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Filtered Rayleigh scattering (FRS) is a non-intrusive, optical measurement technique that can currently provide time-averaged, simultaneous planar measurements of three-component velocity, static temperature, and static density of aerodynamic flows. Development of the FRS technique has typically employed 532 nm Nd:YAG lasers coupled with the use of iodine vapor cells as the molecular filter. One method to improve the effective signal-to-noise ratio (SNR), and therefore the performance of an FRS system, is to use shorter wavelengths. This takes advantage of the dependence of the Rayleigh scattering signal on the inverse of the wavelength of the incident laser light to the fourth power: even small shifts to shorter wavelengths can offer significant gains in SNR as a result. This study explores the implementation of an ultraviolet (UV) FRS system nominally at 387 nm with the use cesium vapor as the molecular filter. The cesium absorption lineshapes (corresponding to the 62S1/2 → 82P3/2 atomic transitions around 387 nm) are considered along with camera specifications to simulate an ultraviolet filtered Rayleigh scattering (UV FRS) measurement of aerodynamic flows. A signal model is developed using numerical functions for the cesium vapor cell transmission, camera specifications, signal-dependent shot noise, and signal-independent electronic detector read noise. Using this noise-inclusive model (over a 2.4 GHz scan bandwidth with a 7.5 cm long cesium vapor cell corresponding to current Virginia Tech FRS capabilities) velocity, static temperature, and static density measurement sensitivities for this proposed configuration are analyzed by evaluating and deriving the Cramér-Rao lower bound (CRLB) for each quantity. The effects of different flow conditions, Mie and geometric scattering levels, cesium vapor cell temperature, and spectral resolution are demonstrated. It is found that the best possible theoretical measurement results are obtained for high-speed wind tunnel flow conditions with high spectral resolution, and that the CRLB for velocity, static temperature, and static density for a 387 nm system approaches or exceeds that of a 532 nm system for a given signal-to-noise ratio (SNR). / Master of Science / One type of non-intrusive measurement technique that can be applied to aerodynamic flows is filtered Rayleigh scattering (FRS). Unlike other non-intrusive techniques such as particle image velocimetry (PIV) and Doppler global velocimetry (DGV), FRS does not require that the flow be seeded with particles and can provide simultaneous measurements of three-component velocity, static temperature, and static density. Current FRS measurement systems commonly use 532 nm green-light lasers and iodine cells for filtering. However, a stronger Rayleigh scattering signal (and therefore better measurement) can be attained by using shorter laser wavelengths as the strength of the Rayleigh scattering is related to the inverse of the incident wavelength to the fourth power. This study takes advantage of this fact to propose an FRS measurement system using ultraviolet laser light at nominally 387 nm. The implementation of a commercially available 387 nm laser system with the use of cesium cells for filtering is investigated. In order to simulate the performance of the system, a signal model is developed that includes both signal-dependent shot noise, and signal-independent electronic detector read noise. The signal model is combined with the transmission profile of cesium vapor, commercially available camera specifications, and typical FRS measurement parameters to simulate a 387 nm FRS system measurement. The measurement sensitives and performance of the proposed UV FRS system at 387 nm are investigated by deriving and evaluating the Cramér-Rao lower bound (CRLB) for velocity, static temperature, and static density. The effects of different flow conditions, Mie and geometric scattering levels, cesium vapor cell temperature, and scan resolution are demonstrated. The best performance is attained at high-speed conditions with high spectral resolution, and this approaches or exceeds the simulated performance of a 532 nm system with an iodine vapor cell over the same range of conditions.

Doctor of Philosophy / Filtered Rayleigh scattering (FRS) is a laser-based measurement technique that makes use of the scattering of light off particles that are much smaller than the wavelength of light that hits them (i.e., Rayleigh scattering of air molecules). The scattered laser light is altered after encountering particles in predictable ways that can be related to changes in velocity, temperature, and density. However, other sources of scattered light interfere with the pure Rayleigh scattering signal such as Mie and background scattering. Mie scattering is the scattering of light off particles that are much bigger than the wavelength of light that hits them (i.e., dust particles suspended in air). Background scattering is the laser light scattered off physical objects that reflect back into the region of interest. The different types of scattering are accounted for with intensive modeling and iterative fitting schemes where the error between simulated data and experimental data is minimized. This fit allows for velocity, temperature, and density information to be extracted from the measured scattered light. This iterative scheme is then applied to experimental measurements on the ground with mini turbojet engines as well as full-scale turbofan engines. A data grouping technique is derived such that the total measured force using FRS can be divided into individual contributions from different parts of the engine. These developed techniques have laid the foundation for future in-flight measurements of engine forces.

<p dir="ltr">From transonic to hypersonic regimes, the characterization of high-speed flow dynamics is critical for the development, testing, and improvement of launch and reentry vehicles, boost-glide vehicles, and thermal protection systems. The design of this technology often relies on computational/empirical models for predictions which make quantitative thermodynamic measurements crucial for numerical validation. Laser diagnostic techniques facilitate non-intrusive, <i>in situ</i> measurements of fluid dynamic properties as well as visualization of flows, shocks, and boundary layer interactions. However, many diagnostics rely on seeding the flow with foreign materials to make measurements, such as the application of particle image velocimetry (PIV), Doppler global velocimetry (DGV), and planar laser-induced fluorescence (PLIF). Molecular filtered Rayleigh scattering (FRS) diagnostics are attractive for flow characterization due to the fact that pressure, temperature, density and velocity measurements can be made directly from air or N₂ molecules without the need for seeding materials. The development of the burst-mode laser (BML) has enabled high-energy pulses generated at the rates necessary to resolve phenomena such as instabilities in boundary-layers and shock-wave evolution using Rayleigh scattering methods. The goal of this dissertation is to advance molecular burst-mode FRS for quantitative, high resolution, multi-parameter measurements. For fixed-wavelength FRS measurements, the spectral characteristics of a BML system were investigated and improved by integrating an etalon for spectral-gating. For multi-parameter measurements, two strategies for wavelength-agility, the ability to quickly switch between two or more laser wavelengths, of the BML were explored: frequency-scanning and frequency-shifting. The frequency-scanning FRS (FS-FRS) technique measurement rate was increased to 1 kHz and demonstrated for 1-ms pressure, temperature, and radial velocity measurements in an underexpanded jet flow. Building upon this, an acousto-optic modulator-based method was implemented to generate frequency-shifted pulses. The rapid frequency-shifting increased the effective FRS multi-parameter measurement rate to 25 kHz and planar pressure, temperature, and radial velocity measurements were captured in an overexpanded jet flow. Finally, design tools for the laser configuration of wavelength-agile FRS were developed for the optimization of relative absolute measurement errors.</p>