TM No. 377 



SUMMARY 



Little is known of how wind stress is imparted to the sea surface and 

 how this stress generates both waves and ocean currents . Most wave studies 

 have concentrated upon measuring free surface fluctuations and have avoided 

 the examination of the dynamic regime beneath the free surface of the waves. 

 The work reported herein deals with making direct Eulerian measurements of 

 the complex motions within the wave regime. 



A historical review and a discussion of the problems of measuring tur- 

 bulent and oscillatory motions are presented. The development of wave meter 

 instrumentation is discussed. The wave motion sensor consists of a cylindri- 

 cal housing containing an axially -mounted impeller. The instrument is sus- 

 pended beneath the free surface, and the component of flow parallel to the 

 cylinder axis imparts to the impeller an angular velocity proportional to 

 the flow speed. The impeller rotation is detected by the interaction of min- 

 iature magnets (mounted at the blade tips) with a small pickup coil on the 

 cylinder. The ducted meter systems consist of a pair of cylinders mounted: 



(1) adjacent and orthogonal to detect the horizontal (u) and vertical (w) 

 flow components simultaneously at a single depth; or (2) spatially separated 

 on a vertical rod whereby both meters are aimed either horizontally or ver- 

 tically to detect simultaneous velocity pairs of u or w. 



Various calibration techniques were used, including a wind tunnel, and 

 rotating boom and towing tank systems. Studies of both accelerative and 

 "off axis" response to flow were made, and an oscillation test simulated 

 wave motion effects upon the ducted meters. It was found that the ortho- 

 gonally mounted ducted meters registered steady flow in the plane defined 

 by the two orthogonal u and w meter axes. The flow components were found 

 to have a positive error, which was a function of the magnitude of the "off 

 angle" but independent of absolute flow speed. A means of correcting the 

 error in u and w is demonstrated. 



The methods of data processing, using high speed digital computers, 

 are presented. Details of the analysis of time series data of wave motions, 

 using the Tukey spectrum techniques, are provided. Wave measurements were 

 made both in Narragansett Bay and on the Buzzards Bay Entrance Light Station. 

 A discussion is presented of the Light Station facilities and of environ- 

 mental measurements made of the current and tide conditions. 



The wave measurements provided important information about the res- 

 ponse of the ducted meters to actual wave motions. In general, it was 

 found that the orthogonally mounted system displayed quite realistically 

 the u and w wave components, but that the u motion at times was somewhat 

 damped. Causes for this effect are associated with: (l) fundamental proper- 

 ties of trains of wind waves and swells moving in different directions; and 



(2) the horizontal wave motions moving the suspended wave meter so as to 

 dampen its response to the u oscillations. 



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