TM No. 377 



In the OMDUM I system the threshold velocity is about 2-3 cm sec 

 (figure II-5) and is determined primarily by the friction bearings. In 

 OMDUM II the threshold velocity is more dependent upon the pole strength 

 of the magnets in the impellers than on the bearing friction. The 

 attraction between the magnets and the iron core of the induction coil is 

 such that, when the impeller is spun, a magnet always comes to rest directly 

 opposite the coil core. (This did not occur with the thin slugs in OMDUM I.) 

 It is this force of attraction that must be overcome before the impeller can 

 commence spinning. 



The ability of the modified system ( OMDUM II) to measure wave motions 

 was assessed by evaluating three distinct dynamic characteristics of the 

 system: (l) the calibration of the impeller response as a function of the 

 speed of a steady rectilinear flow parallel to the cylinder axis (referred 

 to as "end-on flow"); (2) impeller response to flow at a given angle to the 

 cylinder axis (referred to as "off -angle flow"); and (3) the time frequency 

 response of one impeller system "cc fluctuations of flow. 



A calibration of the OMDUM II system to determine iuS response to steady 

 flow in both the end-on and off -angle positions was performed during June, 

 196k, using a rotating boom towing system maintained by the Department of 

 Civil Engineering of the Worcester Polytechnical Institute. The boom system, 

 shown in the figure 11=8, is located in a shallow pond (1.5=2.0 meters deep) 

 at Holden, Mass., near the institute. 



This relatively simple boom system provides an in-water towing facility 

 without the great expense of an elaborate towing tank. Perhaps more signi- 

 ficant is the ability of the system to tow at the desired speed for an un- 

 restricted length of time or distance. The double-ended boom (2k meters 

 long) rotates about a vertical axis shaft. The shaft is supported by 

 bearings set in a concrete platform in the middle of the pond. The boom 

 system is driven by a 117-volt DC motor with a variable speed control. The 

 object to be towed is fixed to one end of the boom and rotated at the proper 

 angular velocity J2^ to give the desired towing (tangential) velocity Vrp. 



The boom structure has two collinear arms that are symmetrical about the 

 vertical rotating axis. One arm is equipped with electrical leads and is 

 used as the towing end whereas the other arm serves as a counterbalance to 

 facilitate smooth rotation. The boom operator is stationed at the center of 

 the boom at the speed controls. The ragged construction permits an additional 

 man to position him self at the instrument (towing) end for observations 

 during rotation. Power is provided to the drive motor and the recording 

 equipment through a slip ring commutator mounted on the boom axis. 



The OMDUM II system was attached rigidly by a vertical shaft which 

 extended into the water at the towing end of the boom. For the various angu- 

 lar velocities of the boom the tangential velocity V m (or quasi-rectilinear 

 velocity) is calculated from the relation 



(11-14) 



26 



