Phase-averaged time series 



The individual wave is defined by the zero-crossing points of the wave 

 pressure signals, and the ensemble averaging uses 10-deg phase slots. 

 Figure 11-6 shows an example of the computed phase-averaged surface eleva- 

 tion (solid curve), fluid velocity measured by LDV (symbol o), and other 

 quantities to be discussed later. For these data the LDV sensing volume was 

 2 cm above the bed, the wave period was 8 sec, and the wave height was 

 0.5 m. For the construction of this figure, data segments from Runs S0609A, 

 S0610A, S0612A, and S0614A, collected under the same wave conditions, are 

 combined. The data length of each test segment is 90 sec and usually contains 

 11 complete wave cycles, therefore, the results shown in Figure 11-6 

 represent the ensemble average of 44 wave cycles. The measured LDV 

 velocity is found to be in good agreement with linear wave theory prediction 

 (dotted curve) except for a phase shift, which is presumably due to frictional 

 effects. 



i 1- 



-50 



O .Pi' O ° 



0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.1 



Phase (if) 



Figure 1 1 -6. Example of phase-averaged surface elevation (solid), velocity (LDV - o, linear 

 theory - dotted), rms turbulent intensity ( + ), and number of sand grains (x) 



224 



Chapter 1 1 LDV in the Bottom Boundary Layer 



