Unseeded velocimetry in nitrogen for high-pressure, cryogenic wind tunnels: part III. Resonant femtosecond-laser tagging

Abstract

Selective two-photon absorptive resonance femtosecond laser electronic excitation tagging (STARFLEET) velocimetry is characterized for the first time at high-pressure, low-temperature conditions. Studies were carried out in the NASA Langley Research Center’s 0.3 meter transonic, cryogenic wind tunnel, with flow conditions spanning the entire operational envelope of the facility; total pressures ranging from 100 kPa to 517 kPa, total temperatures from 80 K to 327 K, and Mach numbers from 0.2 to 0.85. STARFLEET signal intensity and lifetime measurements are examined for their thermodynamic dependencies since both intensity and lifetime have implications for measurement precision. Signal intensity is found to be inversely proportional to density, while lifetime scales nearly linearly with density until approaching the liquid-vapor saturation point of nitrogen. The velocity measurement accuracy and precision are assessed over the full domain of conditions, and standard error was determined to be 1.6%, while precision ranged from roughly 1.5% to 10% of the freestream velocity. The precision was also observed to have a temperature dependence, likely a result of the longer lifetimes experienced at higher densities.