8 sec with 0.5-m wave height. The dimensionless water depth kh (where k is 

 wave number and h is water depth) is 0.424, approaching the shallow- water 

 wave limit. Velocity amplitudes vary only slightly over the whole depth range 

 of measurements (10 percent between surface and bottom based on linear 

 wave theory, considerably less within the 16-cm extent covered by the 

 measurements), as indicated from the two LDV measurements (symbol o) at 

 z = 2 cm (Figure 11 -9a) and z = 16 cm (Figure ll-9b). The LDV velocity 

 realizations appear at unequal time spacing (in multiples of 40 msec), and the 

 BASS (symbol x) was sampled at a constant 1-Hz rate. For reference, the 

 surface elevation measured by the co-located pressure gauge is also plotted in 

 each panel (solid curve). When the LDV data rate is high, the two instru- 

 ments are in good agreement (Figure ll-9a). At z = 16 cm, when the data 

 rate is low, the LDV velocity realization rate is low in the wave-crest region 

 (Figure 11 -9b). 













50 



* x x x x o X x 



° ft 8 ^ ° 8 



i I * x i 



o«> «a °g ff 



rf V. T&V 1 S> TOY 



X T_ 









I- "\ / o\ / \ I \ 1 \ 1 

 \ 1 rtt/° \ 1 *A h °1 / 



\ l\ \ i\ /A 







-50 



v v °y w fj 



x °x °x«k &P *& I®, 

 x **■ °* ft 



<a) 



X/ <\</ ^v/ o»Jo 



** «x* ^ ^ 



6 » 





( 



) 10 20 30 40 



50 60 70 



80 90 



(a) 







50 



X 1 x 1 X ' x 1 



* x 



X x X 



Ax A x A* Ax Ax A 



1 ' X ' A 



X 

 XX o 



A Ax A* A** 



* / \X TV- / \0 / \ 







o 



> \ r \h\L\rbl 

 \ 1 A 1 ft / A 1° \ 1 





L / \ jf - 





-50 



x ° *> x J 



% "x «a *» xx 



-(b) 



w §Vo y v 



*k° *xx ^ » 



x % 





( 



) 10 20 30 40 



50 60 70 



80 9 







(b) 



(sec) 





Figure 1 1-9. LDV (o) and BASS (x) measurements of fluid velocity, Run S0612A; surface 

 elevation (solid line) given for reference; (a) z = 2 cm, (b) z = 1 6 cm 



Chapter 1 1 LDV in the Bottom Boundary Layer 



229 



