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Laser Doppler Anemometry - measurement principles |
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Introduction
The Laser Doppler Anemometer, or LDA, is a widely accepted tool for fluid dynamic investigations in gases and liquids and has been used as such for more than three decades. It is a well-established technique that gives information about flow velocity.
It's non-intrusive principle and directional sensitivity make it very suitable for applications with reversing flow, chemically reacting or high-temperature media and rotating machinery, where physical sensors are difficult or impossible to use. It requires tracer particles in the flow.
The method's particular advantages are: non-intrusive measurement, high spatial and temporal resolution, no need for calibration and the ability to measure in reversing flows.
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| LDA principle. |
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Features
- Non intrusive
- No calibration required
- Velocity range 0 to supersonic
- One, two or three velocity components simultaneously
- Measurement distance from centimeters to meters
- Flow reversals can be measured
- High spatial and temporal resolution
- Instantaneous and time averaged
Principles
The basic configuration of an LDA consists of :
- A continuous wave laser
- Transmitting optics, including a beam splitter and a focusing lens
- Receiving optics, comprising a focusing lens, an interference filtre and a photodetector
- A signal conditioner and a signal processor.
Advanced systems may include traverse systems and angular encoders.
A Bragg cell is often used as the beam splitter. It is a glass crystal with a vibrating piezo crystal attached. The vibration generates acoustical waves acting like an optical grid.
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| The Bragg cell used as a beam splitter. |
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The output of the Bragg cell is two beams of equal intensity with frequencies f0 and fshift. These are focused into optical fibres bringing them to a probe.
In the probe, the parallel exit beams from the fibres are focused by a lens to intersect in the probe volume.
The probe volume
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| The probe and the probe volume. |
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The probe volume is typically a few millimeters long. The light intensity is modulated due to interference between the laser beams. This produces parallel planes of high light intensity, so called fringes. The fringe distance df is defined by the wavelength of the laser light and the angle between the beams: |
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Each particle passage scatters light proportional to the local light intensity.
Flow velocity information comes from light scattered by tiny "seeding" particles carried in the fluid as they move through the probe volume. The scattered light contains a Doppler shift, the Doppler frequency fD, which is proportional to the velocity component perpendicular to the bisector of the two laser beams, which corresponds to the x axis shown in the probe volume.
The scattered light is collected by a receiver lens and focused on a photo-detector. An interference filtre mounted before the photo-detector passes only the required wavelength to the photo-detector. This removes noise from ambient light and from other wavelengths.
Signal processing< |
