Femtosecond Laser Electronic Excitation Tagging for Aerodynamic and Thermodynamic Measurements

Abstract

This thesis presents applications of Femtosecond Laser Electronic Excitation Tagging (FLEET) to a variety of aerodynamic and thermodynamic measurements. FLEET tagged line characteristics such as intensity, width and spectral features are investigated in various flow conditions (pressure, temperature, velocity, steadiness, etc.) and environments (gas composition) for both temporally and spatially instantaneous and averaged data. Special attention is drawn to the nature of first and second positive systems of molecular nitrogen and the ramifications on FLEET measurements. Existing laser-based diagnostic techniques are summarized and FLEET is directly compared with Particle Image Velocimetry (PIV) in various low speed flows. Multidimensional velocity, acceleration, vorticity and other flow parameters are extracted in supersonic free jets and within an enclosed in-draft tunnel test section. Probability distribution functions of the mean and standard deviation of critical flow parameters are unveiled by utilizing a Bayesian statistical framework wherein likelihood functions are established from prior and posterior distributions. Advanced image processing techniques based on fuzzy logic are applied to single-shot FLEET images with low signal-to-noise ratio to improve image quality and reduce uncertainty in data processing algorithms. Lastly, FLEET second positive and first negative emission are considered at a wide range of pressures to correct for changes in select rovibrational peak magnitude and shape due to density from which bulk gas temperature may be extracted.