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Mechanical Engineering

College of Engineering and Applied Science

Laser Doppler Anemometry


TSI s three component Laser Doppler Anemometry system is used to get all three components of velocity simultaneously. The system uses 5W water cooled Argon Ion Laser to provide illumination. Light from the laser passes through a color separator that separates the multi-color laser beam into its components (the three components have 514.5, 488 and 476.5nm in wavelength). Each of the colors is further divided in two using a Bragg cell that also shifts one of each pair of beams by 40MHz. The pairs of beams are routed from the color separator using fiber optic cables to their respective transceivers and through an 83mm wide beam expander of 2.6 expansion ratio. The addition of the beam expander reduces the size of the measuring volume and hence increases the accuracy. The measurement volume in that case is approximately 50 µm in its longest dimension for all three channels.


As for the optics, fringe spacing of 1.91, 1.8183 and 1.7754 µm is produced for the three channels respectively. Particles passing through the probe volume scatter light that passes back through the transceiver and is focused onto a multi-mode receiving fiber. This scattered light is routed to the amplifier module where the light colors are again separated and focused onto different photo-multiplier tubes (PMTs). Electrical signals from the PMTs are then sent to a burst detector where the signals are processed to determine velocity. All these processes are processed inside the top-of-the-line burst collector FSA 4000-3. It is a multibit digital burst collector that has a 175 MHz maximum Doppler frequency, 800 MHz sampling rate and 50ns transit time. One of the many benefits the FSA 4000 has is that it allows a dynamic burst centering as well as dynamic optimum sampling rate. A side addition to the FSA 4000 is a 4 channel analog-to-digital converter that allows us to monitor, for example, the pressure and temperature in the flow. The data is then transmitted through a Firewire interface to the computer where TSI s Flowsizer software controls the acquisition process. During a survey, the probe volume's location in the flow is determined by positioning the transceivers using a home-built three-component traverse system with 60.5 x38 x24 traveling distance. A picture of the LDA system in its 2D configuration mounted on the traverse system with the probe volume near the exit of the jet is shown.

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