2-D velocity and vorticity measurements with FLEET

Femtosecond Laser Electronic Excitation Tagging (FLEET) was introduced recently (Michael, J., Edwards, M., Dogariu, A. and Miles, R., "Femtosecond laser electronic excitation tagging for quantitative velocity imaging in air," Applied Optics, Vol. 50, No. 5158, 2011) and is employed to measure two components of velocity and vorticity in over-expanded and under-expanded supersonic jets. Molecular nitrogen is dissociated in the focal region of a femtosecond laser pulse and recombines over tens of microseconds into an excited electronic state. The subsequent fluorescence of the tagged N₂ is tracked with an intensified CCD camera as it propagates with the flow. Two approximately perpendicular lines are tagged and the displacement of the intersection and along the lines after a prescribed camera delay is used to extract linear and rotational velocity in the image plane. Simulations are performed to analyze the effect of signal-to-noise ratio, image resolution and laser parameters while assessing the uncertainty introduced from various data processing algorithms. Experimental data is obtained for no flow and in both circular and lenticular nozzle configurations at 2 and 5 μs camera delays. Averaged and single-shot velocity and vorticity are calculated employing a variety of techniques including Gaussian least-squares fit across the tagged line profile, one and two dimensional polynomial fits to image intensity and spatial cross-correlation for cross intersection displacement. A detailed statistical analysis based on a Bayesian framework reveals independent degrees of measurement uncertainty due to the experimental apparatus, data processing algorithms and both small and large-scale flow field fluctuations.