2. Fixed-Point Measurements . 



a. Rationale and Needs . At times it is desirable to obtain a time- 

 series record of flow at a given point to (a) assess the contribution of 

 slowly varying ocean parameters, such as connected to tides and seiches, 

 and (b) record the wave and the directional current spectrum with changing 

 barometric pressure, wind velocity, and direction at a particular site 



of interest. In this case deployment of current meters in the vertical 

 is preferred to allow measurement of the propagation of long waves through 

 time and to determine attenuation with depth. 



b. Sensor Configurations . The manner in which sensors are deployed 

 during in situ measurements is a variation of designs discussed previously. 

 The configuration chosen was either an equidistant placement of four sen- 

 sors in the vertical, alined parallel to shore (Fig. 33), or three de- 

 ployed in the same manner with the fourth oriented shore-normal adjacent 

 to the wave gage. The latter design assures that some record of the inci- 

 dent wave orbital velocities is collected while the velocities in the 

 vertical profile are also recorded. 



c. Data Collection . In 260 hours (10.83 days) of continuous record- 

 ing by the system onboard the sea sled, long time-series records can be 

 obtained with TODAS at any site, even if remotely located. This capa- 

 bility can be extended to 65 days if data were to be collected once every 

 hour for 10-minute periods. Because the batteries are regenerative, 80 

 to 90 days of operation may be feasible in this manner. 



3. Fixed-Point Measurements Combined With Lagrangian Techniques . 



The measurement of flow at a fixed point is regarded as the Eulerian 

 technique of measurement. The technique is a record of events passing 

 through the fixed point, and these events can be related to the Lagrangian 

 velocity by: 



Vi(a,t) = Ui (X(a,t),t) , 



where ui is the Eulerian velocity measured at the fixed probe, t is 

 time, Xi(a,0) = a^ is the Lagrangian position, and V"i is the Lagrangian 

 velocity. 



The Lagrangian position of a moving fluid particle in space is: 



Xi (a,t) = a L + ( l V ± (a,f)dt' , 

 ; o 

 and the measurement of V^ requires tagging the particle so that it can 

 be followed and recorded in the interval t-t'. Because the statistics 

 of ui are not related to V^ in a simple way (Tennekes and Lumley, 

 1972) , it is best to record them simultaneously to evaluate both the time- 

 dependency and space-dependency of the moving fluid in the experimental 

 area. 



56 



