TM No. 377 



Unfortunately, the lowest speed obtained for this end-on towing series 

 was 21.9 cm sec~l. When lower speeds were attempted,, an unevenness (jerking) 

 occurred in the towing. It was later found that, with a different boom 

 operator, lower speeds were obtainable, and these were used for the off-angle tests. 



The slope Kf"-'- is 0.215 cm~l compared to a slope of 0.233 cm~l for the 

 OMDUM I system calibration (figure II-5). This change in slope could be caused 

 by the increase in bearing load associated with the replacement of the iron slugs 

 by the relatively heavy alnico magnets. The relationship of the slope constant 

 to the physical dynamic characteristics of the impeller, however, is not 

 obvious. The slope difference is more probably due to the differences in 

 flow characteristics associated with the coupled versus the individual meter 

 configuration. The extrapolation of the line delineated by the data points 

 in figure II-9 to a zero value of f v appears to intersect the origin. 



There is indication of a higher scatter of points with the rotating boom 

 calibration than with the Marine Advisors tests. The scatter seems to increase 

 with towing speed. This is not surprising, since there were uncontrollable 

 ambient motions in the pond caused by slight wind waves and wake generation. 



The second calibration series was conducted on 15 June I96U, and involved 

 the study of system response to off-angle flow. The geometry of the coupled 

 meters and the orientation with respect to flow are shown in figure 11-10. 

 The angle & lies in the vertical XZ plane, and the angle lies in the 

 horizontal XY plane. Thus, represents an azimuthal angle, and © represents 

 an inclination to the horizontal. 



This second series of calibration was made to study the variation of 

 OMDUM II response as a function of the azimuthal angle B. For these tests 

 OMDUM II was suspended from the instrument boom as shown by the schematic 

 plan view in figure 11-11. For each test rotation OMDUM II was oriented 

 so that the axis of the u meter made an angle ±0 with the tangential 

 velocity vector. For each angle OMDUM II was driven at various speeds. 



Note that the negative values of are defined when the u meter is pivoted 

 outward from the center of the boom rotation, and positive values are defined 

 when it is pivoted inward. The instrument was towed at various steady speeds 

 at values of +20 ±1+0°, ±60° and ±80°. The results of the impeller response 

 at the various towing speeds and for the various values of are listed 

 in Appendix C . 



Analysis of the results of these off-angle flow tests may be aided by 

 first considering the following simple model, in which the results of the off -angle 

 response are compared with a theoretical cosine law defined by: 



u. (4) = v T cc5 4> j, (n-16) 



where u(0) is the velocity of the fluid sensed by the impeller. More speci- 

 fically, u(<3) is the speed obtained from the calibration curve for the parti- 



28 



