Femtosecond laser electronic excitation tagging velocimetry in a large-scale hypersonic facility

Femtosecond laser electronic excitation tagging (FLEET) is a nonintrusive optical diagnostic technique that has been demonstrated to provide direct measurement of velocity and other transport properties in unseeded air and nitrogen flows. A portable FLEET system that uses a single laser for production of N₂ tracers and a single camera for tracking their motion in real time has been developed and deployed. The system performance for high-speed velocimetry in unseeded nitrogen flow has been evaluated at the AEDC Hypervelocity Wind Tunnel 9. Instantaneous velocity measurements at 1 kHz repetition rate were performed during 21 tunnel runs at Mach 10 and 14, both in freestream and in a turbulent boundary layer. The freestream velocities measured with FLEET displayed excellent agreement with the velocities computed using the standard Tunnel 9 data reduction methodology. The turbulent boundary-layer velocity profiles measured with FLEET also showed good agreement with PIV measurements and boundary-layer computational modeling.