| Overview: Ozone Tagging Velocimetry (OTV) is a new flow tagging technique under development at Vanderbilt University, in collaboration with MetroLaser, Inc. This technique provides instantaneous, quantitative velocity data as well as qualitative flow visualization, and it is applicable to dry or humid air flows for low (~2m/s) to high speeds (~500m/s).
The technique involves a two-step process: "writing" a line of ozone in an airflow and subsequently "reading" its displaced position. In the writing step, a 193 nm wavelength excimer laser beam is pulsed through the moving air stream. The laser wavelength is tuned to a light absorption line in the O2 Schumann-Runge band. The resulting excited-state O2 molecules quickly dissociate into O atoms and react with O2 to form ozone (O3). The ozone line is then convected downstream. In the reading step, a second pulsed excimer laser operating at a 248 nm wavelength fires a sheet of laser light to detect the ozone line's new location. The ozone is photodissociated at this wavelength, providing a vibrationally-excited O2 photofragment that produces fluorescence by also absorbing the 248 nm laser. Imaging this O2 fluorescence, and knowing the write beam's location and delay time, allows the velocity measurement.
The last figure is an example of an instantaneous OTV image. The transient flowfield is produced by sending a square-wave signal to a loudspeaker coupled to a 27 mm diameter cylindrical nozzle. An ozone cross, created by the intersection of the two write beams, is greatly distorted by the unsteady, rotational flowfield and shows the formation of a vortex ring. The ozone lines' intersection provides an unambiguous point with which to determine displacement and velocity. As shown on the figure, the instantaneous flow velocity near the center of the nozzle is 2 m/s.
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* Exerts from a Vanderbilt Capability Brief written by Dr. Joseph A. Wehrmeyer, Department of Mechanical Engineering, Vanderbilt University, Nashville, TN 37235-1592; September, 1997. |
