TO No. 377 



Since the impellers had to be disassembled several times for circuit 

 maintenance, a later calibration check was performed in the NUWS 

 hydrodynamics laboratory wind tunnel. The in-water calibration data were 

 reproduced well within the limits of experimental error. It was found that 

 as long as the needle bearing mounting was left with at least 1 mm of axial 

 clearance j, the calibration was essentially unaffected by disassembling the 

 impellers. 



The time response of the ducted meter was roughly evaluated by Marine 

 Advisors by means of an acceleration towing test. The results of this 

 test are shown in figure 11=6. The speed of the tow carriage of the tank 

 was monitored as the ducted meter was accelerated and decelerated. The 

 pulse frequency output of the meter was converted to speed, and a comparison 

 of the monitored carriage speed and the speed registered by the ducted 

 meter is depicted in the figure. In general, the indicated speed of the 

 carriage lags behind the response of the ducted meter, both during acceleration 

 (from 0.0 to 1.6 seconds) and deceleration (from 11.0 to 11.2 seconds). It 

 is inferred from these results that the response time is somewhat less than 

 0.1 second (frequency response is above 10 ops). 



It is not possible to derive the exact response time from these data 

 because the frequency response of the meter is apparently faster than the 

 monitoring device. Hence, no time lag is obtained; i.e., the meter response 

 is in equilibrium with the impressed variations in fluid flow. 



A cursory examination was made of impeller response by simulating sudden 

 impulses of flow while the meter was suspended from a wire off a pier. The 

 almost instantaneous coupling with the water when the immersed device was 

 rapidly raised and lowered indicated an extremely fast response time. 



The impeller blades are designed to completely fill the cross-sectional 

 area of the cylinders, thus providing a maximum torque-to-f low ratio. Also, 

 loading on the needle bearings is minimized by buoyancy effects, since the 

 overall density of the micarta impellers and the Teflon shaft is near that 

 of sea water. 



A single series of preliminary wave measurements was made in Narragansett 

 Bay using the OMDUM I system. Upon completion of these measurements the 

 electronics failed, and the system was modified (OMDUM II) before any further 

 calibration was made. These preliminary measurements cast some doubt upon 

 the usefulness of the OMDUM I system for measurement of particle velocity 

 motions in the XZ plane, and on system response to the non-steady motions 

 inherent in the wave regime, particularly when the flow is at off angles to 

 the axis of the impellers. An estimate of system response (for OMDUM I and 

 II) was obtained later and is presented under the next heading. 



The experimentation with OMDUM I, however crude, served as a guide to 

 the modifications that resulted in OMDUM II. OMDUM II was used to examine 

 more thoroughly the laboratory calibration and the ability of the instrument 

 to measure wave motions. 



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