Hydroxyl Tagging Velocimetry
Hydroxyl Tagging Velocimery (HTV) is a molecular tagging technique to yield flow velocity by time of flight; the molecules in the gas flow are tagged (marked) with a laser, and the movement of the tag gives the velocity. Once a laser line (or grid) is tagged, it will move with the flow. There is no particle drag or non-uniformity issues associated with molecular velocimetry. The movement of the tagged regions is imaged by a method dictated by the photochemistry of the tagged molecules (laser-induced fluorescence in the case of HTV). The displacement of the tagged grid over a fixed time period yields the velocity. This time-of-flight measurement of velocity can be easily implemented because it does not require the complex calibrations or corrections necessary for alternate methods (e.g. Doppler shift method).
HTV is a linear method in which an ArF excimer laser photodissociates water in a humid flow. In this single-photon process, photodissociation of vibrationally excited H2O creates lines of OH at a concentration higher than the ambient OH. The OH grid moves for a known period of time and is recorded with laser-induced fluorescence. Hydroxyl is an optimal tag for supersonic applications as OH is formed immediately by the pulsed laser through H2O photodissociation, and OH has an adequate lifetime in the supersonic flow. Also, the use of hydroxyl allows for long lines to be written in a hot flow associated with a scramjet, creating more flexibility and accessibility in the test area setup.