Laser Doppler Velocimeter
Laser Doppler Velocimeter (LDV) is a well-known technique for measuring the velocity of individual particles non-intrusively in a flow field. The method was invented in 1964 by Yeh and Cummins. The particles being measured are either naturally present in the flow or are added to track the flow. Small particles can track the flow with high fidelity and, as a result, measuring the particle velocity provides an accurate estimate of the local flow velocity.
In LDV, two coherent laser beams are crossed to form a small measurement probe volume. Fringes are formed in the probe volume due to the interference between the two coherent beams. A particle entering the measurement probe volume scatters the incident light beams in all directions and the scattered light is detected by a photodetector placed in particular direction (forward-scatter or backscatter). Since the particle is moving with respect to the incident laser beam and the photodetector, the scattered light is shifted in frequency with respect to the incident beam due to the well-known Doppler effect. Furthermore, because of the angular differences between the two laser beams that are simultaneously incident on the particle, the light scattered from each towards the photodetector experience different Doppler shits. Since the incident laser beams are coherent, the scattered light from the two incident beams interfere, and the photodetector provides a signal that is temporally modulated at the Doppler difference frequency – the Doppler burst. The Doppler difference frequency can be directly and linearly related to a velocity component of the moving particle. Additional pairs of crossed beams can be used to measure the second and third components of particle velocity.
Fringes in the measurement volume
Relation of Doppler frequency to particle velocity
Artium continues to advance the state-of-the-art in Laser Doppler Velocimeter (LDV) instrumentation. Artium’s LDV systems offer turnkey operation with a fully automated setup feature. The optical transceiver can be used for the real-time, non-intrusive measurement of individual particle velocity and turbulence measurement (1 or 2 velocity components) in a variety of flow applications. An optional 1-D transceiver can be used along with the 2-D transceiver for the simultaneous measurement of 3-components of velocity.
The complete instrument includes an optical transceiver, ASA signal processors, data management computer and the AIMS system software. The high-powered DPSS lasers built into the transmitter provides stability, compactness, ruggedness, and high reliability; it eliminates the need for inefficient and unreliable fiber optics and bulky Ar-ion lasers.
The Fourier transform based Advanced Signal Analyzer (ASA) incorporates a proprietary digital signal burst detection technique and adaptive Doppler burst sampling approach to provide high accuracy in signal detection and measurement.
The Automated Instrument Management System (AIMS) provides fully automatic setup and operation of the instrument. A variety of standard and user-configurable views are available to analyze the data. It also offers remote operation and monitoring via the Internet.
The compact LDV design incorporates several features aimed at ease-of-use and data accuracy. A new version of the ASA is now available for improved data accuracy at high speeds and in difficult environments. The AIMS software includes an auto-setup feature that automatically selects the processor and optics settings for optimal performance in complex flows.
Artium offers a variety of optical designs, including custom designs, to meet different flow applications. Systems capable of measuring droplet size and 1-, 2-, or 3-components of velocity are available. Laser wavelengths and power can be customized based on the application and user needs.