LDV optical system with multifrequency shifting for simultaneous measurement of flow velocities at several points

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Abstract
The conventional laser Doppler velocimeter (LDV) can only furnish the instantaneous velocity at a single point in a fluid. To obtain the complete flow pattern, the measurement has to be repeated for different regions of the flow field. In the authors' laboratory new optical systems for the simultaneous measurement of flow velocity distribution have been developed (Nakatani et al. 1979). The laser beams were arranged to intersect in a line or plane in the flow region, and optical fibres connected the interfering beams at measured points to the photodetectors. By using unsteady flow in a branch tube, it was demonstrated that the velocimeter could not only be applied to obtain the flow pattern but could also be used as a flowmeter by integrating the velocities. In these systems, however, it was required to use the same number of detectors as measured points.