Hydroxyl tagging velocimetry


Hydroxyl tagging velocimetry 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 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 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 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 can be determined.
Other molecular tagging velocimetry methods have used ozone, excited oxygen and nitric oxide as the tag instead of hydroxyl. In the case of ozone the method is known as ozone tagging velocimetry or OTV. OTV has been developed and tested in many room air temperature applications with very accurate test results. OTV consists of an initial "write" step, where a 193-nm pulsed excimer laser creates ozone grid lines via oxygen UV absorption, and a subsequent "read" step, where a 248-nm excimer laser photodissociates the formed O3 and fluoresces the vibrationally excited O2 product thus revealing the grid lines' displacement.