Hydroxyl tagging velocimetry

Hydroxyl tagging velocimetry (HTV) is a velocimetry method used in humid air flows. Themethod is often used in high-speed combusting flows because the high velocity and temperatureaccentuate 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 areused to create a grid of laser beams. The water in the flow is dissociated only where beams ofsufficient energy pass through the flow, thus creating a grid in the flow where theconcentrations of hydroxyl (OH) are higher than in the surrounding flow. Another laser beam inthe form of a sheet is also passed through the flow in the same plane as the grid. This laserbeam 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. Usingelectronic timing methods the picture of the grid can be captured at nearly the same instantthat the grid is created.

By delaying the pulse of the fluorescence laser and the camera shot an image of the grid thathas now displaced downstream can be captured. Computer programs are then used to compare thetwo images and determine the displacement of the grid. By dividing the displacement by the knowntime 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.