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.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/114750 |
| Date | 21 April 2023 |
| Creators | Pitt, Garrett Christopher |
| Contributors | Aerospace and Ocean Engineering, Lowe, Kevin T., Coutier-Delgosha, Olivier, Nguyen, Vinh |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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