Figure 11-2. 



Principles of LDV operation: (a) single beam system and (b) two-beam system 

 operated in differential mode 



To implement the LDV for the present experiment, as for ocean research, 

 several further issues needed to be addressed. First, the range of turbidity in 

 the ocean can vary greatly. In the nearshore region, as also in the 

 SUPERTANK project, sand or sediment suspension can produce large con- 

 centrations of suspended particle mass. Under these extreme conditions, laser 

 beams penetrate only a short distance in water, attenuated by small particles 

 and blocked by large particles or bubbles. To reduce attenuation, in-water 

 paths are shortened. To minimize flow disturbance from the probe arising 

 from the path shortening, the probe must be small, which is accomplished by 

 using fiber-optic technology. A schematic of the present instrument is shown 

 in Figure 11-3, where the laser source, beam-splitting optics, Bragg cells, 

 optical fiber couplers, and the probe head are shown. The backscattered light 

 is collimated by the same lens, and then focused to a multi-mode fiber tip. 

 The multi-mode fiber brings this signal to the photomultiplier. The probe 

 used in the SUPERTANK project was 3.18 cm in diameter and 20 cm long. 

 The measurement point was 8.9 cm from the probe tip, approximately 

 3 diameters away. 



As mentioned earlier, the amplitude of the scattered light is a function of 

 the particle size. The scattering cross section is determined by two parame- 

 ters, the refractive index and the dimensionless size parameter, 2ira/\, where 

 a is the particle radius and X is the optical wavelength. The refractive index 

 is a function of scattering material, which is uniform for the SUPERTANK 

 data because scattering is mainly from sand. For relatively large particles 

 (2ira/\ >■ 1), the scattering cross section is approximately twice the geometric 

 cross section of the particle (e.g., van de Hulst 1981). The LDV system was 

 operating as a velocimeter sensitive to sand grains only by setting the 

 detection threshold high. The consequences were two-fold: first the data rate 



Chapter 1 1 LDV in the Bottom Boundary Layer 



215 



