TM No. 377 



made under virtually ideal conditions of rectilinear flow; whereas the field 

 measurements were made in actual turbulent ocean waves. The problem of proper 

 or realistic calibration of the wave meters is difficult. It is a manifest 

 impossibility to simulate true ocean wind waves in a laboratory tank. Further, 

 there is no "standard wave", and no one knows how to generate a progressive 

 wave of previously known form (see Kinsman, 1965)* This same problem is well= 

 known by workers making free surface elevation measurements with wave staff 

 devices — experimentally, a much simpler exercise than particle velocity 

 determination. 



The alternative to these laboratory calibrations is, of course, a field 

 test where the response of the wave meters could be directly compared with 

 some other instrument able to measure the same wave motions, preferably by an 

 independent method. But this is not possible, since one is dealing with a 

 unique measurement at the outset, and has to proceed with the handicap than no 

 comparable measurements are readily available for comparison. Because of this 

 unfortunate situation, one is forced to use his intuition as the main guide in 

 interpreting instrument bias. 



The most notable apparent biasing of the OMDUM III system was the damping 

 effect of the horizontal velocity amplitude response. An explanation.!. is 

 presented in chapter V for the ways in which this effect was caused. These 

 explanations, however, are less than completely satisfactory until a more 

 suitable calibration is performed. 



A rough comparison was obtained of the auto-spectra of particle velocity 

 motions and free surface fluctuations, and the spectra displayed strong 

 similarities. But again it is difficult to anticipate intuitively the physical 

 interrelationships of the two functions, other than those indicated by simple 

 small-amplitv.de wave theory. 



Suggested Instrumental Improvements =- More precise calibration is required 

 for a better understanding of the effects of the orthogonal meter arrangement 

 upon oscillatory flow. The U. S. Corps of Engineers has a large wave tank at 

 the Coastal Engineering Research Center in Washington, D„ C. which can generate 

 waves 1-2 meters high in a basin about h meters deep. The OMDUM III and improved 

 systems should be tested with various size waves and at various depths in this 

 tank. This would provide a better understanding of wave meter response to 

 oscillatory motions, even though real wind waves would not be simulated. 



Workers at the Chesapeake Bay Institute of Johns Hopkins University 

 (Pritchard, 196^) are developing an acoustic current meter, mentioned in 

 chapter I, which should be capable of measuring wave motions within a small 

 volume. At the first opportunity, a direct comparison should be made of the 

 two instrument records in real wave conditions. 



A more refined impeller system has been fabricated by the Braincon Corp., 

 Marion, Mass. The new system is similar in dimensions to OMDUM III, but with 

 a lighter six=bladed impeller mounted on all- jewel bearings, and much smaller 

 and lighter magnets and coils. The threshold velocity was found in recent tow 

 tests to be less than 2 cm sec" 1 . This would greatly improve the data record 

 and allow more resolution of smaller scale wave motions. 



163 



