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Laser Doppler Anemometry For Flow Velocity Measurements

Laser Doppler anemometry is a technique for measuring flow velocity using the Doppler effect, using two monochromatic laser beams that converge at an intersection point to form an anemometric measurement volume.

Particles carried by flowing fluid scatter the light of both beams, causing their frequencies to fluctuate based on velocity, which can then be detected using a photomultiplier tube (PMT).

Laser Doppler Anemometry

Laser Doppler Anemometry (LDA) is a non-intrusive way to measure velocity of flowing liquids and gases without disturbing their flow, even in reverse flows, chemically reactive media or high temperature environments. Furthermore, LDA can measure both turbulence intensity and distribution.

LDA system components include a He-Ne laser, beam splitter, receiving lens, photodetector, and signal processing circuit. The beam is split evenly by means of an optical prism or rotating optical grating with half silvered minor, then scattered light from seeding particles is detected by both beams before being measured by photodetector; with signal output directly proportional to Doppler frequency shift and therefore velocity.

For measurement, laser beams are scanned across the surface of a measuring volume. Reflected light collected at various angles is converted into velocity signals by signal processing electronics before being transmitted via cable to a data acquisition system for display and recording.

LDA measurements provide crucial insights for fluid dynamics studies. Their measurements serve as an excellent complement to CFD simulations, helping researchers gain greater insight into fluid behavior under investigation. With this knowledge comes an increased capability of designing better machines or fine-tuning existing products to increase efficiency and quality – for instance in blood pumps this knowledge may reduce instances of thrombosis and hemolysis while simultaneously increasing pump longevity and reliability.


Laser Doppler Anemometry (LDA) is a non-intrusive optical measurement technique that utilizes the Doppler effect in scattered light to accurately determine fluid velocity. LDA utilizes two intersecting beams of monochromatic and coherent laser light, particle seeding systems, and photomultiplier tubes (PMTs), whereby Doppler shift in frequency is directly proportional to liquid or gas velocity; LDA provides an inexpensive yet accurate means of assessing transparent or semitransparent fluid flows or linear or vibratory motion on opaque reflecting surfaces.

LDA operates by splitting a laser beam using a beam splitter into two beams that are collimated by lens L1 into intersecting test sections of flow containing sufficient concentration of particles to scatter light from both beams, producing interference fringes detectable by PMT and producing Doppler shifts proportional to tracer velocity in scattered light frequency from both beams.

By analyzing the time difference between fringes and assuming their geometry is known, one can calculate fluid velocity. Tracer particles’ speed may also be determined through correlations between their concentration and Doppler periods detected; provided their particle size allows multiple cycles.

Phase Doppler particle analyzers offer the unique capability of simultaneously measuring velocity and concentration in spray patterns, providing instantaneous velocity readings from individual points in the spray pattern. This marks a dramatic departure from earlier measurement techniques which could only capture either velocity or concentration at one time – this new capacity opens up many application opportunities, such as MEMS vibration measurement which require high resolution measurements of minute forces.

Measurement Volume

Laser Doppler anemometry’s combination of directional sensitivity and non-intrusive nature make it the ideal measurement technique for flow velocity in industrial settings. It can be utilized in chemically reactive or high temperature media, rotating machinery or similar environments where attaching physical sensors would be difficult or impossible. Tracer particles added into the flow help alter its reflected intensity to enable accurate results from measurements taken with this measurement method.

A basic method involves crossing two beams of collimated monochromatic laser light in the direction of flow at a location of interest, where an interference pattern formed at their intersection contains information about flow velocity components in the vicinity. Doppler shift of scattered light (fD) can be used to estimate perpendicular velocity components by using intensity signals from photodetectors as measurements to calculate this value.

To ensure optimal operation of an LDA, its two beams of light must form an overlap in their measurement volume. This can be accomplished using a Bragg cell as a beam splitter and carefully tuning transmitting optics so they align their waists at the point where two streams of particles intersect – this ensures that intensity signals from photo detectors focus directly onto particles while decreasing interference signals such as ambient lighting or other wavelengths from laser spectrums.

Once particles enter a detector, they produce an intermittent electrical signal which is converted to a Doppler pulse by photodetector. The time delay between adjacent fringes reflects particle velocity; by measuring frequency of Doppler pulses it is possible to calculate frequency deflection factor (fD), thus giving us access to velocity estimation.

Doppler signals generated from flow can also be used to assess particle concentration using statistical correlation between Doppler periods and particle counts. This information is essential for process control as it allows us to account for particle effects on measurement results and adjust accordingly.


Laser Doppler anemometry is an established measurement technique for providing information about flow velocity. Due to its non-intrusive principle and directional sensitivity, it makes an excellent solution for applications involving reverse flows, chemically reacting media or rotating machinery where physical sensors would be difficult to use. However, in order for it to function, tracer particles in the fluid must first be measured for measurement purposes.

This technique works by passing a monochromatic laser beam through a measuring volume in flowing fluid and collecting its reflected radiation. For simplicity’s sake, two beams of light may be crossed at their waists at one point inside of the measurement volume so as to create interference fringes proportional to velocity component perpendicular to beam bisector (as illustrated by Figure 10.1.8). Original and reflected signals are collected with photodetectors.

With single laser diodes, this means an integrated photomultiplier tube (PMT). When original and reflected laser light are recorded onto this same PMT, an increase or decrease in frequency corresponding to flow velocity will become noticeable through its Doppler shift effect.

A regulating circuit is used to maintain a constant frequency difference by controlling current and temperature of the laser diode, then evaluates this measurement signal through signal processing and display device in order to give an indication of actual flow velocity.

At its core, system accuracy depends on two variables – geometry of measurement volume and relationship between two optical beams – making construction of measurement volume as accurate as possible and alignment of optical systems exact; this is particularly crucial in system-on-chip configurations where component sizes on microstrip can vary across units on chip.

As the velocity component of a laser Doppler anemometry is measured by the difference in wavelength between transmitted and received signals, its signal amplitude must remain consistent across each particle in its path. Unfortunately, this requirement can be hard to meet given their various sizes; small particles in particular have greater effects than larger ones on measurement signals.
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