TM Woo 377 

 CHAPTER II 



DEVELOPMENT OF THE WAVE METER 



Any measurement of the motions in ocean waves must take into account the 

 fact that this dynamic environment contains a broad and time -variable spec- 

 trum of complex motions „ This gives rise at once to the unavoidable problems 

 of making fluid flow measurements in a turbulent or quasi -turbulent field 

 of motion o 



In this study "turbulence" is associated with particule motions in ocean 

 waves o The term is defined at the outset because some oscillations of an 

 approximate periodic character-, such as those associated with surface gravity 

 waves , are usually excluded from discussions of oceanic turbulence (see 

 Bowden, 1962-A). This exclusion is justified by Bowden on the basis that 

 wave motions are more predictable than the motions of a highly turbulent 

 regime, such as that in the surf zone of a beach or at the base of a water- 

 fall o This distinction, however, is quite arbitrary „ Since the ocean is 

 not in viscid and is acted upon by rotational stresses, its motions must con- 

 tain major random and non-linear components „ Thus, the particle motions in 

 waves may have strong turbulent, but anisotropid, characteristics. These 

 characteristics are the main point of interest. 



The general principles of measuring turbulent flows are discussed by 

 Hlnze (l959)o There are several general requirements that should be 

 satisfied by any Eulerian flow-detecting apparatus before turbulent motions 

 can be delineated with any degree of reliability. 



1= The sensing elements introduced into the water flow field must cause 

 a minimum of disturbance to the flow pattern. 



2c The instantaneous velocity distribution must be uniform in the 

 region occupied by the element „ la other words, the sensor should be 

 dimensionally much smaller than the scale of turbulence under examination. 



3° The inertia of the sensor response (i.e., the response time) must be 

 much smaller than the period of the turbulent fluctuations . 



k The sensor should respond to fluctuations that may be small compared 

 to the mean motion. 



5o The instrument must have a stable calibration for periods much longer 

 than the sampling periods. 



6. The device must be strong and rigid or otherwise able to absorb 

 oscillations of a periodic motion impressed by the turbulent flow. This 

 requirement may be particularly critical where the sensor support has a 

 tendency to oscillate with the .impressed drag of the fluid. 



7o Most important of all is a clear understanding of the principle of 

 measurement so that the limitations of the instrument system, which is 

 necessarily imperfect, can be assessed, 



Ik 



