

 channel strip charts required laborious editing and hand-reading for conversion 

 to the computer format on punched cards e The preliminary processing was expensive, 

 time consuming, and liable to human error. More advanced procedures are desirable. 



The threshold velocity of the impellers (used in OMDUM III and LIMDUM i) was 

 found to be about ]' a sec*-*-. Thus, oscillatory wave motion is not detected 

 between + 7 cm see « This presents, by virtue of the interpolation procedures, 

 some oias in the record and its associated statistics, particularly when the wave 

 motions are of small amplitude. 



The wave meters, as with, all current meters, have inherent imperfections in 

 their ability to measure fluid flow. They impress their own signature into the 

 wave data. Calibration normally allows one to assess the instrument bias in order 

 to eliminate it from the instrument record. Unfortunately, all calibrations were 



162 



