Information about Molecular Tagging Velocimetry
Molecular Tagging Velocimetry (MTV) is a specific form of velocimetry. 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. 
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 correlated to determine the displacement. If the flow is three-dimensional or turbulent the line will not only be displaced, it will also be deformed.
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.
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 technique that holds promise for the future is Hydroxyl tagging velocimetry (HTV). It is based on photo-dissociation of water vapor followed by visualisation of the resulting OH radical using LIF.
Laser-induced fluorescence (LIF) is a spectroscopic method used for studying structure of molecules, detection of selective species and flow visualization and measurements.
..... Click the link for more information.
)
Schematic setup of an Molecular Tagging Velocimetry experiment
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 correlated 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: fluorescence, phosphorescence and laser induced fluorescence (LIF). In all three cases molecules relax to a lower state and their excess energy is released as photons. 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.
MTV schemes
MTV techniques have proven to allow measurements of velocities in inhospitable environments, like jet engines, 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 1980's 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 technique that holds promise for the future is Hydroxyl tagging velocimetry (HTV). It is based on photo-dissociation of water vapor followed by visualisation of the resulting OH radical using LIF.
See also
- Particle image velocimetry
- Laser Doppler velocimetry
- Hot-wire anemometry
References
- Elenbaas, Thijs (2005). Writing lines in turbulent air using Air Photolysis and Recombination Tracking. Eindhoven: Technische Universiteit Eindhoven. ISBN 90-386-2401-8.
- R.B. Miles, J. Grinstead, R.H. Kohl, and G. Diskin (2000). "The RELIEF flow tagging technique and its application in engine testing facilities and for helium–air mixing studies". Measurement Science and Technology 11: 1272–1281.
- L.A. Ribarov, J.A. Wehrmeyr, R.W. Pitz, and R.A. Yetter (2002). "Hydroxyl tagging velocimetry (HTV) in experimental air flows". Applied Physics B 74: 175–183.
- C.P. Gendrich, M.M. Koochesfahani, and D.G. Nocera (1997). "Molecular tagging velocimetry and other novel applications of a new phosphorescent supramolecule". Experiments in Fluids 23: 361–372.
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.
..... Click the link for more information.
..... Click the link for more information.
laser is a mechanical device that produces coherent radiation. The term "laser" is an acronym: Light Amplification by Stimulated Emission of Radiation.
..... Click the link for more information.
..... Click the link for more information.
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.
..... Click the link for more information.
..... Click the link for more information.
turbulence or turbulent flow is a flow regime characterized by chaotic, stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time.
..... Click the link for more information.
..... Click the link for more information.
Fluorescence is a luminescence that is mostly found as an optical phenomenon in cold bodies, in which the molecular absorption of a photon triggers the emission of another photon with a longer wavelength.
..... Click the link for more information.
..... Click the link for more information.
Phosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs.
..... Click the link for more information.
..... Click the link for more information.
molecule is defined as a sufficiently stable electrically neutral group of at least two atoms in a definite arrangement held together by strong chemical bonds.[1][2] In organic chemistry and biochemistry, the term molecule
..... Click the link for more information.
..... Click the link for more information.
Photon
Photons emitted in a coherent beam from a laser
Composition: Elementary particle
Family: Boson
Group: Gauge boson
Interaction: Electromagnetic
Theorized: Albert Einstein (1905–17)
Symbol: or
Mass: 0[1]
..... Click the link for more information.
Photons emitted in a coherent beam from a laser
Composition: Elementary particle
Family: Boson
Group: Gauge boson
Interaction: Electromagnetic
Theorized: Albert Einstein (1905–17)
Symbol: or
Mass: 0[1]
..... Click the link for more information.
Fluorescence is a luminescence that is mostly found as an optical phenomenon in cold bodies, in which the molecular absorption of a photon triggers the emission of another photon with a longer wavelength.
..... Click the link for more information.
..... Click the link for more information.
Atmospheric pressure is the pressure at any point in the Earth's atmosphere. In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point.
..... Click the link for more information.
..... Click the link for more information.
Phosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs.
..... Click the link for more information.
..... Click the link for more information.
A forbidden mechanism or forbidden line is a concept in physics/chemistry. It is a spectral line emitted by atoms undergoing energy transitions not normally allowed by the selection rules of quantum mechanics.
..... Click the link for more information.
..... Click the link for more information.
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.
..... Click the link for more information.
..... Click the link for more information.
In physics, wavelength is the distance between repeating units of a propagating wave of a given frequency. It is commonly designated by the Greek letter lambda (λ). Examples of wave-like phenonomena are light, water waves, and sound waves.
..... Click the link for more information.
..... Click the link for more information.
- For other uses, see LIF.
Laser-induced fluorescence (LIF) is a spectroscopic method used for studying structure of molecules, detection of selective species and flow visualization and measurements.
..... Click the link for more information.
Photonics is the science of generating, controlling, and detecting photons, particularly in the visible and near infra-red spectrum, but also extending to the ultraviolet (0.2 - 0.
..... Click the link for more information.
..... Click the link for more information.
Particle image velocimetry (PIV) is an optical method used to measure velocities and related properties in fluids. The fluid is seeded with particles which, for the purposes of PIV, are generally assumed to faithfully follow the flow dynamics.
..... Click the link for more information.
..... Click the link for more information.
Laser Doppler velocimetry (LDV, also known as laser Doppler anemometry, or LDA) is a technique for measuring the direction and speed of fluids like air and water. In its simplest form, LDV crosses two beams of collimated, monochromatic, and coherent laser light in the flow of the
..... Click the link for more information.
..... Click the link for more information.
Doppler effect, named after Christian Doppler, is the change in frequency and wavelength of a wave as perceived by an observer moving relative to the source of the waves. For waves that propagate in a wave medium, such as sound waves, the velocity of the observer and of the source
..... Click the link for more information.
..... Click the link for more information.
Doppler effect, named after Christian Doppler, is the change in frequency and wavelength of a wave as perceived by an observer moving relative to the source of the waves. For waves that propagate in a wave medium, such as sound waves, the velocity of the observer and of the source
..... Click the link for more information.
..... Click the link for more information.
Scattering is a general physical process whereby some forms of radiation, such as light, sound or moving particles, for example, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which it passes.
..... Click the link for more information.
..... Click the link for more information.
jet engine is an engine that discharges a fast moving jet of fluid to generate thrust in accordance with Newton's . This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets and pump-jets, but in common usage, the term generally refers to a
..... Click the link for more information.
..... Click the link for more information.
A Pitot (IPA: [pito]) tube is a pressure measuring instrument used to measure fluid flow velocity, and more specifically, used to determine the airspeed of an aircraft.
..... Click the link for more information.
..... Click the link for more information.
Nitric oxide or Nitrogen monoxide is a chemical compound with chemical formula NO. This gas is an important signaling molecule in the body of mammals including humans and is an extremely important intermediate in the chemical industry.
..... Click the link for more information.
..... Click the link for more information.
Hydroxyl tagging velocimetry (HTV) is a velocimetry method used in humid air flows. The method is often used in high-speed combusting flows because the high velocity and temperature accentuate its advantages over similar methods.
..... Click the link for more information.
..... Click the link for more information.
Particle image velocimetry (PIV) is an optical method used to measure velocities and related properties in fluids. The fluid is seeded with particles which, for the purposes of PIV, are generally assumed to faithfully follow the flow dynamics.
..... Click the link for more information.
..... Click the link for more information.
Laser Doppler velocimetry (LDV, also known as laser Doppler anemometry, or LDA) is a technique for measuring the direction and speed of fluids like air and water. In its simplest form, LDV crosses two beams of collimated, monochromatic, and coherent laser light in the flow of the
..... Click the link for more information.
..... Click the link for more information.
This article is copied from an article on Wikipedia.org - the free encyclopedia created and edited by online user community. The text was not checked or edited by anyone on our staff. Although the vast majority of the wikipedia encyclopedia articles provide accurate and timely information please do not assume the accuracy of any particular article. This article is distributed under the terms of GNU Free Documentation License.
Herod_Archelaus
