Hydroxyl tagging velocimetry method optimization: signal intensity and spectroscopy.

The previously demonstrated nonintrusive time-of-flight molecular velocity tagging method, hydroxyl tagging velocimetry (HTV), has shown the capability of operating both at room temperature and in flames. Well-characterized jets of either air (nonreacting cases) or hydrogen-air diffusion flames (reacting cases) are employed. A 7 x 7 OH line grid is generated first through the single-photon photodissociation of H2O by a approximately 193 nm pulsed narrowband ArF excimer laser and is subsequently revealed by a read laser sheet through fluorescence caused by A2sigma+(v' = 3) <-- X2pi(i)(v'' = 0), A2sigma+(v' = 1) <-- X2pi(i)(v'' = 0), or A2sigma+(v' = 0) < or = X2pi(i)(v'' = 0) pumping at approximately 248, approximately 282, or approximately 308 nm, respectively. A detailed discussion of the spectroscopy and relative signal intensity of these various read techniques is presented, and the implications for optimal HTV performance are discussed.

Hydroxyl tagging velocimetry in a supersonic flow over a cavity.

Hydroxyl tagging velocimetry (HTV) measurements of velocity were made in a Mach 2 (M 2) flow with a wall cavity. In the HTV method, ArF excimer laser (193 nm) beams pass through a humid gas and dissociate H2O into H + OH to form a tagging grid of OH molecules. In this study, a 7 x 7 grid of hydroxyl (OH) molecules is tracked by planar laser-induced fluorescence. The grid motion over a fixed time delay yields about 50 velocity vectors of the two-dimensional flow in the plane of the laser sheets. Velocity precision is limited by the error in finding the crossing location of the OH lines written by the excimer tag laser. With a signal-to-noise ratio of about 10 for the OH lines, the determination of the crossing location is expected to be accurate within +/- 0.1 pixels. Velocity precision within the freestream, where the turbulence is low, is consistent with this error. Instantaneous, single-shot measurements of two-dimensional flow patterns were made in the nonreacting M 2 flow with a wall cavity under low- and high-pressure conditions. The single-shot profiles were analyzed to yield mean and rms velocity profiles in the M 2 nonreacting flow.