Molecular tagging velocimetry
Encyclopedia
Molecular Tagging Velocimetry (MTV) is a specific form of flow velocimetry
, a technique for determining the velocity of currents in fluids such as air and water. In its simplest form, a single "write" laser
beam is shot once through the sample space. Along its path an optically induced chemical process
is initiated, resulting in the creation of a new chemical species or in changing the internal energy state of an existing one, so that the molecules struck by the laser beam can be distinguished from the rest of the fluid. Such molecules are said to be "tagged."
This line of tagged molecules is now transported by the fluid flow. To obtain velocity information, images at two instances in time are obtained and analyzed (often by correlation
of the image intensities) to determine the displacement. If the flow is three-dimensional or turbulent the line will not only be displaced, it will also be deformed.
, phosphorescence
and laser induced fluorescence (LIF). In all three cases molecule
s relax to a lower state and their excess energy is released as photon
s. In fluorescence
this energy decay occurs rapidly (within
s to 
s at atmospheric pressure
), thus making "direct" fluorescence impractical for tagging. In phosphorescence
the decay is slower, because the transition is quantum-mechanically forbidden
.
In some "writing" schemes, the tagged molecule ends up in an excited state
. If the molecule relaxes through phosphorescence, lasting long enough to see line displacement, this can be used to track the written line and no additional visualisation step is needed. If during tagging the molecule did not reach a phosphorescing state, or relaxed before the molecule was "read", a second step is needed. The tagged molecule is then excited using a second laser beam, employing a wavelength
such that it specifically excites the tagged molecule. The molecule will fluoresce and this fluorescence is captured by means of a camera. This manner of visualisation is called Laser Induced Fluorescence (LIF).
Optical techniques are frequently used in modern fluid velocimetry but most are opto-mechanical in nature. Opto-mechanical techniques do not rely on photonics
alone for flow measurements but require macro-size seeding. The best known and often used examples are Particle Image Velocimetry
(PIV) and Laser Doppler Velocimetry
(LDV). Within the field of all-optical techniques we can distinguish analogous techniques but using molecular tracers. In Doppler
schemes, light quasi-elastically scatters off molecules and the velocity of the molecules convey a Doppler shift to the frequency of the scattered light
. In molecular tagging techniques, like in PIV, velocimetry is based on visualizing the tracer displacements.
s, flames, high pressure vessels, where it is difficult for techniques like Pitot
, hotwire velocimetry and PIV to work. The field of MTV is fairly young; the first demonstration of implementation emerged within the 1980s and the number of schemes developed and investigated for use in air is still fairly small. These schemes differ in the molecule that is created, whether seeding the flow with foreign molecules is necessary and what wavelength of light are being used.
The most thorough fluid mechanics studies have been performed using the RELIEF scheme and the APART scheme. Both techniques can be used in ambient air without the need of additional seeding.
In RELIEF, excited oxygen is used as tracer. The method takes advantage of quantum mechanical properties that prohibit relaxation of the molecule, so that the excited oxygen has a relatively long lifetime.
APART is based on the "photosynthesis" of nitric oxide
. Since NO is a stable molecule, patterns written with it can, in principle, be followed almost indefinitely.
Another well-developed and widely documented technique that yields extremely high accuracy is Hydroxyl tagging velocimetry
(HTV). It is based on photo-dissociation of water vapor followed by visualisation of the resulting OH radical using LIF. HTV has been successfully demonstrated in many test conditions ranging from room air temperature flows to Mach 2 flows within a cavity.
Velocimetry
Velocimetry is the measurement of the velocity of fluids, as often used to solve fluid dynamics problems, or to study fluid networks, as well as in industrial and process control applications, or in the creation of new kinds of fluid flow sensors...
, a technique for determining the velocity of currents in fluids such as air and water. In its simplest form, a single "write" laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
beam is shot once through the sample space. Along its path an optically induced chemical process
Chemical process
In a "scientific" sense, a chemical process is a method or means of somehow changing one or more chemicals or chemical compounds. Such a chemical process can occur by itself or be caused by somebody. Such a chemical process commonly involves a chemical reaction of some sort...
is initiated, resulting in the creation of a new chemical species or in changing the internal energy state of an existing one, so that the molecules struck by the laser beam can be distinguished from the rest of the fluid. Such molecules are said to be "tagged."
Correlation
In statistics, dependence refers to any statistical relationship between two random variables or two sets of data. Correlation refers to any of a broad class of statistical relationships involving dependence....
of the image intensities) to determine the displacement. If the flow is three-dimensional or turbulent the line will not only be displaced, it will also be deformed.
Description
There are three optical ways via which these tagged molecules can be visualized: fluorescenceFluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
, phosphorescence
Phosphorescence
Phosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum...
and laser induced fluorescence (LIF). In all three cases molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s relax to a lower state and their excess energy is released as photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s. In fluorescence
Fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
this energy decay occurs rapidly (within
s to s at atmospheric pressureAtmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...
), thus making "direct" fluorescence impractical for tagging. In phosphorescence
Phosphorescence
Phosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum...
the decay is slower, because the transition is quantum-mechanically forbidden
Forbidden mechanism
In physics, a forbidden mechanism or forbidden line is a spectral line emitted by atoms undergoing nominally "forbidden" energy transitions not normally allowed by the selection rules of quantum mechanics. In formal physics, this means that the process cannot proceed via the most efficient route...
.
In some "writing" schemes, the tagged molecule ends up in an excited state
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
. If the molecule relaxes through phosphorescence, lasting long enough to see line displacement, this can be used to track the written line and no additional visualisation step is needed. If during tagging the molecule did not reach a phosphorescing state, or relaxed before the molecule was "read", a second step is needed. The tagged molecule is then excited using a second laser beam, employing a wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
such that it specifically excites the tagged molecule. The molecule will fluoresce and this fluorescence is captured by means of a camera. This manner of visualisation is called Laser Induced Fluorescence (LIF).
Optical techniques are frequently used in modern fluid velocimetry but most are opto-mechanical in nature. Opto-mechanical techniques do not rely on photonics
Photonics
The science of photonics includes the generation, emission, transmission, modulation, signal processing, switching, amplification, detection and sensing of light. The term photonics thereby emphasizes that photons are neither particles nor waves — they are different in that they have both particle...
alone for flow measurements but require macro-size seeding. The best known and often used examples are Particle Image Velocimetry
Particle image velocimetry
Particle image velocimetry 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...
(PIV) and Laser Doppler Velocimetry
Laser Doppler velocimetry
Laser Doppler Velocimetry , also known as Laser Doppler Anemometry , 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.-Technology origin:With the...
(LDV). Within the field of all-optical techniques we can distinguish analogous techniques but using molecular tracers. In Doppler
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
schemes, light quasi-elastically scatters off molecules and the velocity of the molecules convey a Doppler shift to the frequency of the scattered light
Light scattering
Light scattering is a form of scattering in which light is the form of propagating energy which is scattered. Light scattering can be thought of as the deflection of a ray from a straight path, for example by irregularities in the propagation medium, particles, or in the interface between two media...
. In molecular tagging techniques, like in PIV, velocimetry is based on visualizing the tracer displacements.
MTV schemes
MTV techniques have proven to allow measurements of velocities in inhospitable environments, like jet engineJet engine
A jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...
s, flames, high pressure vessels, where it is difficult for techniques like Pitot
Pitot tube
A pitot tube is a pressure measurement instrument used to measure fluid flow velocity. The pitot tube was invented by the French engineer Henri Pitot Ulo in the early 18th century and was modified to its modern form in the mid-19th century by French scientist Henry Darcy...
, hotwire velocimetry and PIV to work. The field of MTV is fairly young; the first demonstration of implementation emerged within the 1980s and the number of schemes developed and investigated for use in air is still fairly small. These schemes differ in the molecule that is created, whether seeding the flow with foreign molecules is necessary and what wavelength of light are being used.
The most thorough fluid mechanics studies have been performed using the RELIEF scheme and the APART scheme. Both techniques can be used in ambient air without the need of additional seeding.
In RELIEF, excited oxygen is used as tracer. The method takes advantage of quantum mechanical properties that prohibit relaxation of the molecule, so that the excited oxygen has a relatively long lifetime.
APART is based on the "photosynthesis" of nitric oxide
Nitric oxide
Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...
. Since NO is a stable molecule, patterns written with it can, in principle, be followed almost indefinitely.
Another well-developed and widely documented technique that yields extremely high accuracy is Hydroxyl tagging velocimetry
Hydroxyl tagging velocimetry
Hydroxyl tagging velocimetry is a velocimetry method used in humid air flows. Themethod is often used in high-speed combusting flows because the high velocity and temperature...
(HTV). It is based on photo-dissociation of water vapor followed by visualisation of the resulting OH radical using LIF. HTV has been successfully demonstrated in many test conditions ranging from room air temperature flows to Mach 2 flows within a cavity.