Instruments 



LDV. The LDV is a two-axis backscatter system developed under the 

 Office of Naval Research sponsorship for ocean deployment. For the 

 SUPERTANK deployment, the LDV measured the two horizontal velocity 

 components of the flow field. In this report, the cross-channel velocity com- 

 ponent is not discussed; only the longitudinal velocity component data are 

 presented. 



An LDV measures flow velocity by sensing the Doppler shift in light 

 scattered by particles (Agrawal and Belting 1988). Figure ll-2a is a simple 

 sketch showing the operating principle. The Doppler shift is fundamentally 

 given by 





where 



214 



f D = Doppler frequency shift (Hz) 



k t = wave number vector of the incident beam (rad/cm) 



k s = wave number vector of the scattered light (rad/cm) 



v = particle velocity (cm/sec) 



If two laser beams are used to illuminate the particle (Figure 11 -2b), the 

 differential Doppler shift, derived from taking the difference of the two Dopp- 

 ler shift equations similar to Equation 11-1 is given by 



fn = (ha ~ V i~ t 11 ' 2 ) 



Z7C 



where the subscripts 1 and 2 denote the two distinct incident laser beams. 

 This is the differential mode of operation, in which the direction of 

 observation does not enter into the measurement. The velocity component in 

 the plane of the two laser beams and normal to their bisector is sensed. To 

 measure two velocity components, it is sufficient to use three beams, one 

 common and two others for the two orthogonal velocity components. A 

 detecting optics arrangement consisting of imaging lenses and a pinhole senses 

 the light scattered from the beam intersection region (the sensing volume). 

 Light entering the pinhole is sensed by a photomultiplier. The optical signal 

 is converted to electrical, with amplitudes proportional to the particle 

 scattering intensity and frequency representing the Doppler shift. The former 

 is a function of particle size, and the latter is linearly proportional to the 

 particle velocity. To prevent signal crosstalk between the two velocity axes, 

 the two velocity components are distinguished by shifting the individual laser 

 beams into different frequency bands. This is accomplished by Bragg cells. 

 In the present design, one velocity axis is put in a 1-MHz band with a 

 ±300-kHz bandwidth, and the other in a 4-MHz band, also with a ±300-kHz 

 bandwidth. 



Chapter 1 1 LDV in the Bottom Boundary Layer 



