<p dir="ltr">Proper development of hypersonic vehicles requires knowledge of thermal loads, which are primarily dictated by the turbulent kinetic energy [1]. Accurate measurements of these values require measurements of velocity fluctuations, which are difficult to obtain using conventional seeded velocimetry methods [2]. However, molecular tagging velocimetry methods such as femtosecond laser electronic excitation tagging (FLEET) have been shown to be capable of measuring mean velocity within highly varying flows [3].</p><p dir="ltr">This work extends the available measurements provided by FLEET, through combination with optical thermometry and novel analysis methods of the signal. By performing FLEET velocimetry alongside thermometry, this shows capability to make instantaneous measurements of Mach number within supersonic flows [4]. Additionally, by tracking multiple characteristics of FLEET image signals, the ability to both capture instantaneous velocity fluctuations and improve measurement of mean velocity are demonstrated.</p><p dir="ltr">Furthermore, the uncertainty intrinsic to the analysis of FLEET signals is investigated. This is done using a combination of both classical statistical methods and uncertainty calculation methods commonly used in particle imaging velocimetry [5]. This is necessary to provide the best possible estimate of velocity fluctuations for the validation of computational fluid dynamic (CFD) models of boundary layer heat transfer.</p><p dir="ltr">Beyond simply improving the quality of velocity measurements, frequency analysis tools are developed and extended to analysis of fluid dynamic problems. These tools have been used prior for detecting extreme transitions within a signal [6], but are applied here to demonstrate their ability to detect physics captured within flow fields. These tools show promise in the ability to detect frequency couplings in time and can potentially be implemented to improve current control strategies in the field of fluid dynamics.</p>

No description available.

<div>Hypersonic flows consist of harsh environments where chemistry effects are relevant and low speed assumptions such as the ideal gas law and the continuum hypothesis</div><div>begin to break down. Because of these processes, computer models do a poor job of predicting behavior of vehicles in hypersonic flight. High fi?delity ground test</div><div>measurements are necessary to anchor and extrapolate CFD simulations so that flight vehicle designs can continue to improve. Due to the harsh conditions and complexities</div><div>of test facilities, implementing experimental measurements can prove challenging. Molecular tagging methods such as Femtosecond Laser Electronic Excitation Tagging</div><div>(FLEET) are attractive for use in hypersonic ground test facilities for many reasons. They are generally considered non-intrusive, since they require no physical probes or seed particles to be placed in the flow. This both keeps the facility safe from damage and minimizes the disturbance imparted on the flowfi?eld by the measurement. Since the tracer is comprised of molecules already present in the flow, the measurement is reliable and can track velocities over a wide dynamic range. The optical arrangement for FLEET is rather simple, requiring only a focused laser beam and a camera to capture the signal. The method can even be applied as a one-sided measurement requiring only one direction of optical access. The current state-of-the-art for the FLEET method is point-wise measurements made at 1 kHz with a</div><div>commercially available laser system. The basis for this thesis is to identify and address current limitations in the implementation of FLEET to relevant flow facilities in terms of the useful aerodynamic information that can be extracted. Fundamental advances to the spatial extent and temporal resolution of FLEET are investigated, and novel applied measurements in high speed flow facilities are presented. Considerations of the precision, spatial resolution and ability to implement fundamental advances to harsh and more complex environments are discussed. A custom-built burst-mode femtosecond laser system is used to enable FLEET measurements at 1 MHz, an improvement of three orders</div><div>of magnitude in measurement rate. New optical arrangements including microlens arrays and holographic beamsplitters are developed to allow multi-dimensional grids</div><div>to be tracked to instantaneously measure velocity gradients. Shock wave and shear measurements in a supersonic bladeless turbine and boundary layer measurements</div><div>on a Mach 6 cone-cylinder-flare are demonstrated. Additionally, an adapted method, Femtosecond Laser Activation and Sensing of Hydroxyl (FLASH) is developed and applied to measure velocity in reacting environments such as a Rotating Detonation Engine (RDE). These innovations provide a path forward for improving the spatiotemproal fi?delity of velocity measurements and extending the capability for investigation high-speed reacting and non-reacting flows in hypersonic ground test facilities.</div><div><br></div>

The Multidisciplinary University Research Initiative (MURI) research at Texas A and M University is concerned with the experimental characterization of non-thermal and non-chemical equilibrium systems in hypersonic (Mach greater than 5) flowfields using experimental diagnostics, and is an interdisciplinary collaboration between the Chemistry and Aerospace Engineering departments. Hypersonic flight conditions often lead to non-thermochemical equilibrium (NTE) state of air, where the timescale of reaching a single (equilibrium) Boltzmann temperature is much longer than the timescale of the flow, meaning that certain molecular modes such as vibrational modes, may be much more excited than the translational or rotational modes of the molecule leading to thermal-nonequilibrium. A nontrivial amount of energy is therefore contained within the vibrational mode, and this energy cascades into the flow as thermal energy, affecting flow properties through the process of various vibrational-vibrational (V-V) and vibrational-translational (V-T) energy exchanges between the flow species. The research is a fundamental experimental study of these NTE systems and involves the application of advanced laser and optical diagnostics towards hypersonic flowfields. The research is broken down into two main categories: the application and adaptation of existing laser and optical techniques towards characterization of NTE, and the development of new molecular tagging velocimetry techniques which have been demonstrated in an NTE flowfield, but may be extended towards a variety of flowfields.

Ce travail de thèse porte sur le développement de la technique de Vélocimétrie par Marquage Moléculaire (Molecular Tagging Velocimetry - MTV) pour l’étude expérimentale des micro-écoulements gazeux internes. Les micro-écoulements gazeux sont des écoulements raréfiés, caractérisés par un nombre de Knudsen non négligeable. L’analyse de la littérature montre un besoin crucial de données expérimentales de grandeurs locales relatives aux micro-écoulements gazeux. Ces données permettraient une discussion pertinente de la précision et des limites d’applicabilité des différents modèles théoriques proposés dans la littérature pour l’étude du régime de glissement, régime raréfié le plus souvent rencontré en microfluidique gazeuse. Dans cette optique, un banc d’essais expérimental a été développé pour la mesure de champs de vitesses par MTV. La technique consiste à suivre des molécules traceuses d’acétone introduites dans le gaz en écoulement et qui deviennent phosphorescentes lorsqu’elles sont excitées par une source lumineuse UV. Les différents compromis pris en compte pour le développement de ce banc (choix du traceur et du matériau, conception du canal instrumenté,…), ainsi que les techniques d’acquisition et de traitement de signal sont détaillés dans le manuscrit. L’analyse expérimentale commence par une étude du signal de phosphorescence de l’acétone. Ensuite, la technique de vélocimétrie par marquage moléculaire est validée par la mesure de champs de vitesses dans des écoulements laminaires confinés en régime non raréfié. Les résultats obtenus sont comparés à des profils de vitesse théoriques d’un écoulement de Poiseuille à pression atmosphérique. Enfin, les premiers résultats obtenus à basse pression sont présentés et commentés. La détection du signal à un niveau de pression de 1kPa est encourageante et offre de nombreuses perspectives pour l’exploration d’écoulements en régime raréfié / This thesis focuses on the development of Molecular Tagging Velocimetry (MTV) technique for the experimental analysis of internal microflows of gases. Gaseous microflows are rarefied flows characterized by a non-negligible Knudsen number. A literature review highlights a crucial need of experimental data on velocity fields within gaseous microflows. These data are required for a relevant discussion on the validity and limits of applicability of the different boundary conditions proposed in the slip flow, which is a regime often encountered in gaseous microsystems. An experimental setup has been designed for analyzing by MTV the velocity distribution in microchannels. The technique consists in detecting the displacement of acetone molecules, introduced as tracers in a gas flow; these molecules exhibit phosphorescence once excited by a UV light source. The various compromises taken into account for the setup design (choice of tracer, laser, channel material and design, camera and intensifier…), as well as the acquisition and processing techniques are detailed in the manuscript. The experimental analysis starts with a study of the acetone phosphorescence signal. Then, the MTV technique is validated by velocity field measurements in internal laminar flows through a rectangular minichannel in non-rarefied regime. The obtained results are successfully compared to the theoretical velocity profile of a Poiseuille flow. Finally, preliminary results obtained at lower pressures are presented and commented. The signal detection at a pressure level as low as 1 kPa is encouraging and draws various perspectives for the exploration of rarefied regimes