TM No, 377 



Design Characteristics — The new system was designed to reduce the mean 

 cross-sectional area and volume and to streamline the overall geometry. The 

 aim was to decrease the form drag of the system and thereby reduce any ten- 

 dency to interfere with the flow, both around and through the wave meter. 

 Reduction in size would also enable the new wave meter to spatially resolve 

 smaller scale orbital motions than those resolved with OMDUM I and II. More- 

 over, reduction in form drag would reduce the wave forces on the instrument 

 when it was not rigidly suspended (as in measurements from Buzzards Bay 

 Entrance Light Station), thus decreasing its tendency to swing. 



An effort was made to further inhibit horizontal reaction to the wave 

 motions by using a more efficient wire suspension system. The possibility 

 of using the individual ducted cylinders to measure parallel velocity com- 

 ponents in waves at two (or more) depths simultaneously was also examined. 

 This would require a simple method of altering the geometric placement of 

 the two meters with respect to one another. 



Two new ducted meters were designed and fabricated to meet these goals. 

 Figures 11-15 and II-l6 show the meters mounted in the orthogonal position 

 ( OMDUM III). The cylinders are constructed from bronze tubing 15.3 cm long, 

 with an 8.8 cm outside diameter and a 0.2 cm wall thickness. Instead of 

 the bulky pipe support and U-bolts used in the previous devices, the cylin- 

 ders are held in the orthogonal configuration by a set of l/2-inch bolts 

 attached to a piece of flat brass stock 6 cm long, 10 cm wide, and about 

 O.lj- cm thick. This flat plate is brazed tangent to each cylinder on the 

 side opposite the pillboxes (see figure II-l6). The bolts hold the two 

 plates to a center plate which is, in turn, brazed at each end to a brass 

 rod 2 cm in diameter. This rod is about 130 cm long and, as shown in 

 figure 11-15, has a large pad eye brazed onto each end. 



The cylindrical housing, support braces, and shaft were fabricated at 

 NUWS. The impellers and bearing mounts were built and installed by the 

 Braincon Corp., Marion, Mass. The impellers are constructed of micarta, 

 similar to the OMDUM I and II systems; however, they contain six blades. 

 As in OMDUM II, small alnico magnets are potted into the blade tips. The 

 pillboxes housing the induction coils are about 3.0 cm high and 3° 5 cm in 

 diameter. The miniature coils (similar to those used for OMDUM II) are 

 oriented within the pillboxes so as to provide a signed voltage pulse 

 unique to the sense of impeller rotation. The inductance of the coil is 

 high enough to produce a pulse of about ^5 millivolts when the impeller 

 is spun at 10 cps (equivalent to 180 cm sec for water flow). 



Within the open-top pillboxes each output lead of the inductance coil 

 is soldered to one end of a 30 cm long, single-pin, watertight connector 

 (manufactured by Mecca Cable and Service, Inc., Houston, Texas). The 

 solder joints are coiled in the pillbox, which is filled with expoxy resin 

 (no. 5, manufactured by Minnesota Mining and Manufacturing Corp,, Needham, 

 Mass.)* When potted, the resin forms a very strong and rigid watertight 

 seal. The male ends of the connectors are led around the cylinder (figure 

 II-16) and taped tightly to the support shaft. 



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