Unseeded velocimetry in nitrogen for high-pressure, cryogenic wind tunnels: part I. Femtosecond-laser tagging

: Picosecond laser electronic excitation tagging (PLEET) is implemented in a large-scale wind tunnel for the first time. High-speed, unseeded velocimetry is performed in the NASA Langley 0.3-m Transonic Cryogenic Tunnel; repetition rates up to 25 kHz are tested. Velocity measurements are assessed for accuracy and precision. Measurement errors vary in the range of 0.6-1.2%, while the instrument precision is found to lie between 1.2 m/s and 2 m/s and exhibits little variation over the full operating range of the facility. An examination of the signal intensity reveals little to no thermodynamic dependence, and the signal lifetimes exhibit an inverse dependence on both pressure and temperature. The PLEET signal is demonstrated to be largely unaffected by buoyancy despite the large temperature rise. The velocity dynamic range of the measurements is found to be a factor of at least 200 in these experiments with the capacity to measure much higher velocities as well. The spatial resolution of the velocity measurements is found to lie between 2 and 2.7 mm, and the maximum frequency response is 12.5 kHz with the ability to resolve up to 50 kHz with the current measurement system. Overall measurement uncertainties in the streamwise velocity are found to lie between 4% and 4.8% for high to low velocities, while the uncertainty in the transverse velocity is less than 6 m/s. The measurement uncertainties are found to be dominated by systematic errors in the calibration procedure, which could be improved in future experiments.