In axial compressors with a relatively large blade tip clearance, an unsteady phenomenon denoted as rotating instability (RI) can be detected when the compressor is throttled to the operating points near the stability limit. In the frequency domain, RIs are shown as a hump lower than the blade passing frequency. This indicates an increase in noise level and might cause blade vibration and other undesirable structural issues. In this thesis, a comprehensive study on RIs is performed based on an axial compressor rotor row of the Low Speed Research Compressor at Technische Universität Dresden. Three blade tip clearances are investigated, and a groove casing treatment is mounted over the shroud for flow control. Methods of numerical modeling are evaluated, and zonal large eddy simulation is selected as the numerical model. By analyzing the flow properties and applying the dynamic mode decomposition, the coherent flow structure corresponding to the dominant frequency of RIs is extracted and visualized as the waves located in the blade tip region. The criteria for the appearance of RIs in the investigated research object are concluded.

<p>This dissertation covers a computational investigation into hypersonic flight vehicle geometric imperfections, with a focus on wing-elevon-cove configurations. The primary region of focus for the overall research was the cove region at the juncture of the main wing element and the elevon. This region is associated with the shock-wave/boundary-layer interaction produced by the control surface deflection. There also exists a centrifugal instability at the cove, due to streamline curvature, which is associated with the production of Görtler vortices. The content includes three projects revolving around hypersonic wing-elevon-cove flows. These flows were computed with improved delayed detached-eddy simulation.</p> <p><br></p> <p>The first project was a computational investigation simulating the NASA experimental study done by W.D. Deveikis and W. Bartlett in 1978. This experiment consisted of hypersonic high Reynolds number wind tunnel tests for a shuttle-type reentry vehicle. The computational aerothermodynamic surface loadings for this project were compared to the experimental published data. Grounded with the agreement with mean surface data, this project expanded on the topics explored in the experimental study to include topics such as flow visualization and statistical analysis. The second and third project are extensions of this work and were done in collaboration with Purdue University and the University of Tennessee Space Institute (UTSI). A swept wing-elevon-cove model was designed by Carson Lay, of Purdue University, and is currently being employed in ongoing experiments in the Purdue Boeing/AFOSR Mach 6 Quiet Tunnel (BAM6QT) and at the Tennessee Aerothermodynamics Laboratory (TALon). A computational investigation on hypersonic high Reynolds number wing-elevon-cove flows was conducted with this model, where both corresponding experimental facility conditions were employed. At this time, the experimental data are limited; however, future experimental and computational collaboration is expected.</p> <p><br></p> <p>The motivation behind this research was to expand the knowledge on hypersonic wing-elevon-cove flows, gap heating, and the low-frequency unsteadiness in shock-wave/boundary-layer interactions. Therefore, the intended goal of this work was to provide an accurate characterization of the three hypersonic wing-elevon-cove flows. This was accomplished by using computational data to produce flowfield visualizations, analyze aerothermodynamic loadings, and conduct statistical flow analyses. The results on the three hypersonic wing-elevon-cove computations are presented, analyzed, and discussed throughout this dissertation.</p>

No description available.

In day to day life we encounter many different materials which are intermediate between crystalline solids and simple liquids that include paints , glues , suspensions, polymers, surfactants, food and cosmetic products and so on. ‘Soft condensed matter’ is an emerging field of science that aims to generalize the flow and various deformation mechanisms in this apparent diverse class of materials from a ‘mesoscopic’ point of view (important length scales for these systems is usually 10nm-1μm) where the actual atomic and molecular details governed by various quantum mechanical laws are not very important. These soft systems are held together by weaken tropic forces and therefore can be perturbed easily (the typical elastic modulus of these materials is many orders of magnitude lower compared to metallic solids). Moreover, very long relaxation times in these systems(∼10−3 to 1 s) have made them ideal candidates to study non-equilibrium physics. The present Thesis is an endeavor to understand linear and non-linear flow behavior and low Reynolds number instabilities in various soft matter systems like suspensions of flocculated carbon nanotubes and carbon black, surfactant gels, colloidal glasses, Langmuir monolayers etc probed mainly by bulk and interfacial rheology, in-situ light scattering, particle image velocimetry(PIV) techniques and Fourier transform rheology. We also use dynamic light scattering techniques for particle sizing and characterization of Brownian systems. Chapter 1 gives a general introduction to soft condensed matter, particularly, the important length and time scales, various interactions and the rich phase behavior emerged from the delicate balance between energy and entropy in these systems. In this context, We describe the detailed phase behavior of two such systems studied in this thesis. We next describe briefly a few important concepts which motivate the main problems studied in the present thesis like the shear-thickening in suspensions of Brownian and non-Brownian particles, non-equilibrium steady state fluctuation relations in driven systems, elasticity driven instabilities in complex fluids, jamming transitions and aging behavior. This is followed by a discussion of the experimental techniques like linear and nonlinear rheology, including the Fourier transform rheology. Chapter 2 discusses the experimental techniques used by us in detail. We first describe the different components and mode of operations of the MCR-300 stress-controlled rheometer (Paar Physica, Germany) and various experimental geometries. Next we discuss the set up for two dimensional rheological measurements. The homebuilt imaging set up for in-situ polarized light scattering and direct imaging studies is described along with the in-situ particle image velocimetry (PIV) to map out the exact spatially resolved velocity profiles in 2D systems. We give a brief account of the techniques of Fourier transform rheology. At the end of this chapter, we briefly describe the angle resolved dynamic light scattering (DLS) set up (Brookhaven Instruments, USA). In Chapter 3, we study colossal discontinuous shear-thickening transition in confined suspensions of fractal clusters formed by multi-wall carbon nanotubes (MWNT) by rheology and in-situ imaging experiments. Monotonic decrease in viscosity with increasing shear stress, known as shear thinning, is a known rheological response to shear flow in complex fluids in general and for flocculated suspensions in particular. In the present experiments we demonstrate a discontinuous shear thickening transition where the viscosity jumps sharply above a critical shear stress by four to six orders of magnitude in flocculated suspensions of MWNT even at very low weight fractions(∼0.5%). Rheo-optical observations reveal the shear-thickened state as a percolated structure of MWNT flocs spanning the system size. We present a dynamic phase diagram of the non-Brownian MWNT dispersions revealing a starting jammed state followed by shear-thinning and shear-thickened states. The present study further suggests that the shear-thickened state obtained as a function of shear stress is likely to be a generic feature of fractal clusters under flow, albeit under confinement. An understanding of the shear thickening phenomena in confined geometries is pertinent for flow controlled fabrication techniques in enhancing the mechanical strength and transport properties of thin films and wires of nanostructured composites as well as in lubrication issues. We try to understand the flow of jammed and shear-thickened states under constant applied strain rate by studying the building up and relaxation of individual stress fluctuation events similar to the flow in dense granular materials. We also characterize the metastable shear thickened states by superposing a small sinusoidal stress component on a steady applied stress as well as by studying the a thermal entropy consuming fluctuations which are also observed for other jammed systems under an applied steady shear stress as described in the next chapter. Chapter 4 reports the study of non-equilibrium fluctuations in concentrated gels and glassy systems(in jammed state), the nature of fluctuations and their systemsize dependence in the framework of fluctuation relation and Generalized Gumbel distribution. In the first part, we show that the shear rate at a fixed shear stress in a micellar gel in a jammed state exhibits large fluctuations, showing positive and negative values, with the mean shear rate being positive. The resulting probability distribution functions (PDFs) of the global power flux to the system vary from Gaussian to non-Gaussian, depending on the driving stress and in all cases show similar symmetry properties as predicted by Gallavotti-Cohen steady state fluctuation relation. The fluctuation relation allows us to determine an effective temperature related to the structural constraints of the jammed state. We have measured the stress dependence of the effective temperature. Further, experiments reveal that the effective temperature and the standard deviation of the shear rate fluctuations increase with the decrease of the systemsize. In the second part of this chapter, we report a universal large deviation behavior of spatially averaged global injected power just before the rejuvenation of the jammed state formed by an aging suspension of laponite clay under an applied stress. The probability distribution function (PDF) of these entropy consuming strongly non-Gaussian fluctuations follow an universal large deviation functional form described by the Generalized Gumbel (GG) distribution like many other equilibrium and non-equilibrium systems with high degree of correlations but do not obey Gallavotti-Cohen Steady State Fluctuation Relation (SSFR). However, far from the unjamming transition (for smaller applied stresses) SSFR is satisfied for both Gaussian as well as non-Gaussian PDF. The observed slow variation of the mean shear rate with system size supports a recent theoretical prediction for observing GG distribution. We also establish the universality of the observations reported in this chapter in the light of other jammed systems under shear. We examine in the first part of Chapter 5, the shear-thinning behavior of a two dimensional yield stress bearing monolayer of sorbitan tristearate at air/water interface. The flow curve (stress vs shear rate) consists of a linear region at low shear stresses/shear rates, followed by a stress plateau at higher values. The velocity profile obtained from particle imaging velocimetry indicates that shear banding occurs showing coexistence of fluidized region near the rotor and solid region with vanishing shear-rate away from the rotor. In the fluidized region, the velocity profile which is linear at low shear rates becomes exponential at the onset of shear-thinning, followed by a time varying velocity profile in the plateau region. At low values of constant applied shear rates, the viscosity of the film increases with time, thus showing aging behavior like in soft glassy three-dimensional (3D) systems. Further, at the low values of the applied stress in the yield stress regime, the shear-rate fluctuations in time show both positive and negative values, similar to that observed in sheared 3D jammed systems. By carrying out a statistical analysis of these shear-rate fluctuations, we estimate the effective temperature of the soft glassy monolayer using the Galavatti-Cohen steady state fluctuation relation. In the second part of this chapter, we study in detail the non-linear viscoelastic behavior of Langmuir monolayers. Under oscillatory shear usually observed in many 3D metastable complex fluids with large structural relaxation times. At large strain amplitudes(γ), the storage modulus (G”) decreases monotonically whereas the loss modulus (G”) exhibits a peak above a critical strain amplitude before it decreases at higher strain amplitudes. The power law decay exponents of G” and G” are in the ratio 2:1. The peak in G” is absent at high temperatures and low concentration of sorbitan tristearate. Strain-rate frequency sweep measurements on the monolayers do indicate a strain-rate dependence of the structural relaxation time. The present study on sorbitan tristearate monolayers clearly indicates that the nonlinear viscoelastic behavior in 2D Langmuir monolayers is very general and exhibits many of the features observed in 3D complex fluids. We report in the first part of Chapter 6 scattering dichroism experiments to quantify the spatio-temporal nematodynamics of shear-thinning worm like micellar gels of surfactant Cetyltrimethylammonium Tosylate (CTAT) in the presence of salt sodium chloride (NaCl) enroute to rheochaos. For shear rates past the plateau onset, we observe a presence of alternating bright and dark‘ intertwined’ birefringent structures along the vorticity direction. The orientational order corresponding to these structures are predominantly oriented at +45deg and−45deg to the flow (v) in the (v,∇v) plane. The orientational dynamics of the nematics especially at the interface between the structures, has a one-to-one correspondence with the temporal behavior of the stress. Experiments show that the spatial motion of the vorticity structures depend on the gap thickness of the Couette cell. We next discuss the random temporal flow behavior of this system at high values of applied shear rate/stress in the framework of elastic turbulence in the second part of this chapter. Here, we study the statistical properties of spatially averaged global injected power fluctuations for the worm-like micellar system described above. At sufficiently high Weissenberg numbers (Wi) the shear rate and hence the injected power p(t) at a constant applied stress shows large irregular fluctuations in time. The nature of the probability distribution function (PDF) of p(t) and the power-law decay of its power spectrum are very similar to that observed in recent studies of elastic turbulence for polymer solutions. Remarkably, these non-Gaussian pdf scan be well described by an universal large deviation functional form given by the Generalized Gumbel (GG) distribution observed in the context of spatially averaged global measures in diverse classes of highly correlated systems. We show by in-situ rheology and polarized light scattering experiments that in the elastic turbulent regime the flow is spatially smooth but random in time, in agreement with a recent hypothesis for elastic turbulence. In Chapter 7, we study the vorticity banding under large amplitude oscillatory shear (LAOS) in a dilute worm-like micellar gel formed by surfactant CTAT by Fourier transform rheology and in-situ polarized light scattering. Under LAOS we found the signature of a non-trivial order-disorder transition of Taylor vortices. In the non-linear regime, higher harmonicde composition of the resulting stress signal reveals that the third harmonic I3 shows a very prominent maximum at the strain value where the number density (nv) of the Taylor vortices is maximum for a wide range of angular frequencies both above and below the linear crossover point. Subsequent increase in applied strain results in distortions of the vortices and a concomitant decrease in nv when I3 also drops very sharply and acts like an order parameter for this order-disorder transition. We further quantify the transition by defining an independent order parameter like quantity from the spatial correlation function of the scattered intensity and equivalently its Fourier transform which essentially captures the non monotonous third harmonic behavior. Lissajous plots indicate an intra-cycle strain hardening for the values of γ corresponding to the peak of I3 similar to that observed for hard-sphere glasses. Our study is an important step forward to correlating the structures developed in the system under LAOS to the appearances of the higher harmonics in the non-linear regime. The Thesis concludes with a summary of the main results and a brief account on the scope of future work as described in Chapter 8.

Several types of instability can affect the interface between two fluids. For instance, a Rayleigh-Taylor instability (or density fingering) is encountered when a heavier fluid is placed upon a lighter one in the gravity field and double diffusive instabilities can be triggered by differential diffusivity of the different species present in the fluid. In this context our work aims to understand theoretically in which way a chemical reaction can induce and influence such instabilities in a fluid initially at rest. To understand the dynamics resulting from the coupling between chemical reactions and hydrodynamical instabilities we use chemical fronts as model systems. These fronts result from the coupling between autocatalytical chemical reactions and diffusion and they allow to create a self-organized interface between the products and the reactants. As during a chemical reaction the density may vary due to solutal and thermal effects, the products and the reactants can have different densities which may trigger convection movements leading to the destabilization of the fronts. We have in particular studied the influence of the exothermicity of the reaction on the fingering of chemical fronts, focusing first on the influence of heat losses through the walls of the set-up. These leaks have a marked influence on the dynamics because they affect the temperature profiles and hence the density profiles too. We have also classified the various types of instabilities that may appear dues to solutal and thermal effects. We have found a new type of hydrodynamic instability of statically stable fronts induced by the chemical reaction. We have furthermore analyzed an isothermal model with two chemical species. If they diffuse at different rates the front can be subject to diffusive instabilities as well. We have shown that the coupling between such a diffusive instability and fingering can trigger complex dynamics. We have eventually studied the influence of an external electric field on the diffusive instabilities and on fingering underlying the possibility to destabilize otherwise stable fronts./ Différents types d'instabilités hydrodynamiques peuvent affecter les interfaces entre deux fluides comme par exemple, une instabilité de Rayleigh-Taylor (ou digitation de densité) quand un fluide plus dense se trouve placé au-dessus d'un fluide moins dense dans le champ de gravité ou des instabilités de double diffusion induites par des différences entre les diffusivités d'un soluté et de la chaleur contenus dans les fluides. Dans ce contexte, notre thèse s'attache à comprendre de manière théorique comment une réaction chimique peut influencer ces instabilités voire les générer dans un fluide initialement au repos. Pour étudier les dynamiques résultant du couplage entre réactions chimiques et instabilités hydrodynamiques, nous utilisons des systèmes modèles: les fronts chimiques de conversion résultant de la compétition entre réactions chimiques autocatalytiques et diffusion créant une interface auto-organisée entre les réactifs et les produits. Comme au cours d'une réaction chimique la densité peut varier par des effets solutaux et thermiques, les produits et les réactifs de densités différentes peuvent générer des mouvements de convection qui conduisent à la déstabilisation des fronts. Nous avons en particulier étudié l'influence de l'exothermicité de la réaction sur les instabilités de digitation de fronts chimiques, en nous focalisant dans un premier temps sur l'influence des pertes de chaleur par les parois du réacteur. Ces fuites ont un effet marqué sur les instabilitités car elles affectent les profils de température et donc les profils de densité dans le système. Nous avons également classifié les différentes instabilités qui peuvent apparaître via des changements de densité dûs à des effets thermiques et solutaux et mis en évidence un nouveau type de déstabilisation hydrodynamique de fronts statiquement stables induit par une réaction chimique. Nous avons ensuite analysé un modèle isotherme impliquant deux espèces chimiques. Si ces dernières diffusent a des vitesses différentes le front peut être sujet à une instabilité diffusive. Nous avons montré qu'un couplage entre une telle instabilité diffusive et de la digitation peut être à l'origine de dynamiques complexes. Nous avons ensuite considéré l'influence d'un champ électrique sur les instabilité diffusives et de digitation en soulignant la possibilié de déstabiliser via ce champ des fronts initialement stables.

Approved for public release, distribution is unlimited / Inter-domain routing connects individual pieces of Internet topology, creating an integral, global data delivery infrastructure. Currently, this critical function is performed by the Border Gateway Protocol (BGP) version 4 [RFC1771]. Like all routing protocols, BGP is vulnerable to instabilities that reduce its effectiveness. Among the causes of these instabilities are those which are maliciously induced. Although there are other causes, e.g., natural events and network anomalies, this thesis will focus exclusively on maliciously induced instabilities. Most current models that attempt to predict a BGP routing instability confine their focus to either macro- or micro-level metrics, but not to both. The inherent limitations of each of these forms of metric gives rise to an excessive rate of spurious alerts, both false positives and false negatives. It is the original intent of this thesis to develop an improved BGP instability prediction model by statistically combining BGP instability metrics with user level performance metrics. The motivation for such a model is twofold. 1) To provide sufficient prior warning of impending failure to facilitate proactive protection measures. 2) To improve warning reliability beyond existing models, by demonstrably reducing both false positives and false negatives. However, our analysis of actual network trace data shows that a widely used BGP instability metric, the total number of update messages received in a time period, is not a good indicator of future user level performance. / Civilian, Department of Defense

Les réglementations toujours plus drastiques sur les émissions de polluants ont conduit au développement de systèmes de combustion opérant en régimes pauvres qui sont malheureusement sujet aux instabilités thermo acoustiques. La capacité de la Simulation aux Grandes Echelles (SGE) à simuler des turbines à gaz industrielles complexes de grande puissance est mise en évidence au cours de ce travail de thèse. Tout d’abord, la SGE est appliquée à un brûleur académique et validée par comparaison à des mesures effectuées à l’Université de Berlin ainsi qu’à des simulations SGE effectuées avec OpenFOAM chez Siemens. Afin de déterminer la stabilité de ce bruleur le couplage entre l’acoustique et la combustion est modélisé par l’approche de type fonction de transfert de flamme (FTF). Suite à ces calcules et l’évaluation de la FTF les fluctuations du nombre de swirl sont identifiées comme un paramètre à même de modifier cette réponse de flamme. Après cette première étape de validation, une turbine à gaz industrielle est simulée en SGE pour deux géométries différentes du brûleur et pour deux points de fonctionnement. La FTF issue de ces calculs est peu influencée par les deux points de fonctionnement. A l’inverse, des légères modifications de la géométrie du swirler modifient les caractéristiques de la FTF montrant que plusieurs mécanismes sont en jeu. Ces mécanismes sont identifiés comme étant la vitesse d’entrée, les fluctuations de swirl et les fluctuations de fraction de mélange. Cette dernière est causée par: 1) la pulsation du débit de carburant injecté et 2) la trajectoire fluctuante des jets de carburant. Bien que le swirler soit conçu pour fournir un mélange le plus homogène possible, d’importantes hétérogénéités de mélange à l’entrée de la chambre de combustion sont présentes. Les perturbations de mélange se combinent avec les fluctuations de vitesse (et donc avec les fluctuations de swirl) aboutissant à des résultats de FTF différents. Un modèle étendu pour la FTF reliant le dégagement de chaleur à la vitesse d’entrée et à la fluctuation de fraction de mélange (modèle MISO) se révèle être une bonne solution pour ces systèmes complexes. Une analyse non linéaire montre en outre que l’amplitude de forçage conduit non seulement à une saturation de la flamme, mais aussi à un changement de la réponse de flamme. La saturation de la flamme n’est vérifiée que pour la FTF globale et le gain augmente localement avec une amplitude croissante. Pour ce système on notera enfin que la flamme linéaire, comme la flamme non linéaire, ne sont pas compactes: certaines zones pourtant situées l’une à coté de l’autre, ont des différences significatives de délai de FTF, montrant que certaines parties de la flamme amortissent l’excitation alors que d’autres l’amplifient. / Modern pollutant regulation have led to a trend towards lean combustion systems which are prone to thermo-acoustic instabilities. The ability of Large Eddy Simulation (LES) to handle complex industrial heavy-duty gas turbines is evidenced during this thesis work. First, LES is applied to an academic single burner in order to validate the modeling against measurements performed at TU Berlin and against OpenFoam LES simulations done at Siemens. The coupling between acoustic and combustion is modeled with the Flame Transfer Function (FTF) approach and swirl number fluctuations are identified changing the FTF amplitude response of the flame. Then, an industrial gas turbine is analyzed for two different burner geometries and operating conditions. The FTF is only slightly influenced for the two operating points but slight modifications of the swirler geometry do modify the characteristics of the FTF showing that a simple model taking only into account the flight time is not appropriate and additional mechanisms are at play. Those mechanisms are identified being the inlet velocity, the swirl and the inlet mixture fraction fluctuations. The latter is caused by two mechanisms: 1) the pulsating injected fuel flow rate and 2) the fluctuating trajectory of the fuel jets. Although the diagonal swirler is designed to provide good mixing, effects of mixing heterogeneities at the combustion chamber inlet occur. Mixture perturbations phase with velocity (and hence with swirl) fluctuations and combine with them to lead to different FTF results. Another FTF approach linking heat release to inlet velocity and mixture fraction fluctuation (MISO model) shows further to be a good solution for complex systems. A nonlinear analysis shows that the forcing amplitude not only leads to a saturation of the flame but also to changes of the delay response. Flame saturation is only true for the global FTF and the gain increases locally with increasing forcing amplitude. Both, the linear and the nonlinear flames, are not compact: flame regions located right next to each other exhibited significant differences in delay meaning that at the same instant certain parts of the flame damp the excitation while others feed it.

L’objectif de ce travail est de concevoir un outil d’atomisation adapté à la production de poudres de carburede tungstène dans l’entreprise Technogenia, située à Saint-Jorioz (74). Plus généralement, nous avons étudiéune nouvelle conception d’atomiseur rotatif pour lequel les surfaces d’atomisation sont orientées selon un angled’incidence non nul. Une telle orientation permet alors d’exploiter de façon plus efficace la fréquence de rotationde l’atomiseur ainsi que les forces de centrifugation et de Coriolis. A partir de cette base de réflexion, troisgéométries de surface d’atomisation ont été étudiées : des surfaces lisses, des surfaces pourvues de structurationsde long de leur bord de fuite et des surfaces matricées de perforations.Les différents modes de déstabilisation des écoulements liquides ont été observés grâce à l’acquisition de vidéosultra-rapides des différentes étapes de l’impact d’une goutte unique sur les différentes géométries de surfacesd’atomisation étudiées. Les processus d’atomisation peuvent ainsi varier entre la rupture de jets liquides régulierscausée par des instabilités de type Rayleigh-Plateau et la rupture de nappes liquides causée par des instabilitésde type Rayleigh-Taylor et/ou l’initiation de perforations au coeur des nappes et films liquides. Les vidéos dessprays aqueux ainsi que l’observation au microscope des poudres métalliques nous permettent de caractériser lessprays produits. Sur l’ensemble de nos expériences nous observons que la finesse de l’atomisation s’améliore àmesure que la fréquence de rotation augmente ; ce qui constitue l’objectif prioritaire du procédé développé dans lathèse. Cependant, les surfaces texturées, certes compatibles avec les liquides classiques (aqueux ou organiques),ne le sont plus avec les contraintes induites par du métal à haute température de fusion, tel le carbure detungstène fondu objet de la thèse. En conséquence, seules les surfaces lisses sont aujourd’hui retenues et fontl’objet de développements dans le cadre de la production de poudres métalliques au sein de Technogenia. / The aim of this work is to design a spray tool for the production of tungsten carbide powder for the companyTechnogenia, located in Saint-Jorioz (74). Thus, we have studied a new design of rotary atomizer based onatomization surface oriented at a non-zero incidence angle. Such an orientation can then allow to use moreeffectively the rotation frequency of the atomizer and the centrifugal and Coriolis forces. From this base reflection,three geometries of atomization surface were studied : smooth surfaces, surfaces with serrated structurationsalong their trailing edge and multi-perforated surfaces.The different destabilization modes of liquid flows were observed through high-speed videos of the different stagesof the single drop impact on each atomization surfaces. The atomization process can thus vary between liquid jetbreakup caused by Rayleigh-Plateau instabilities and liquid sheet breakup caused by Rayleigh-Taylor instabilitiesand/or initiating of holes in liquid films or sheets. Videos of aqueous sprays and microscopic observation of metalpowders allow us to characterize sprays. On all of our experiments, we observe that the atomisation gets better(producing finer spray), as the frequency of rotation increases. Although the textured surfaces are compatiblewith usual liquids (aqueous or organical), they can’t stand the constraints imposed by metal with a high meltingtemperature such as tungsten carbide, the object of this thesis. Therefore, only the smooth surfaces are nowthe subject of an industrial development to produce metallic powder with Technogenia company.

<div>In order to better understand and quantify the effect of instabilities in systems utilizing flow boiling heat transfer, the present study explores dynamic results for pressure drop, mass velocity, thermodynamic equilibrium quality, and heated wall temperature to ascertain and analyze the dominant modes in which they oscillate. Flow boiling experiments are conducted for a range of mass velocities with both subcooled and saturated inlet conditions in vertical upflow, vertical downflow, and horizontal flow orientations. High frequency pressure measurements are used to investigate the influence of individual flow loop components (flow boiling module, pump, pre-heater, condenser, etc.) on dynamic behavior of the fluid, with fast Fourier transforms of the same used to provide critical frequency domain information. Conclusions from this analysis are used to isolate instabilities present within the system due to physical interplay between thermodynamic and hydrodynamic effects. Parametric analysis is undertaken to better understand the conditions under which these instabilities form and their impact on system performance. Several prior stability maps are presented, with new stability maps provided to better address contextual trends discovered in the present study.</div><div>Further, this study utilizes experimental results for vertical upflow boiling of FC-72 in a rectangular channel with finite inlet quality to investigate Density Wave Oscillations (DWOs) and assess their potential impact on design of two-phase systems for future space missions. High-speed flow visualization image sequences are presented and used to directly relate the cyclical passage of High and Low Density Fronts (HDFs and LDFs) to dominant low-frequency oscillations present in transient pressure signals commonly attributed to DWOs. A methodology is presented to determine frequency and amplitude of DWO induced pressure oscillations, which are then plotted for a wide range of relevant operating conditions. Mass velocity (flow inertia) is seen to be the dominant parameter influencing frequency and amplitude of DWOs. Amplitude of pressure oscillations is at most 7% of the time-averaged pressure level for current operating conditions, meaning there is little risk to space missions. Reconstruction of experimental pressure signals using a waveform defined by frequency and amplitude of DWO induced pressure fluctuations is seen to have only moderate agreement with the original signal due to the oversimplifications of treating DWO induced fluctuations as perfectly sinusoidal in nature, assuming they occur at a constant frequency value, and neglecting other transient flow features. This approach is nonetheless determined to have potential value for use as a boundary condition to introduce DWOs in two-phase flow simulations should a model be capable of accurately predicting frequency and amplitude of oscillation.</div><div>Additionally, this study presents a new mechanistic model for Density Wave Oscillations (DWOs) in vertical upflow boiling using conclusions drawn from analysis of flow visualization images and transient experimental results as a basis from which to begin modeling. Counter to many prior studies attributing DWOs to feedback effects between flow rate, pressure drop, and flow enthalpy causing oscillations in position of the bulk boiling boundary, the present instability mode stems primarily from body force acting on liquid and vapor phases in a separated flow regime leading to liquid accumulation in the near-inlet region of the test section, which eventually departs and moves along the channel, acting to re-wet liquid film along the channel walls and re-establish annular, co-current flow. This process was modeled by dividing the test section into three distinct control volumes and solving transient conservation equations for each, yielding predictions of frequencies at which this process occurs as well as amplitude of associated pressure oscillations. Values for these parameters were validated against an experimental database of 236 FC-72 points and show the model provides good predictive accuracy and capably captures the influence of parametric changes to operating conditions.</div><div>Also, this study shows analysis of pressure signals in condensing systems reveal the presence of relevant oscillatory phenomena during flow condensation as well, which may impact performance in applications concerned with precise system control. Towards this end, the present study presents results for oscillatory behavior observed in pressure measurements during flow condensation of FC-72 in a smooth circular tube in vertical upflow, vertical downflow, and horizontal flow orientations. Dynamic behavior observed within the test section is determined to be independent of other components within the flow loop, allowing it to be isolated and interpreted as resulting from physical aspects of two-phase flow with condensation. The presence of a peak oscillatory mode (one of significantly larger amplitude than any others present) is seen for 72% of</div><div>vertical upflow test cases, 61% of vertical downflow, and 54% of horizontal flow. Relative intensities of this peak oscillatory mode are evaluated through calculation of Q Factor for the corresponding frequency response peak. Frequency and amplitude of peak oscillatory modes are also evaluated. Overall, vertical upflow is seen to exhibit the most significant oscillatory behavior, although in its maximum case amplitude is only seen to be 7.9% of time-averaged module inlet pressure, indicating there is little safety risk posed by oscillations under current operating conditions. Flow visualization image sequences for each orientation are also presented and used to draw parallels between physical characteristics of condensate film behavior under different operating conditions and trends in oscillatory behavior detected in pressure signals</div><div>Further, the present work outlines a new methodology utilizing temperature and pressure measurements to identify condensation flow regimes. For vertical upflow condensation, amplitude of dynamic temperature and pressure oscillations are shown to clearly indicate transition from counter-current flow regimes (i.e., falling film, oscillating film, flooding) to annular, co-current flow (climbing film flow regime). In horizontal flow condensation, standard deviation between multiple thermocouple measurements distributed around the tube circumference was calculated at all axial (stream-wise) measurement locations. High values of standard deviation are present for stratified flow (stratified flow, wavy-stratified, plug flow), while axisymmetric flow regimes (i.e., slug flow, annular flow) yield significantly lower values. Successful development of this technique represents a valuable contribution to literature as it allows condensation flow regime to be identified without the often-costly restriction of designing a test section to allow optical access. Identified flow regimes in both vertical upflow and horizontal flow orientations are compared to regime maps commonly found in the literature in pursuit of optimum performing maps.</div><div>Finally, the present study aims to better analyze the influence of body force on flow condensation heat transfer by conducting tests at multiple orientations in Earth’s gravity. Dielectric FC-72 is condensed in a smooth stainless-steel tube with 7.12 mm diameter and 574.55 mm condensing length by counterflow of cooling water across the outer surface of the tube. Test conditions span FC-72 mass velocities of 50.3 – 360.3 kg/m2s, test section inlet pressures of 127.0 – 132.1 kPa, and test section inlet thermodynamic equilibrium qualities of 0.13 – 1.15. A subset of data gathered corresponding to axisymmetric, annular condensation heat transfer is identified and a detailed methodology for data reduction to calculate heat transfer coefficient presented. Uncertainty analysis is also presented and indicates channel average heat transfer coefficients are calculated within ±3.6% to ±26.7% (depending on operating conditions). Analysis of parametric trends for condensation heat transfer reveals the dominant influence of mass velocity (flow inertia), secondary influence of vapor mass fraction (thermodynamic equilibrium quality), and strong dependence on orientation (body force) at low mass velocities. At higher mass velocities results for all orientations investigated begin to converge, indicating body force independent annular condensation heat transfer is achieved. Separated Flow Model predictions of vertical downflow condensation heat transfer provide reasonable agreement with experimental results, evidence by a Mean Absolute Error (MAE) of 31.2%. Evaluation of condensation heat transfer correlations for horizontal flow reveal most correlations struggle for cases with high liquid content. Specific correlations are identified for superior accuracy in predicting the measured data.</div>

Le procédé d'hydroformage est largement utilisé dans les industries automobile et aéronautique. L'optimisation déterministe a été utilisée pour le contrôle et l'optimisation du procédé durant la dernière décennie. Cependant,dans des conditions réelles, différents paramètres comme les propriétés matériaux,les dimensions géométriques, et les chargements présentent des aléas qui peuvent affecter la stabilité et la fiabilité du procédé. Il est nécessaire d'introduire ces incertitudes dans les paramètres et de considérer leur variabilité. L'objectif principal de cette contribution est l'évaluation de la fiabilité et l'optimisation du procédé d'hydroformage en présence d'incertitudes.La première partie de cette thèse consiste à proposer une approche générale pour évaluer la probabilité de défaillance spatiale du procédé d'hydroformage, principalement dans les régions critiques. Avec cette approche, il est possible d'éviter les instabilités plastiques durant une opération d'hydroformage. Cette méthode est basée sur des simulations de Monte Carlo couplée avec des métamodèles. La courbe limite de formage est utilisée comme critère de défaillance pour les instabilités plastiques potentielles.La seconde partie de cette thèse est l'optimisation avec prise en compte d'incertitudes dans le procédé d'hydroformage. En utilisant des exemples illustratifs, on montre que l'approche probabiliste est une méthode efficace pour l'optimisation du procédé pour diminuer la probabilité de défaillance et laisser le procédé insensible ou peu sensible aux sources d'incertitudes. La difficulté est liée à la considération des contraintes fiabilistes qui nécessitent d'énormes efforts de calcul et impliquent des problèmes numériques classiques comme la convergence, la précision et la stabilité. Pour contourner ce problème, la méthode de surface de réponse couplée à des simulations Monte Carlo est utilisée pour évaluer les contraintes probabilistes.L'approche probabiliste peut assurer la stabilité et la fiabilité du procédé et minimise considérablement le pourcentage des pièces défectueuses. Dans cette partie, deux méthodes sont utilisées : l'optimisation fiabiliste et l'optimisation robuste.La dernière partie consiste à optimiser le procédé avec une stratégie Multi-Objectif(MO) avec prise en compte d'incertitudes. Le procédé d'hydroformage est un problème MO qui consiste à optimiser plus d'une performance simultanément.L'objectif principal est d'étudier l'évolution du front de Pareto lorsque des incertitudes affectent les fonctions objectifs ou les paramètres. Dans cette partie, on propose une nouvelle méthodologie qui présente les solutions dans un nouvel espace et les classifie suivant leurs probabilités de défaillances. Cette classification permet d'identifier la meilleure solution et fournit une idée sur la fiabilité de chaque solution. / Hydroforming process is widely used in automotive and aerospace industries. Deterministic design optimization have been used to control and optimize this process in the last decade. However, under realistic conditions, different parameters such as material properties, geometric dimensions, and load exhibits unavoidable scatter that can affect the stability and the reliability of the process.It is interesting to introduce the uncertainties in parameter and to consider their variability. The main objective of this contribution is to evaluate the reliability and optimization of the hydroforming process in the presence of uncertainties.The first part of this thesis proposes a general approach to evaluate the spatial probability of failure in hydroforming process mainly in the critical region. With the proposed approach it is possible to avoid failure during hydroforming process.This method is based on Monte Carlo simulation coupled with metamodels, the forming limit curve is used as failure criteria for potential plastic instabilities.The second part of this thesis is the optimisation of the hydroforming process under uncertainties. Using illustrative examples, it is shown that probabilistic approach is an efficient method to optimize the process, to decrease the probability of failure and make the process insensitive or less sensitive to sources of variability. The difficulty lies in the considerations of the reliability constraints, which require a large computational effort and involve classical numerical problems, such as convergence,accuracy and stability. To overcome this problem, response surface method with Monte Carlo simulations were used to evaluate the probabilistic constraints.Probabilistic approach can ensure a stable and reliable process and decrease the percentage of defects parts significantly. Through this part, two methods were used : Reliability-Based Design Optimization and robust optimization.The last part consists of optimizing the process with Multi-Objective (MO) strategy taking account of the uncertainty. Metal forming process is MO problem that consists of optimizing more than one performance simultaneously. The main goal isto study the evolution of the Pareto front when some uncertainties can affect the objective functions or the parameters. In this part, a new methodology is proposed which presents the solutions in a new space and classify the whole solution with their probability of failure. This classification allows to identify the best solutionand can provide an idea about the reliability of each solution.