This thesis studies the prediction and reduction of installed jet noise, combining both analytical and experimental techniques. In the prediction part, it starts with formulating a low-order but robust isolated jet noise prediction model, based on which a remarkably fast code with pre-informed data is developed. A semi-empirical low-order model is then developed to predict installed jet noise. The model consists of two parts, the first of which is based on the Lighthill's acoustic analogy theory. The second part embraces Amiet's approach to model the sound due to the scattering of jet instability waves. It is shown that the significant low-frequency noise enhancement observed in installed jet experiments is due to the scattering of near-field instability waves. The trailing edge scattering model can successfully predict noise spectra at all distinct angles. The quadrupole-induced high-frequency sound is either efficiently shielded at $90^\circ$ to the jet axis on the shielded side or enhanced by around $3$ dB at $90^\circ$ on the reflected side. But these effects gradually diminish as the observer angle decreases. The high-frequency spectra can be robustly predicted at large observer angles while deviation occurs at low observer angles due to jet refraction effects. An experimental study on installed jet noise is then conducted. The effects of plate positions and Mach numbers are studied. Excellent agreement between the experimental results and model predictions is achieved at low frequencies for all plate positions and Mach numbers tested. At high frequencies, the noise spectra at $90^\circ$ on the reflected side can also be correctly predicted. At lower observer angles, deviations occur due to jet refraction effects. In the noise reduction part, an experimental study is firstly carried out to study the effects of lobed nozzles on installed jet noise at constant flow rates. It is found that lobed nozzles do not noticeably change the installed jet noise spectra at low frequencies. However, they do result in a slight noise reduction at high frequencies. To understand why lobed nozzles hardly change low-frequency installed jet noise, an analytical stability analysis for lobed vortex sheets is performed. The results show that lobed jets change both the convection velocity and the temporal growth rate of instability waves. The changes become more pronounced as the number of lobes $N$ and the penetration ratio $\epsilon$ increase. A second set of experiments is carried out to explore the possibility of reducing installed jet noise by using two pylons. The results show that even in the most conservative case installed jet noise is reduced by around $2\sim3$ dB at low frequencies. It is concluded that using two pylons to reduce installed jet noise has significant practical potential.

Transverse jets in crossflow are widely used to enhance mixing between two flow streams. Such jets exhibit complex flow features, and are highly sen- sitive to a wide variety of operating conditions. The focus of this work is the mixing of relatively low Reynolds number jets that are often encountered in the chemical processing industry. The main objective is to determine if the the jet mixing characteristics can be sufficiently altered by changing the nature of the jet inflow. In particular, we study the effect of jet shape and inflow veloc- ity profile on the mixing properties. Four different jet shapes including circle, square, upstream triangle, and downstream triangle are considered. It is found that the jet shape has tremendous impact on the near field dynamics, gener- ating unique vortical structures for each shape. However, the overall mixing rate is unaffected and is controlled by the evolution of the coherent vortex pair (CVP) in the far-field of the jet. Analyses of turbulence modeling constraints and structure of reaction zones for consecutive-competitive reactions are also presented. / text

Flameless combustion technology has been developed over the past twenty years achieving low-NOx emissions and high energy efficiency for industrial applications. In the present work, three aspects of flameless combustion were examined based on a burner employing the Strong-Jet/Weak-Jet (SJ/WJ) configuration. In the first part of the work, a 3-D SJ/WJ physical model was developed in the Lagrangian perspective for an isothermal pair of free jets. The model was used to predict the WJ trajectory, identify important design/operation factors, and estimate the extent of mixing in the main combustion region (confluence region). The model was also validated with experimental data and showed excellent agreement over a wide range of flow conditions. In the second part of the work, a simplified chemical kinetic model was developed for the flameless combustion of natural gas. A detailed chemical reaction mechanism (GRI Mech 3.0) was successfully reduced to a skeletal chemical reaction mechanism under flameless combustion conditions by Principal Component Analysis, sensitivity analysis and reaction flow analysis. The skeletal mechanism was further simplified to a set of 2-D manifolds by Trajectory-Generated Low-Dimensional Manifolds (TGLDM) method. The set of 2-D manifolds was tested by the Batch Reactor (BR) and Perfect Stirred Reactor (PSR) models. From the BR model test, it was found that the chemical reaction rates were well represented by the 2-D manifolds. The effect of the physical perturbation, tested by PSR model, could be handled by the perpendicular projection instead of the orthogonal projection because both showed similar discrepancies with the skeletal mechanism. In the final part of the work, the steady-state Reynolds-Averaged Navier-Stokes (RANS) simulation was conducted for the turbulent flameless combustion in the SJ/WJ furnace, based on the Probability Density Function (PDF)/Mixing approach. The set of 2-D manifolds and Conditional Source-term Estimate (CSE) method were used for the combustion reaction and the estimation of the mean production/destruction rate, respectively. This CSE-TGLDM model provided good predictions of major species concentrations. However, the gas temperatures and CO concentrations were highly over-predicted. / Thesis (Ph.D, Chemical Engineering) -- Queen's University, 2010-09-23 11:05:21.884

L’atomisation assistée d’une phase liquide lente par un co-courant gazeux rapide est un sujet largement étudié dans la littérature, et des avancées notables sont intervenues notamment sur les mécanismes de brisure, la structure du jet atomisé ainsi que sur les caractéristiques des gouttes formées. En revanche, peu d’études traitent d’une configuration où la phase lente consiste en un jet diphasique gaz-liquide. Cette situation se rencontre par exemple lors du transitoire d’allumage des moteurs cryotechniques de fusée durant lequel la fraction volumique de gaz passe continument de 1 (jet interne purement gazeux) à 0 (jet interne purement liquide), de sorte que pratiquement tous les régimes d’écoulements diphasiques, allant du régime à bulles jusqu’aux écoulements annulaires, peuvent être observés.L’objectif est donc de comprendre comment la fraction volumique de gaz et/ou le régime d’écoulement diphasique du jet interne impactent les modes d’atomisation et in fine les caractéristiques du spray.Pour répondre à ces questions, des expérimentations ont été menées avec comme fluides de substitution de l’air et de l’eau en conditions ambiantes et sous gravité terrestre. Les trois paramètres de contrôle principaux sont la vitesse superficielle du liquide qui a été variée de 0,17 m/s à 2 m/s, la vitesse superficielle du gaz dans le jet interne qui a été fixée de telle sorte que la fraction de débit gaz balaye la plage 0 à 0,99 et enfin la vitesse du gaz externe qui a évolué entre de 20 à 200 m/s. Trois géométries d’injecteurs axisymétriques ont été utilisées pour d’une part accéder à tous les régimes d’écoulements diphasiques souhaités excepté l’écoulement à brouillard, et d’autre part pour varier le diamètre du jet central d’un facteur d’environ deux. Deux types de campagnes expérimentales ont été réalisées : une campagne à rapport des pressions dynamiques gaz-liquide fixé à 16 pour des fractions de débit volumique gaz variables, ainsi que des campagnes à fractions de débit volumique gaz fixe et M variable .Les caractéristiques structurelles du spray, sa longueur de brisure et l’angle du spray formé ont été mesurés par imagerie rapide alors que les caractéristiques de la phase dispersée, c’est-à-dire tailles, vitesses et flux de gouttes ont été mesurés par sonde optique.Les cartographies des régimes d’écoulements dans l’injecteur et des structures du jet diphasique avec et sans assistance par le gaz externe que nous avons établies ont permis de démontrer que ces structures étaient étroitement liées au régime d’écoulement du jet central. Trois modes d’atomisation principaux ont été identifiés et leur frontières établies. A faible fraction de débit gaz, l’atomisation de jets liquides chargés en petites bulles est sujet à l’épluchage de surface et aux battements latéraux à grande échelle comme sur un jet monophasique liquide. A très grande fraction de débit gaz, l’écoulement annulaire donne lieu à l’atomisation d’une nappe annulaire. Pour des valeurs intermédiaires, de nouvelles structures de type "parapluie" se forment à l’arrivée des bouchons de gaz caractérisées par une grande amplitude et un développement orthogonal au jet. L’atomisation des écoulements à régimes « churn » et annulaire donne lieu à des sprays à caractère intermittent du fait de passage de blocs liquides issus de l’écoulement interne.La longueur de brisure est réduite par l’ajout de gaz interne jusqu’à devenir très petite pour les fractions de débit gaz élevées. Le comportement de l’angle du spray est différent selon le diamètre du jet atomisé et le régime d’écoulement interne : il peut augmenter ou réduire selon la configuration.Les pdf centrées sur la taille goutte moyenne sont peu sensibles à la fraction de débit gaz. En revanche les tailles de gouttes moyennes et le flux volumique montrent des évolutions marquées : ils peuvent selon la fraction de débit gaz et donc selon la structure du jet atomisé réduire ou augmenter. / Assisted atomization of a liquid phase slow by a rapid gas co-current is a topic widely studied in the literature, and significant advances have occurred especially on the breakup mechanisms, the structure of the atomized jet as well as the characteristics of formed drops. However, few studies deal with a configuration where the slow phase consists of a two-phase liquid-gas jet. This situation occurs for example during the transitional ignition of cryogenic rocket engines during which the volumic gas fraction decreases continuously from 1 (purely gas) to 0 (purely liquid), so that almost all two-phases flow regimes, from bubbly flow to annular flow can be observed.The goal is to understand how the volumic gas fraction and/or two-phase flow regime of internal jet impact the atomization modes and the characteristics of the spray.To answer these questions, experiments were conducted with as fluid of substitution air and water under ambient conditions and under gravity. The three main control parameters are the superficial velocity of the liquid which was varied from 0.17 m/s to 2 m/s, the superficial gas velocity in the internal jet that has been set so that the gas flow rate fraction sweeps the range 0 to 0.99 and finally the external gas velocity that has evolved between 20 to 200 m/s. Three geometries of axisymmetric injectors were used to firstly access any desired phase flow regimes except mist flow, and also to vary the diameter of the central jet by a factor of about two. Two types of experimental campaigns were carried out: a campaign where the gas-liquid dynamic pressure ratio was set at 16 for varied gas flow rate fraction, as well as campaigns with fixed gas flow rate fraction and variable M.The structural characteristics of the spray, its breakup length and the angle of spray were measured by high speed imaging while the characteristics of the dispersed phase, that is to say, sizes, velocities and flows of the drops were measured by optical probe.Mapping of flow regimes in the injector and two-phase jet structures with and without assistance by external gas that we have established have shown that these structures were closely related to the flow regime of the central jet. Three main atomization modes were identified and its borders established. For small gas flow rate fraction, the atomization of liquid jets laden bubbles is subject to surface peeling and large-scale lateral beats like a single phase liquid jet. For very large gas flow rate fraction, the annular flow results in the atomization of an annular liquid sheet. For intermediate values, new structures type of umbrella form at the arrival of gas slugs characterized by high amplitude and orthogonal development with respect to the jet. Atomization of “churn" flow and annular flow gives rise to intermittent sprays because of passage of "liquid blocks" from the internal flow.The breakup length is reduced by the addition of internal gas and become very small for the high gas flow rate fractions. The behavior of the angle of the spray is different depending on the diameter of the atomized jet and the internal flow regime. It may therefore increase or decrease depending on configuration.Centred pdf on mean drop size are not much sensitive to the gas flow rate fraction. However mean drop sizes and volumic fluxes show marked evolution: they can according to the gas flow rate fraction and therefore the atomized jet structure decrease or increase.

Cette Thèse de Doctorat est consacrée à l'étude des écoulements cisaillés réactifs supersoniques et plus particulièrement à la dynamique des jets compressibles fortement sous détendus. Ce type d'écoulement est rencontré dans un certain nombre d'applications relatives, par exemple, à l'injection d'hydrogène dans les superstatoréacteurs ou bien à l'allumage de moteurs fusées. Il est aussi représentatif du percement de réservoirs à haute pression. Ce travail s'appuie sur l'emploi d'un outil de simulation numérique haute fidélité: CREAMS (Compressible REActive Multi-species Solver). Ce code de calcul met en oeuvre des schémas numériques d'ordre élevé: schéma d' intégration d'ordre 3 en temps et d'ordre 7 et 8 en espace. Il s'appuie sur une description des termes de transport moléculaire et des termes sources chimiques la plus précise possible (transport détaillé et chimie complexe). Les simulations réalisées dans des conditions inertes permettent de caractériser l'importance des interactions choc/turbulence avec une attention particulière accordée au mélange turbulent à petite échelle. Les simulations réactives de jets fortement sous détendus d' hydrogène montrent les difficultés d'allumage et de stabilisation de la combustion pour ce type de conditions, même en présence d'un apport externe d' énergie. Enfin, l'analyse d'un jet représentatif d'un allumeur de moteur fusée révèle certaines spécificités de l'auto-allumage dans ces conditions non-prémélangées rapides. / This dissertation is devoted to the study of sheared supersonic reactive flows and more specifically the dynamics of highly underexpanded jets. Such complex compressible turbulent flow conditions are of praclical interest for scramjets as well as rocket engines applications. Similar condit ions may also be found during the accidentai releases of flammable substances into the atmosphere during high pressure vessel rupture or venting. This work is conducted with a high fidelity computational solver: CREAMS (Compressible Multi-reactive species Solver). It uses high precision numerical schemes third-order Runge Kutta scheme for time integration, plus a combination of seventh and eighth-order centered and WENO schemes for spatial integration. The molecular transport terms and chemical sources terms are handled with the most accurate descriptions, i. e., including detailed transport and chemistry. Inert flow simulations allow to characterize the importance of shock/turbulence interactions with a special emphasis placed on the small-scale scalar mixing. Highly under-expanded reactive hydrogen jet simulations underline the specific difficulties associated to ignition and combustion stabilization even in the presence of an external deposit of energy. Finally, the analysis of the rocket engine igniter jet reveals some specific features of self-ignition phenomena in such non-premixed conditions.

The thesis describes experimental techniques used and results obtained in the investigation of the pure tone components of sound radiation from a commercial 19 inch axial flow fan. The causes and extent of the discrete tone sources were investigated by several methods: cross-correlation of fan surface pressure fluctuations with far field sound, spectral analysis of surface pressure, and examination of surface pressure waveforms. A unique feature involved the design of an apparatus for detecting and transmitting fan-borne pressure fluctuations off the rotating blades. "Causality Correlations" with the on-axis far field sound rendered dipole source strength distribution functions over a span wise line at 15 per cent chord from the leading edge of the fan blade and around a circumferential ring on the motor support strut at a fan radius of 89 per cent. Results indicate that the on-axis discrete tones are a result of source mechanisms causing force fluctuations on the blades and struts which in turn lead to sound radiation which is dipole in nature and is most intense on the axis of the fan. These mechanisms include ingestion of a concentrated vortex, modulation of the clearance between the blade tips and the fan shroud, flow separation around the inlet bell mouth, and fluctuations in the inflow velocity due to the proximity of the fan to the wedged wall of the Anechoic chamber. Crude integral approximations of source strength distributions over the surfaces of the blades and the struts indicated that sound radiation at the blade passage frequency from these two contributors to the overall sound would be about equal, although more sound radiation is expected to originate at the rotor. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

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published_or_final_version / Mechanical Engineering / Master / Master of Science in Engineering

The evaluation of the noise produced by flows has reached a high level of importance in the past years. The physics surrounding flow-induced noise is quite complex and sensitive to various flow conditions like temperature, shape. Empirical models were built in the past for some special geometries but they cannot be used in a general case for a shape optimization for instance. Experimental aeroacoustic facilities represent the main tool for acoustic analyses of flow fields, but are quite expensive because extreme care must be exercised not to introduce acoustic perturbations in the flow (silent facilities). These tools allow a good analysis of the physical phenomena responsible for noise generation in the flow by a comparison of the noise sources and the flow characteristics (pressure, turbulence,...). Nevertheless, the identification and location of noise sources to compare with flow structures requires quite complex methods. The numerical approach complements the experimental one in the sense that the flow characteristics are deeply analyzed where experiments suggest noise production. For the numerical approach, the turbulence modeling is quite important. In the past, some models were appreciated for their good prediction of some aerodynamic parameters as lift and drag for instance. The challenge is now to tune these models for a correct prediction of the noise sources. In the low subsonic range, the flow field is completely decoupled from acoustics, and noise sources can be computed from a purely hydrodynamic simulation before this information is transferred to an acoustical solver which will compute the acoustic field at the listener position. This post processing of the aerodynamic results is not obvious since it can introduce non-physical noise into the solution. This project considers the aspect of noise generation in turbulent jets and especially the noise generated by vortex pairing, as it occurs for instance in jet flows. The axisymmetric version of the flow solver SFELES has been part of this PhD research, and numerical results obtained on the jet are similar to the experimental values. Analyses performed on the numerical results are interesting to go to complete turbulence modeling for aeroacoustics since vortex pairing is one of the basic acoustical processes in vortex dynamics. Currently, a standard static Smagorinski model is used for turbulence modeling. However, this model has well known limitations, and its influence on the noise sources extracted from the flow field is not very clear. For this reason, it is planned to adopt a dynamic procedure in which the subgrid scale model automatically adapts to the flow. We planned also to perform simulations with the variational multiscale approach to better simulate the different interactions between large and unresolved scales. The commercial software ACTRAN distributed by Free Field Technologies is used for the computation of sound propagation inside the acoustic domain.

No description available.