What is Laser Doppler Velocimetry (LDV)?

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- GoPhotonics

Oct 30, 2023

Laser Doppler Velocimetry (LDV), also known as Laser Doppler Anemometry (LDA), is an optical technique used to measure the instantaneous point velocity of fluid flows. It is based on the principles of the Doppler effect, which is the change in frequency or wavelength of a wave in relation to an observer moving relative to the source of the wave.

LDV requires the introduction of small non-buoyant particles into the flow to serve as markers for the specific phase whose velocity is being tracked. In liquids, a large number of naturally occurring suspensions can scatter light, but in gaseous flows, the use of a seed generator, such as the atomization of a non-volatile oil like glycerine, is required. The seed particles, whether solid or liquid, scatter light much more efficiently than the fluid itself. In the presence of very small seed particles, they closely follow the path of the fluid, resulting in their velocity matching the fluid velocity. This particle velocity can be directly correlated with the observed Doppler shift in frequency by a receiver, which is the fundamental principle in LDV. The Doppler shift is the difference in frequencies between the incident light and scattered light.

LDV is capable of measuring velocity in both isothermal and non-isothermal flows, and it remains unaffected by fluctuations in fluid temperature.

In the 1960s, LDV technique was developed subsequent to the development of the first continuous He-Ne laser source at Bell Laboratories in 1962. Similar to Particle Image Velocimetry (PIV), this method is non-intrusive and capable of measuring all three velocity components (axial, radial, and tangential).

Construction of Laser Doppler Velocimeter

Figure: 1

The Laser Doppler Velocimeter system consists of several key components such as laser, transmission optics (e.g. Bragg cell, lenses, beam expanders, beam splitter, mirrors, prisms, fiber cable link with laser beam manipulator), receiving optics (e.g. lenses, pinhole, interference filter, photomultiplier), signal processor units (e.g.. fringe-counting, spectral analysis, photon-correlation), traversing mechanism (manual or automated) for transmitting and receiving optics, oscilloscope, seeding generation (solid or liquid vapor) and computer (large capacity hard disk) with a data acquisition board and data handling software. The beam splitter is placed at 45° to the laser beam. The more compact and user-friendly version of the LDV system employs fiber-optic transmission and receiving optics. 

Seed Particles and LDV System Functionality

Seed particles are small particles or tracers that are introduced into a fluid flow to help visualize and study the flow patterns. These seed particles are typically very small and lightweight, often on the order of micrometers or smaller, and they are designed to follow the motion of the fluid without significantly affecting it. One common example of a seed particle used is a tiny spherical glass or polymer microsphere.

They are introduced into a fluid flow alongside small, neutrally buoyant particles that scatter light. They facilitate the LDV system's ability to non-invasively measure and analyze fluid flow velocities with high precision and accuracy.

When illuminated by the laser light, they scatter the light in a way that allows their movement to be tracked and analyzed. The scattered light contains information about the particles' velocities, which are influenced by the flow velocity of the fluid they are suspended in.

Working of Laser Doppler Velocimeter

The laser Doppler velocimeter operates by projecting a monochromatic coherent laser beam towards the intended target and collects the radiation that is reflected. According to the Doppler effect, the change in wavelength of the reflected radiation is dependent on the relative velocity of the targeted object. Hence, the velocity of the object can be determined by measuring the change in wavelength of the reflected laser light, which is done by forming an interference fringe pattern.

Figure: 2

The laser beam emitted from the source passes through the beam splitter, dividing it into two distinct parts. These two beams then get refracted after passing through the focusing lens L1, which converges them onto a specific point within the flow stream where the velocity is to be determined. At this focal point which is the measurement volume (Figure 2), the two beam components intersect, leading to the formation of an interference fringe pattern characterized by alternating regions of low and high intensity.

The instantaneous fluid velocity in a given area is determined by measuring the frequency of light scattered by small seed particles, such as microscopic dust or other particulate matter, moving within the flowing substance. These particles are assumed to accurately represent the flow velocity as they pass through a confined space. According to the Doppler effect, the frequency of scattered light is shifted proportionally to the flow velocity. 

L2 collects the scattered light from particles in the measurement volume and focuses on the photodetector. A photodetector is used to collect the scattered light, which manifests as a varying electrical signal. The frequency of this signal is directly proportional to the rate at which the dust particles cross the interference fringes. By analyzing the wavelength, the velocity of the particles can be calculated. 

Measurement of Scattered Light and Velocity Determination

The particles are illuminated by a specific frequency of light. The difference in frequencies between the incident and scattered light is known as the Doppler shift. Through the analysis of the Doppler-equivalent frequency of the scattered light by the seeded particles within the flow, the local velocity of the fluid can be determined.

The Doppler shift ‘f’, depends on the speed of the object, or speed ‘V’ of the seeding particles, direction of the particle motion, wavelength ‘λ’ of the light, and the orientation of the observer.

The orientation of the observer is defined by the angle ‘α’ between the incident light and the detector. Whereas, the direction of particle motion is defined by ‘β’, the angle between the velocity vector and the bisector of TOD (transmitting optics-object-detector).

Configurations Techniques of LDV

The LDV system is referred to as forward scattering when the angle 'β' between the transmitting optics and the receiving optics is 180 degrees, and as backscattering when this angle is 0 degrees.

Off-axis forward scattering:

  • Angle (β) = between 0-180 degrees (opposite direction).
  • Good signal quality and sample volume definition.
  • Receiving optics are placed off-axis.

Backward scattering:

  • Angle (β) = 0 degrees (same direction).
  • Transceiver detects backscattered light.
  • Lower signal strength, but convenient.
  • Lower scattered light intensity.
  • Poorer sample volume definition.
  • Receiving optics are placed on the same side and near the transmitting optics.

On-axis forward scattering:

  • Angle (β) = 180 degrees (opposite direction).
  • Best signal quality but poorer sample volume definition.
  • Detector positioned on the opposite side of the tunnel from the laser source.
  • Requires optically flat clean windows on both sides of the tunnel test section.

Off-axis backscattering: lower signal, good sample volume definition

  • Lower signal strength but good sample volume definition.
  • Requires a high-power laser.
  • Only requires a single window.
  • Both the detector and the laser are positioned on the same side of the tunnel.

90o light scattering: very low light scatter, do not use

  • Very low light scattering.
  • Typically not used for LDV, as it doesn't provide sufficient signal for velocity measurements.

Particle Velocity Determination and Fluid Flow Analysis

By observing the changes in the frequency of the scattered light (Doppler shift), the LDV system can determine the velocities of the particles, and consequently, the fluid. This information is crucial for understanding the behavior of the fluid flow, identifying turbulence, studying vortices, and obtaining valuable insights into various fluid dynamics phenomena.

Seeding and Cleanliness Considerations

An atomized light oil spray is a common choice for seeding in an LDV system. The assumption that the particles follow the flow holds true for smaller particles. However, if particles grow too large (around 10 - 15 x 10-6 m), they lose this ability to accurately follow the flow's movement. Indeed, the act of spraying the seed into the tunnel brings about a cleanliness challenge when the test is completed.

The incident laser beam can affect the velocity measurement in several ways:

  • Scattering angle: The scattering angle of light by the seed particles can influence the detected signal. Different scattering angles might be collected and analyzed, depending on the experimental setup. The choice of scattering angle affects the sensitivity and accuracy of the velocity measurement.
  • Particle displacement: The laser beam exerts a radiation pressure force on the particles, causing them to move slightly. This displacement can affect the particle's velocity, especially in very fine particles. In some cases, this effect might need to be corrected or compensated for in the analysis.
  • Heating and vaporization: Intense laser beams can heat the particles, particularly if they are absorbing the laser light. This localized heating might lead to changes in the particle's behavior, potentially impacting the velocity measurement. Additionally, if the laser power is high enough, it can vaporize the particles, altering the flow dynamics.
  • Multiple scattering: In dense particle-laden flows, multiple scattering events can occur. This means that particles might scatter light more than once, leading to complex scattering patterns. Proper experimental design and analysis techniques are necessary to account for multiple scattering effects accurately.
  • Alignment and focus: Proper alignment and focusing of the laser beam are critical to ensure accurate measurements. Misalignment or poor focus can result in inaccurate velocity measurements as the scattered light might not be collected optimally.

Advantages of LDV

The Laser Doppler Velocimetry (LDV) method, highly regarded for its precision and non-intrusiveness, offers distinct advantages in fluid flow measurement. One of its key attributes is its non-contact nature, as it does not disturb the fluid flow being studied. Moreover, LDV eliminates the need for extensive calibration processes to determine flow rates accurately. An additional advantage is its complete avoidance of physical contact with the fluid, ensuring that the measurements remain untainted.

The level of accuracy LDV provides is exceptional, typically within the range of ±0.2%. This high degree of precision makes LDV a valuable tool for researchers and engineers seeking reliable flow measurements. Furthermore, LDV's versatility shines through, as it can be effectively employed to measure the flow rates of both liquids and gases, making it a valuable asset in a wide range of fluid dynamics applications.

Disadvantages of LDV

Laser Doppler Velocimetry (LDV) has certain limitations that should be taken into consideration when choosing it as a measurement method. Firstly, it is primarily suited for applications where the flow is passing through transparent channels, making it less suitable for opaque environments. Additionally, LDV may not be the ideal choice for monitoring exceptionally clean fluids, as its sensitivity may be affected

Moreover, LDV operation requires a skilled operator who is well-versed in its principles and intricacies, ensuring accurate and reliable results. Lastly, it's worth noting that the cost associated with implementing LDV can be relatively high, which should be weighed against the specific requirements and budget constraints of the intended application. While LDV offers exceptional precision and capabilities, these considerations should guide its selection in appropriate fluid dynamics scenarios.

Applications of Laser Doppler Velocimeter

Laser Doppler Velocimetry (LDV) finds diverse applications across scientific, engineering, and industrial sectors due to its precise non-invasive measurement of fluid velocities. Some notable applications include:

  • Fluid Dynamics Research: LDV helps researchers understand intricate flow patterns, turbulence, and vortices in gases and liquids. It aids in refining designs, validating theories, and optimizing fluid behavior analysis.
  • Aerospace Engineering: The technique aids in investigating airflow around aircraft and spacecraft components, contributing to optimized aerodynamic designs for heightened efficiency and performance.
  • Hydrodynamic Studies: It is crucial for examining water flow patterns, underwater vehicle propulsion, and marine ecosystem dynamics. It informs the interaction between structures and aquatic environments.
  • Combustion Analysis: Scrutinizing combustion processes in engines, turbines, and furnaces, enhancing combustion efficiency and reducing emissions.
  • Biomechanical Research: In fields like cardiology and biomechanics, LDV measures blood flow in arteries, muscle contractions, and joint movements. This has implications for diagnostics and medical understanding.
  • Environmental Monitoring: LDV contributes to tracking fluid flows in natural settings like rivers, oceans, and the atmosphere. This enhances insights into ecological systems and pollutant dispersion.
  • Medical Imaging: Applications in ophthalmology and cardiology by measuring blood flow in retinal vessels and heart chambers, assisting in diagnosing diseases and assessing circulatory health.
  • Industrial Quality Control: The technique ensures the efficiency of industrial processes involving fluids, such as liquid mixing, pipeline analysis, and spray pattern examination.
  • Material Science: It supports the investigation of material behaviors, particularly fluid flow around and within porous materials. It informs processes involving heat and mass transfer.
  • Wind Tunnel Experiments: Provides accurate airflow velocity measurements for wind tunnel tests, aiding in optimizing designs for vehicles, aircraft, and structures.
  • Microfluidics and Nanotechnology: LDV's microchannel velocity measurements advance microfluidics and nanotechnology, contributing to the development of small-scale devices.