FLEET Boundary Layer Velocity Profile Measurements
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abstract
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 velocity profiles in an over-expanded supersonic jet. A laser pulse is focused to within approximately 20 microns of a curved razor surface at the nozzle exit where it dissociates a line of molecular nitrogen in air. The atomic nitrogen recombines in an elevated electronic energy state and the subsequent fluorescence is tracked by an intensified-CCD camera over tens of microseconds. Assuming one-dimensional flow, the velocity is calculated by determining the displacement of the tagged line center and dividing by the camera shutter delay. The data is averaged over a series of laser pulses and fluctations between single shots are used to quantify measurement uncertainty. Both the transverse and perpendicular velocity profiles are compared to laminar and turbulent boundary layer models with the Meador-Smart reference temperature for compressibility and Van Driest transformed velocity to account for wall heat transfer. Transition from a laminar to turbulent boundary layer is apparent across the curvature of the surface while interaction with the shock cone and shear layer downstream may explain deviation from expected behavior.
