Effect Of Low Temperatures And Pressures On Signal, Lifetime, Accuracy And Precision Of Femtosecond Laser Tagging Velocimetry

Femtosecond laser electronic excitation tagging velocimetry is experimentally characterized in pure nitrogen at pressures (0.228-101 kPa) and temperatures (72-298 K) relevant to high-speed ground testing. Evaluated metrics of performance include signal intensity/lifetime and velocity accuracy/precision. A fast-framing camera lens-coupled to a bandpass-filtered image intensifier acquired data at six principal time delays (0.1-5.0 mu s). Results show signal intensity depends primarily on density and secondarily on temperature. Relative to atmospheric conditions, signal intensity is optimized at reduced densities and cryogenic temperatures. In agreement with previous experiments, the density dependence breaks down at extended time delays. Signal lifetime depends largely on density with a mild to moderate inverse correlation to initial signal strength. In contrast to previous work in pure nitrogen at above atmospheric densities, initial signal and lifetime exhibit different, more complicated trends at sub-atmospheric densities. Although accuracy lacks clear thermodynamic dependencies, precision strongly depends on density and temperature, largely driven by its more fundamental (though inverse) relationship to signal intensity. Precision also inversely depends on the length of the time interval used to determine velocity, in agreement with previous studies. Normalized accuracies and precisions of less than a percent and a few percent, respectively, are obtained in the supersonic free jet.