NON- INTRUSIVE FLOW DIAGNOSTICS:

Laser Doppler anemometry:

Laser Doppler velocimetry (LDV) , also known as laser Doppler anemometry (LDA), is the technique of using the Doppler shift in a laser beam to measure the velocity in transparent or semi-transparent fluid flows, or the linear or vibratory motion of opaque, reflecting, surfaces.

Operating principles:

In its simplest and most presently used form, LDV crosses two beams of collimated, monochromatic, and coherent laser light in the flow of the fluid being measured. The two beams are usually obtained by splitting a single beam, thus ensuring coherence between the two. Lasers with wavelengths in the visible spectrum (390–750 nm) are commonly used; these are typically He-Ne, Argon ion, or laser diode, allowing the beam path to be observed. A transmitting optics focuses the beams to intersect at their waists, where they interfere and generate a set of straight fringes. As particles entrained in the fluid pass through the fringes, they reflect light that is then collected by a receiving optics and focused on a photo detector.

The reflected light fluctuates in intensity, the frequency of which is equivalent to the Doppler shift between the incident and scattered light, and is thus proportional to the component of particle velocity which lies in the plane of two laser beams. If the sensor is aligned to the flow such that the fringes are perpendicular to the flow direction, the electrical signal from the photo detector will then be proportional to the full particle velocity. By combining three devices (e.g.; He-Ne, Argon ion, and laser diode) with different wavelengths, all three flow velocity components can be simultaneously measured.

Another form of LDV, particularly used in early device developments, has a completely different approach akin to an interferometer. The sensor also splits the laser beam into two parts; one (the measurement beam) is focused into the flow and the second (the reference beam) passes outside the flow. A receiving optics provides a path that intersects the measurement beam, forming a small volume. Particles passing through this volume will scatter light from the measurement beam with a Doppler shift; a portion of this light is collected by the receiving optics and transferred to the photoelectron. The reference beam is also sent to the photo detector where optical heterodyne detection produces an electrical signal proportional to the Doppler shift, by which the particle velocity component perpendicular to the plane of the beams can be determined.

Similar arrangements using optical heterodyning are also used in laser Doppler sensors for measuring the linear velocity of solids and for measuring vibrations of surfaces; the latter sensor is usually called a laser Doppler vibrometer, also abbreviated LDV.

Applications:

In the decades since the LDV was first introduced, there has been a wide variety of laser Doppler sensors developed and applied.

Flow research:

Laser Doppler velocimetry is often chosen over other forms of flow measurement because the equipment can be outside of the flow being measured and therefore has no effect on the flow. Some typical applications include the following:

Wind tunnel velocity experiments for testing aerodynamics of aircraft, missiles, cars, trucks, trains, and buildings and other structures

Velocity measurements in water flows (research in general hydrodynamics, ship hull design, rotating machinery, pipe flows, channel flow, etc.).

Fuel injection and spray research where there is a need to measure velocities inside engines or through nozzles

Environmental research (combustion research, wave dynamics, coastal engineering, tidal modeling, river hydrology, etc.).

One disadvantage has been that LDV sensors are range-dependent; they have to be calibrated minutely and the distances where they measure have to be precisely defined. This distance restriction has recently been at least partially overcome with a new sensor that is range independent.

Vibration and acoustics:

Laser Doppler velocimetry is effective in measuring surface vibrations via reflection of the laser light from the vibrating surface. The technology, adapted to include a scanning capability (to provide measurement of the vibration over an array of points), has been used to measure vibration generation and propagation for ultrasonic motors and acoustic and ultrasonic microfluidics. Remarkably, it is possible to measure the deformation of capillary wave as well using a laser Doppler vibrometer.

Particle image velocimetry:

Particle image velocimetry (PIV) is an optical method of flow visualization used in education and research. It is used to obtain instantaneous velocity measurements and related properties in fluids. The fluid is seeded with tracer particles which, for sufficiently small particles, are assumed to faithfully follow the flow dynamics (the degree to which the particles faithfully follow the flow is represented by the Stokes number). The fluid with entrained particles is illuminated so that particles are visible. The motion of the seeding particles is used to calculate speed and direction (the velocity field) of the flow being studied.

Other techniques used to measure flows are laser Doppler velocimetry and hot-wire anemometry. The main difference between PIV and those techniques is that PIV produces two dimensional or even three dimensional vector fields, while the other techniques measure the velocity at a point. During PIV, the particle concentration is such that it is possible to identify individual particles in an image, but not with certainty to track it between images. When the particle concentration is so low that it is possible to follow an individual particle it is called Particle tracking velocimetry, while Laser speckle velocimetry is used for cases where the particle concentration is so high that it is difficult to observe individual particles in an image.

Typical PIV apparatus consists of a camera (normally a digital camera with a CCD chip in modern systems), a strobe or laser with an optical arrangement to limit the physical region illuminated (normally a cylindrical lens to convert a light beam to a line), a synchronizer[disambiguation needed] to act as an external trigger for control of the camera and laser, the seeding particles and the fluid under investigation. A fiber optic cable or liquid light guide may connect the laser to the lens setup. PIV software is used to post-process the optical images.

Advantages:

The method is, to a large degree, nonintrusive. The added tracers (if they are properly chosen) generally cause negligible distortion of the fluid flow.

Optical measurement avoids the need for Pitot-tubes, hotwire anemometers or other intrusive Flow measurement probes. The method is capable of measuring an entire two-dimensional cross section (geometry) of the flow field simultaneously.

High speed data processing allows the generation of large numbers of image pairs which, on a personal computer may be analyzed in real time or at a later time, and a high quantity of near-continuous information may be gained.

Sub pixel displacement values allow a high degree of accuracy, since each vector is the statistical average for many particles within a particular tile. Displacement can typically be accurate down to 10% of one pixel on the image plane.

Dis-advantages:

In some cases the particles will, due to their higher density, not perfectly follow the motion of the fluid (gas/liquid). If experiments are done e.g. in water, it is easily possible to find very cheap particles (e.g. plastic powder with a diameter of ~60 µm) with the same density as water. If the density still does not fit, the density of the fluid can be tuned by increasing/ decreasing its temperature. This leads to slight changes in the Reynolds number, so the fluid velocity or the size of the experimental object has to be changed to account for this.

Particle image velocimetry methods will in general not be able to measure components along the z-axis (towards too/away from the camera). These components might not only be missed, they might also introduce interference in the data for the x/y-components caused by parallax. These problems do not exist in Stereoscopic PIV, which uses two cameras to measure all three velocity components.

Since the resulting velocity vectors are based on cross-correlating the intensity distributions over small areas of the flow, the resulting velocity field is a spatially averaged representation of the actual velocity field. This obviously has consequences for the accuracy of spatial derivatives of the velocity field, vortices, and spatial correlation functions that are often derived from PIV velocity fields.

Laser-induced fluorescence:

Laser-induced fluorescence or LED induced fluorescence (LIF) is a spectroscopic method used for studying structure of molecules, detection of selective species and flow visualization and measurements.

The species to be examined is excited with a laser. The wavelength is often selected to be the one at which the species has its largest cross section. The excited species will after some time, usually in the order of few nanoseconds to microseconds, de-excite and emit light at a wavelength longer than the excitation wavelength. This fluorescent light is typically recorded with a photomultiplier tube (PMT) or Filtered Photodiodes.

Two different kinds of spectra exist, disperse spectra and excitation spectra.

The disperse spectra are performed with a fixed lasing wavelength, as above and the fluorescence spectrum is analyzed. Excitation scans on the other hand collect fluorescent light at a fixed emission wavelength or range of wavelengths. Instead the lasing wavelength is changed.

An advantage over absorption spectroscopy is that it is possible to get two- and three-dimensional images since fluorescence takes place in all directions (i.e. the fluorescence signal is usually isotropic). The signal-to-noise ratio of the fluorescence signal is very high, providing a good sensitivity to the process. It is also possible to distinguish between more species, since the lasing wavelength can be tuned to a particular excitation of a given species which is not shared by other species.

LIF is useful in the study of the electronic structure of molecules and their interactions. It has also been successfully applied for quantitative measurement of concentrations in fields like combustion, plasma, spray and flow phenomena (such as Molecular tagging velocimetry), in some cases visualizing concentrations down to nanomolar levels. LED induced fluorescence has been used in situ to delineate aromatic hydrocarbon contamination as a cone penetrometer adds on module and also as a percussive capable asset.

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