Information about Hydroxyl Tagging Velocimetry
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. HTV uses a laser (often an argon-fluoride
excimer laser) to dissociate the water in the flow into H + OH. Before entering the flow optics are
used to create a grid of laser beams. The water in the flow is dissociated only where beams of
sufficient energy pass through the flow, thus creating a grid in the flow where the
concentrations of hydroxyl (OH) are higher than in the surrounding flow. Another laser beam in
the form of a sheet is also passed through the flow in the same plane as the grid. This laser
beam is tuned to a wavelength that causes the hydroxyl molecules to fluoresce in the UV spectrum. The fluorescence is then captured by a charge-coupled device (CCD) camera. Using
electronic timing methods the picture of the grid can be captured at nearly the same instant
that the grid is created.
By delaying the pulse of the fluorescence laser and the camera shot an image of the grid that has now displaced downstream can be captured. Computer programs are then used to compare the two images and determine the displacement of the grid. By dividing the displacement by the known time delay the two dimensional velocity field (in the plane of the grid) can be determined.
Other Molecular tagging velocimetry (MTV) methods have used ozone, excited oxygen and nitric oxide as the tag instead of hydroxyl. In this case the method is known as ozone tagging velocimetry or OTV.
By delaying the pulse of the fluorescence laser and the camera shot an image of the grid that has now displaced downstream can be captured. Computer programs are then used to compare the two images and determine the displacement of the grid. By dividing the displacement by the known time delay the two dimensional velocity field (in the plane of the grid) can be determined.
Other Molecular tagging velocimetry (MTV) methods have used ozone, excited oxygen and nitric oxide as the tag instead of hydroxyl. In this case the method is known as ozone tagging velocimetry or OTV.
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.
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An excimer laser (sometimes, and more correctly, called an exciplex laser) is a form of ultraviolet chemical laser which is commonly used in eye surgery and semiconductor manufacturing.
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Optics (ὀπτική appearance or look in Ancient Greek) is a branch of physics that describes the behavior and properties of light and the interaction of light with matter.
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Hydroxyl in chemistry stands for a molecule consisting of an oxygen atom and a hydrogen atom connected by a covalent bond. The neutral form is a hydroxyl radical and the hydroxyl anion is called a hydroxide.
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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.
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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.
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charge-coupled device (CCD) is an analog shift register, enabling analog signals (electric charges) to be transported through successive stages (capacitors) controlled by a clock signal.
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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
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