EXTENSION OF HYBRID FEMTOSECOND/PICOSECOND COHERENT ANTI-STOKES RAMAN SCATTERING TO HIGH-SPEED FLOWS

High-speed flows are important for defense, national security, and transportation applications and generate harsh environments where simplifying assumptions such as the ideal gas law are not valid due to nonequilibrium and chemistry effects. These flows are difficult and expensive to replicate experimentally, so the development and improvement of high-speed vehicles often relies on high-fidelity computational fluid dynamics (CFD) models. The successful modeling of complicated phenomena, such as heat transfer in a turbulent boundary layer, relies on validation by experimental data taken with high spatiotemporal resolution, precision, and accuracy. Precise experimental measurement of temperature, an important thermodynamic property for CFD models, is difficult with physical probes which are typically slow and perturb the flow. Instead, hybrid femtosecond/picosecond (fs/ps) coherent anti-Stokes Raman scattering (CARS) allows for non-intrusive, spatially-resolved, collision-free thermometry at kHz repetition rates with high precision and accuracy. 

The goal of this thesis is to advance hybrid fs/ps CARS for extension to high-speed flows, with particular improvements to the spatial extent, probe characteristics, and precision of the technique. A novel method for multipoint measurements in a simple and effective optical arrangement is demonstrated, enabling single-shot and averaged measurements of temperature and O₂/N₂ concentration along a linear array of probe volumes. The generation of a variable-pulsewidth probe beam by a ps slicer, electro-optic modulator, fiber amplifier, and custom narrowband amplifier system is used for improved signal-to-noise ratios at low pressure. Simultaneous CARS thermometry and femtosecond laser electronic excitation tagging (FLEET) velocimetry are performed in the freestream of Mach 3 and Mach 4 nitrogen flows. These measurements reveal the need to quantify and establish the ultimate precision of the hybrid fs/ps CARS technique. Sources of uncertainty in hybrid fs/ps CARS thermometry are determined through a theoretical uncertainty analysis and the predicted precision of the technique is confirmed experimentally in room temperature nitrogen. Benchtop measurements in a supersonic nozzle are used to indicate spatial and temporal simultaneity between FLEET and CARS measurements and hybrid fs/ps CARS thermometry is performed in a high-speed, low temperature flow.