Cette thèse est consacrée à l’étude mathématique des écoulements complexes dans des tubes minces. Les difficultés ne sont pas seulement liées à la rhéologie complexe, mais aussi aux conditions au bord sur la pression en entrée et en sortie (qui sont moins habituelles, mais réalistes du point de vue physique). Dans une première partie, des écoulements quasi-newtoniens stationnaires sont étudiés. D’abord, on utilise la petitesse du domaine pour montrer l’existence de la solution. Ensuite, on écrit un développement asymptotique de cette solution et on calcule formellement ses coefficients. Finalement, on justifie rigoureusement la validité de ce développement en démontrant des estimations d’erreur. Dans une deuxième partie, on considère des écoulements de fluides visco-élastiques décrits par la loi d’Oldroyd en régime stationnaire. Le modèle que nous avons choisi contient un terme diffusif en contrainte, dont l’ordre de grandeur est lié à la petitesse du domaine. Similairement à la première partie, un développement asymptotique est complètement justifié du point de vue mathématique. Dans le cas particulier de domaines axisymétriques une solution numérique est cherchée afin de la comparer à la solution obtenue via la technique asymptotique. Dans une dernière partie, on étudie les équations de Navier-Stokes non stationnaires. Un résultat d’existence des solutions fortes pour des données petites est démontré. Malheureusement, la méthode directe ne nous a pas permis pas d’avoir suffisamment de contrôle par rapport à la petitesse du domaine. Pour obtenir le résultat désiré, on utilise l’approche à la Kato, basé sur la théorie de C0 semigroupes. / This thesis is devoted to the mathematical analysis of complex flows in thin pipes. The difficulties stem not only from the complex rheology, but also from the boundary conditions used involving the pressure (which are rather atypical, but realistic from a physical point of view).In the first part, we study stationary, quasi-newtonian flows. The existence of a solution is shown using the smallness of the domain as a key ingredient. Furthermore, an asymptotic expansion of this solution is sought and its coefficients are formally computed. Lastly, the validity of this expansion is rigorously justified by proving error estimates. In the second part, we consider visco-elastic flows represented by Oldroyd’s law in stationary regime. The model which we have chosen contains a diffusive stress term, whose order of magnitude is related to the smallness of the domain. Similarly to the first part, a complete asymptotic expansion in mathematically justified. For the special case of axisymmetric domains a numerical solution is sought in order to compare it against the one obtained via the asymptotic technique. In the last part we study the non stationary Navier-Stokes equations. An existence result of strong solutions for small initial data is proven. Unfortunately, the direct method – based on energy estimates – doesn’t give us an optimal control of the smallness constant with respect to the size of the domain. To obtain the desired result, we employ the method of C 0 semigroups of linear operators.

L'interaction dynamique sol-structure a été largement explorée en supposant le comportement linéaire du sol. Néanmoins, pour des séismes d'intensité modérée à forte, la contrainte de cisaillement maximale peut facilement atteindre la limite élastique du sol. Du point de vue de l'interaction sol-structure, les effets non linéaires peuvent modifier la rigidité du sol à la base de la structure ainsi que la quantité d'énergie dissipée dans le sol. En conséquence, ignorer les caractéristiques non linéaires du sol dans l'interaction dynamique sol-structure (IDSS) peut conduire à des prédictions erronées de la réponse de la structure. Le but de ce travail est d'implémenter dans un code numérique une loi de comportement non linéaire pour le sol afin d'examiner l'effet de la nonlinéarité du sol sur l'interaction dynamique sol-structure. De plus, différents aspects sont pris en compte tels que l'effet de la contrainte de confinement sur le module de cisaillement du sol, les conditions statiques initiales, les conditions d'interface entre le sol et la structure, etc. Durant ce travail, une méthode simple de couche absorbante basée sur une formulation de Rayleigh / Caughey pour l'amortissement, qui est généralement disponible dans les logiciels existants d'éléments finis, a également été développée. Les conditions de stabilité des problèmes de propagation d'onde sont étudiées et on montre que les comportements linéaire et non linéaire sont très différents en ce qui concerne la dispersion numérique. La règle habituelle de 10 points par longueur d'onde, recommandée dans la littérature pour les milieux élastiques, apparaît pas suffisante dans le cas non linéaire.Le modèle implémenté est d'abord vérifié numériquement en comparant les résultats avec ceux d'autres codes numériques connus. Après cela, une étude paramétrique est menée pour différents types de structures et des profils de sol variés afin de caractériser les effets non linéaires. Différentes caractéristiques de l'IDSS sont comparées à celles du cas linéaire: modification de l'amplitude et du contenu fréquentiel des ondes se propageant dans le sol, fréquence fondamentale, dissipation de l'énergie dans le sol et réponse du système sol-structure. A travers ces études paramétriques nous montrons qu'en fonction des propriétés du sol, le contenu fréquentiel de la réponse du sol peut changer significativement à cause des nonlinéarités de comportement. Les pics de la fonction de transfert entre le champ libre et le rocher affleurant se décalent vers les basses fréquences et l'amplification se produit dans cette gamme de fréquences. Une réduction de l'amplification pour les hautes fréquences et même une dé-amplification peuvent se produire pour un fort niveau des mouvements d'entrée. Ces changements influencent la réponse de la structure. Ce travail montre également que la proximité des fréquences fondamentales de la structure et du sol influence fortement l'interaction sol-structure. Enfin, l'effet du poids de la structure et du balancement de la superstructure peut être significatif. Finalement, le bassin de Nice est utilisé comme un exemple de propagation d'onde dans un milieu non linéaire hétérogène et d'interaction dynamique sol-structure. La réponse du bassin dépend fortement de la combinaison de la nonlinéarité du sol, des effets topographiques et du contraste d'impédance entre les couches de sol. Pour les structures et les profils de sol sélectionnés dans ce travail, les simulations numériques réalisées montrent que le décalage de la fréquence fondamentale n'est pas un bon indicateur pour distinguer le comportement linéaire du sol du comportement non linéaire / The interaction of the soil with the structure has been largely explored the assumption of material and geometrical linearity of the soil. Nevertheless, for moderate or strong seismic events, the maximum shear strain can easily reach the elastic limit of the soil behavior. Considering soil-structure interaction, the nonlinear effects may change the soil stiffness at the base of the structure and therefore energy dissipation into the soil. Consequently, ignoring the nonlinear characteristics of the dynamic soil-structure interaction (DSSI) this phenomenon could lead toerroneous predictions of structural response. The goal of this work is to implement a fully nonlinear constitutive model for soils into anumerical code in order to investigate the effect of soil nonlinearity on dynamic soil structureinteraction. Moreover, different issues are taken into account such as the effect of confining stress on the shear modulus of the soil, initial static condition, contact elements in the soil-structure interface, etc. During this work, a simple absorbing layer method based on a Rayleigh / Caughey damping formulation, which is often already available in existing. Finite Element softwares, is also presented. The stability conditions of the wave propagation problems are studied and it is shown that the linear and nonlinear behavior are very different when dealing with numerical dispersion. It is shown that the 10 points per wavelength rule, recommended in the literature for the elastic media is not sufficient for the nonlinear case. The implemented model is first numerically verified by comparing the results with other known numerical codes. Afterward, a parametric study is carried out for different types of structures and various soil profiles to characterize nonlinear effects. Different features of the DSSI are compared to the linear case : modification of the amplitude and frequency content of the waves propagated into the soil, fundamental frequency, energy dissipation in the soil and the response of the soil-structure system. Through these parametric studies we show that depending on the soil properties, frequency content of the soil response could change significantly due to the soil nonlinearity. The peaks of the transfer function between free field and outcropping responsesshift to lower frequencies and amplification happens at this frequency range. Amplificationreduction for the high frequencies and even deamplication may happen for high level inputmotions. These changes influence the structural response.We show that depending on the combination of the fundamental frequency of the structureand the the natural frequency of the soil, the effect of soil-structure interaction could be significant or negligible. However, the effect of structure weight and rocking of the superstructurecould change the results. Finally, the basin of Nice is used as an example of wave propagation ona heterogeneous nonlinear media and dynamic soil-structure interaction. The basin response isstrongly dependent on the combination of soil nonlinearity, topographic effects and impedancecontrast between soil layers. For the selected structures and soil profiles of this work, the performed numerical simulations show that the shift of the fundamental frequency is not a goodindex to discriminate linear from nonlinear soil behavior

Lors de séismes forts, la propagation des ondes sismiques dans les sols met en jeu des non linéarités de comportement qui se manifestent différemment selon le niveau de sollicitation. En effet, pour de faibles déformations (généralement <10^{-6}), une loi de comportement linéaire (i.e. module et amortissement indépendants du niveau de sollicitation) permet de reproduire les observations expérimentales sur site. En revanche, pour des déformations plus élevées, une loi de comportement non linéaire hystérétique est nécessaire pour décrire l'évolution de la rigidité et des dissipations énergétiques au cours de la sollicitation sismique. De plus, comme les séismes forts sont caractérisés par des amplitudes et des durées plus importantes, le rôle de la pression interstitielle ne peut pas être négligé pour les sols saturés sous fortes sollicitations (mobilité cyclique et liquéfaction). Ces phénomènes peuvent conduire à l'annulation des contraintes effectives et devenir cause de dommages sévères pour les structures et les ouvrages. L'analyse proposée élargit l'applicabilité des modèles de calcul actuels pour une analyse plus fine du risque sismique. En partant d'une formulation aux éléments finis décrivant la propagation des ondes sismiques suivant la direction verticale en prenant en compte le chargement en 3D (l'approche « 1D-3C» : une direction-trois composantes) dans les sols nonlinéaires secs, de nouvelles stratégies pour la prise en compte du rôle de l'eau sont développées. Le modèle de comportement est basé sur la relation entre la pression interstitielle et le travail de la contrainte de cisaillement. Ce modèle décrivant l'évolution de la pression interstitielle considère l'état de contrainte tridimensionnel du matériau. Le modèle est validé par comparaison avec des résultats expérimentaux. L'approche « 1D-3C » a été utilisée pour modéliser la réponse des sols pour 4 séismes réels : le séisme de Superstition Hills en 1987 aux États-Unis (M_{w}=6.7); le séisme de Tohoku en 2011 au Japon (M_{w}=9.1 ); le séisme de Kushiro en 1993 au Japon (M_{w}=7.8) et le séisme d'Emilie Romagne en 2012 en Italie (M_{w}=5.9). Pour les trois premiers séismes, des enregistrements en profondeur et en surface sont disponibles. L'étude de ces trois premiers cas rend possible la validation du modèle par comparaison des données enregistrées et calculées. Le modèle peut donc être considéré comme un outil fiable pour la prédiction de la réponse sismique des sols saturés / During strong earthquakes, the seismic wave propagation in soils involves nonlinear behaviors strongly depending on the strain level. Indeed, for small strain (typically <10^{-6}), a linear constitutive law (modulus and damping independent on the load level) can reproduce the experimental observations on site. However, for larger strains, a nonlinear hysteretic constitutive law is needed to describe the evolution of stiffness and energy dissipation during seismic loading. In addition, as strong earthquakes are characterized by larger amplitudes and durations, the role of pore pressure cannot be neglected for saturated soils. Indeed pore water pressure controls phenomena such as cyclic mobility and liquefaction due to the loss of soil strength. This can lead to a fast decrease of effective stresses and permanent deformations in the soil causing severe damage to structures. This work extends the applicability of existing calculation models for a more detailed analysis of seismic risk. Starting from a FEM approach describing the propagation of seismic waves in the vertical direction, taking into account 3D loading (so-called "1D-3C" approach: 1 direction - 3 components) in nonlinear dry soils, new strategies to consider the role of water are developed. The model is based on the relationship between the pore pressure and the shear work. The three-dimensional stress state of the material is considered. The model is validated by comparison with experimental results. The "1D-3C" approach was used to model the response of soils for four real earthquakes: the Superstition Hills earthquake in 1987 in the United States (M_{w}=6.7), the Tohoku earthquake in 2011 in Japan (M_{w}=9.1), the Kushiro earthquake in Japan in 1993 (M_{w}=7.8) and the Emilia Romagna earthquake in Italy in 2012 (M_{w}=5.9). For the first three earthquakes, records at depth and on the surface are available. The study of the first three cases makes possible the validation of the model by comparing the calculated accelerations on the surface with the available records. The model can then be considered as an advanced tool for the prediction of the seismic soil response

Un nouveau logiciel CFD, LaBS, basé sur la méthode de lattice Boltzmann sur Réseau a été développé dans le cadre d'un projet entre universités et industries. LaBS est utilisé pour la simulation numérique des écoulements thermiques avec un nouveau modèle de frontière immergée pour les conditions limites thermiques. Ce modèle est basé sur la méthode de reconstruction de la fonction de distribution et est évalué pour des conditions limites coincidentes et non-coincidentes avec le maillage, sur le phénomène de diffusion thermique et de convection naturelle.Renault s'intéresse aux situations d'arrêt péage ou de contact coupé, pour lesquelles sont considérés un véhicule roulant à une vitesse soutenue, sur une autoroute par exemple, et qui subit un arrêt ou un ralentissement brutal (avec ou sans contact coupé).Dans ce genre de situation le refroidissement du compartiment moteur qui était assuré par le phénomène de convection forcé durant le roulage laisse place au phénomène de convection naturelle durant les phases de base vitesse ou de vitesse nulle.Le phénomène de convection naturelle est un phénomène lent, qui peut prendre plusieurs minutes à évacuer la chaleur accumulée dans le compartiment moteur. La présence de température élevée pendant une durée trop importante dans le compartiment moteur peut endommager certains composants qui possèdent des seuils de température critique.Pour anticiper ce problème de surchauffe du compartiment moteur, dans lequel un grand nombre de pièces à géométries complexes sont présentes, le phénomène de convection naturelle est étudié avec le nouveau modèle de frontière immergée thermique.%Ce modèle est d'abord testé sur des cas test académique pour validation et est ensuite appliqué au cas d'une voiture réelle.La modélisation des écoulements thermiques avec la méthodes de lattice Boltzmann sur Réseau (LBM) peut-être classée en trois catégories: l'approche multi-vitesse, l'approche hybride et l'approche à deux fonctions de distribution (DDF: Double-Distribution-Function).L'approche multi-vitesse, utilise une équation pour résoudre le champ de vitesse, de densité et de température qui sont résolus avec la LBM. Tandis que l'approche hybride et l'approche DDF utilise un jeux de deux équations, un pour résoudre le champ de vitesse et de densité et l'autre pour résoudre le champ de température.L'approche hybride résout le champ de vitesse et de densité avec la LBM et utilise une méthode de différence finie ou de volume fini pour résoudre le champ de température. L'approche DDF résout quand à elle les deux équations avec la LBM.Le modèle thermique utilisé dans LaBS est basé sur l'approche DDF où les deux équations sont couplées par l'hypothèse de Boussinesq. Le champ de vitesse et de densité est résolu avec un réseau de dix-neuf vitesses discrètes (D3Q19) et champs de température est résolut soit par un réseau à dix-neuf vitesses discrètes (D3Q19) soit par un réseau à sept vitesses discrètes (D3Q7).Le nouveau modèle de frontière immergée décompose la fonction de distribution aux noeuds frontière en sa partie à l'équilibre et hors équilibre. La partie hors équilibre est calculée à partir d'une formulation théorique issus du développement de Chapman-Enskog.La validation du modèle DDF implémenté dans LaBS est faite sur un ensemble de cas test de complexité croissante. Les résultats obtenus avec LaBS sont comparés aux solutions analytiques ou encore à des articles de référence et sont en accord avec les résultats attendus. Ils montrent que qualitativement les résultats sont aussi bons pour le modèle D3Q19/D3Q19 que pour le modèle D3Q19/D3Q7 mais que quantitativement le modèle D3Q19/D3Q19 reste meilleur. / A new three-dimensional CFD solver, LaBS, based on the lattice Boltzmann alogorithms has been developed in a framework of university and industry consortium. In this thesis, this solver is used to simulate thermal flows, with a new thermal boundary condition for immersed solid boundary. The new proposed thermal boundary condition is based on the reconstruction method of the distribution function and is evaluated for immersed solid with coincident and non-coincident wall on the case of diffusion and natural convection phenomena.Renault case study, deals with a vehicle moving at constant speed (highway) that suddently slows down and stops (with or without a cut off contact). In such situation the cooling of the engine compartment first driven by forced convection during taxiing stage, abruptly switches to natural convection in low velocity stages. As natural convection is a slow process, it can take several minutes to remove the accumulated heat in the engine compartment. Such duration could be damaging for some components of the engine compartement which do not tolerate high temperature.In order to anticipate overheating of the engine compartment, where a lot of automotive parts with complex geometry are present and to avoid the above mentioned damages, the phenomenon of natural convection is here studied with the new thermal boundary condition.%The new proposed thermal boundary condition is first tested on academic case studies for validation, and then applied to the case of a real car.The modelling of thermal flows with the lattice Boltzmann method (LBM) can be classified into three categories: the multispeed approach, the hybrid approach and the double-distribution-function (DDF) approach. The multispeed approach, uses only one equation to resolve velocity, density and temperature field, which is solved by the LBM. Whereas the hybrid approach and the DDF approach utilize two sets of equations, one to resolve velocity field and density field and another to resolve temperature field. The hybrid approach solves velocity field and density field by the LBM method and the temperature field by finite-different or finite-volume methods. On the other hand the DDF approach solves the two equations with LBM.The thermal model used in the solver LaBS is based on the coupled DDF approach. In this model, the flow field is solved by a D3Q19 velocity model while the temperature field is solved by a D3Q19 or a D3Q7 velocity model. The coupling between the momentum and the energy transport is made by the boussinesq approximation. The new proposed thermal boundary condition decomposes the distribution function at the boundary node into its equilibrium and non-equilibrium part. The non-equilibrium part is calculated from the theoretical solution based on Chapman-Enskog developement.LaBS thermal model based on the coupled DDF approach is evaluated on a set of cases with increasing complexity. The results obtained with LaBS are compared with analytical solutions or with reference articles and are in a good agreement with the results expected. Results show that the model D3Q19/D3Q7 is qualitatively as good as the model D3Q19/D3Q19 but quantitatively the model D3Q19/D3Q19 remains the best.

Cette thèse est consacrée à l'analyse mathématique et numérique d'un problème d'interaction fluide-structure stationnaire, couplant un fluide newtonien, visqueux et incompressible, modélisé par les équations de Stokes 2D et une structure déformable, décrite par les équations d'une poutre 1D. Le fluide et la structure sont couplés via une condition aux limites de type frottement à l'interface.Dans l'étude théorique, nous montrons un résultat d'existence et unicité de solutions faibles, dans le cadre de petits déplacements, du problème de couplage fluide structure avec une condition de glissement de type Tresca en utilisant le théorème de point fixe de Schauder.Dans l'analyse numérique, nous étudions d'abord, l'approximation du problème de Stokes avec la condition de Tresca par une méthode d'éléments finis mixtes à quatre champs. Nous montrons ensuite une estimation d'erreur a priori optimale pour des données régulières et nous réalisons des tests numériques. Enfin, nous présentons un algorithme de point fixe pour la simulation numérique du problème couplé avec des conditions aux limites non linéaires. / This PHD thesis is devoted to the theoretical and numerical analysis of a stationary fluid-structure interaction problem between an incompressible viscous Newtonian fluid, modeled by the 2D Stokes equations, and a deformable structure modeled by the 1D beam equations.The fluid and structure are coupled via a friction boundary condition at the fluid-structure interface.In the theoretical study, we prove the existence of a unique weak solution, under small displacements, of the fluid-structure interaction problem under a slip boundary condition of friction type (SBCF) by using Schauder fixed point theorem.In the numerical analysis, we first study a mixed finite element approximation of the Stokes equations under SBCF.We also prove an optimal a priori error estimate for regular data and we provide numerical examples.Finally, we present a fixed point algorithm for numerical simulation of the coupled problem under nonlinear boundary conditions.

A method for solving the inverse geometrical problem is presented by reconstructing the unknown subsurface cavity geometry using boundary element methods, a genetic algorithm, and Nelder-Mead non-linear simplex optimization. The heat conduction problem is solved utilizing the boundary element method, which calculates the difference between the measured temperature at the exposed surface and the computed temperature under the current update of the unknown subsurface flaws and cavities. In a first step, clusters of singularities are utilized to solve the inverse problem and to identify the location of the centroid(s) of the subsurface cavity(ies)/flaw(s). In a second step, the reconstruction of the estimated cavity(ies)/flaw(s) geometry(ies) is accomplished by utilizing an anchored grid pattern upon which cubic spline knots are restricted to move in the search for unknown geometry. Solution of the inverse problem is achieved using a genetic algorithm accelerated with the Nelder-Mead non-linear simplex. To optimize the cubic spline interpolated geometry, the flux (Neumann) boundary conditions are minimized using a least squares functional. The automated algorithm successfully reconstructs single and multiple subsurface cavities within two dimensional mediums. The solver is also shown to accurately predict cavity geometries with random noise in the boundary condition measurements. Subsurface cavities can be difficult to detect based on their location. By applying different boundary conditions to the same geometry, more information is supplied at the boundary, and the subsurface cavity is easily detected despite its low heat signature effect at the boundaries. Extensions to three-dimensional applications are outlined

<p> Understanding the behavior of liquid hydrocarbon propellants under high pressure and temperature conditions is a crucial step towards improving the performance of modern-day combustion engines (liquid rocket engines, diesel engines, gas turbines and so on) and designing the next generation ones. Under such harsh thermodynamic conditions (high P&T) propellent droplets may experience anywhere from sub-to-trans-to-supercritical regime. The focus of this research is to explore the dynamics of the vapor-liquid two phase system formed by a liquid hydrocarbon fuel (n-heptane or n-dodecane) and ambient (nitrogen) over a wide range of P&T leading up to the mixture critical point and beyond. Molecular dynamics (MD) has been used as the primary tool in this research along with other tools like: phase stability calculations based on Gibb’s work, Peng Robinson equation of state, density gradient theory and neural networks.</p>

The increasing use of turbochargers is leading to an outstanding research to understand the internal flow in turbomachines. In this frame, computational fluid dynamics (CFD) is one of the tools that can be applied to contribute to the analysis of the fluid-dynamic processes occurring in a turbine. The objective of this thesis is the development of a methodology for performing simulations of radial turbomachinery optimizing the available computational resources. This methodology is used for the characterization of a vaned-nozzle turbine under steady and pulsating flow conditions. An important effort has been devoted in adjusting the case configuration to maximize the accuracy achievable with a certain computational cost. Concerning the cell size, a local mesh independence analysis is proposed as a procedure to optimize the distribution of cells in the domain, thus allowing to use a finer mesh in the most suitable places. Particularly important in turbomachinery simulations is the influence of the approach for simulating rotor motion. In this thesis two models have been compared: multiple reference frame and sliding mesh. The differences obtained using both methods were found to be significant in off-design regions. Steady flow CFD results have been validated against global measurements taken on a gas-stand. The modeling of a turbine, installed either on a turbocharger test rig or an engine, requires the calculation of the flow in the ducts composing the system. Those ducts could be simulated assuming a one-dimensional (1D) approximation, and thus reducing the computational cost. In this frame of ideas, two CFD boundary conditions have been developed. The first one allows performing coupled 1D-3D simulations, communicating the flow variables from each domain through the boundary. The second boundary condition is based in a new formulation for a stand-alone anechoic end, which intends to represent the flow behavior of an infinite duct. Finally, the turbine was simulat / Fajardo Peña, P. (2012). Methodology for the Numerical Characterization of a Radial Turbine under Steady and Pulsating Flow [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/16878

<p dir="ltr">The CFD-CAA method combines computational fluid dynamics (CFD) and computational aeroacoustics (CAA) techniques to analyze the interaction between fluid flow and the generation and propagation of sound. CFD is primarily concerned with simulating fluid flow patterns, while CAA focuses on predicting noise generation and its propagation in fluids. The CFD-CAA method provides a powerful tool for understanding and predicting the acoustic behavior of turbulent flows. By combining the strengths of CFD and CAA, this approach provides more precise and comprehensive analyses across various fields, thereby contributing to enhanced designs and noise control strategies.</p><p dir="ltr">Within industrial applications, a primary concern is noise source identification. This process enables engineers to locate and quantify the strength of noise sources within a system, facilitating the implementation of more effective strategies during the design process. A novel methodology, computational statistically optimized near-field acoustic holography (C-SONAH), is proposed to virtually identify aeroacoustic sources. Initially, sound pressure is obtained using the CFD-CAA method, followed by the application of the SONAH algorithm to locate acoustic sources and predict the sound field. C-SONAH offers computational advantages over direct CAA methods for simulating sound produced by systems with rotating elements, as CAA analyzes sources on the moving elements, making sound field calculation computationally expensive. The SONAH procedure converts these rotating sources into a series of equivalent stationary planar or cylindrical waves, reducing the number of sources and the time required to compute the sound field from each source. This methodology was demonstrated by characterizing the aerodynamic noise produced by a bladeless fan. The sound pressure level obtained by C-SONAH method was validated by the data predicted by the direct CFD-CAA method. Acoustic maps were reconstructed at different locations and frequencies, revealing that the C-SONAH method can predict noise sources generated by airflow and rotating components within the fan. Thus, it serves as an effective tool for understanding the aeroacoustic noise generation mechanism and guiding the design optimization of similar products.</p><p dir="ltr">The CFD-CAA method is also a powerful tool for design optimization. Computational simulations are typically less expensive and time-consuming than building and maintaining experimental setups, particularly for large or complex projects. Additionally, simulations reduce the need for multiple physical prototypes, which can shorten the development cycle. CFD-CAA simulations provide detailed flow and acoustic field data, including variables that may be difficult or impossible to measure experimentally, such as pressure distributions, velocity fields, and turbulent structures. In this dissertation, aeroacoustic characteristics and flow field information of vortex whistles were investigated using the CFD-CAA method. The simulation results clearly illustrate the swirling motion created in the vortex whistle cylinder and also demonstrate the linear frequency versus flow rate relationship characteristic of the whistle. The design of the vortex whistle was optimized based on the acoustic response and flow resistance by both simulations and experiments. The results suggest that the whistle with a thin inlet exhibits the best performance at high flow rates, while the whistle with a scale of 0.5 is the most sensitive to low flow rates, making it suitable for pediatric applications.</p><p dir="ltr">In CFD-CAA simulations, the time step typically cannot be too small due to limited computational resources. This constraint results in an aliasing error in spectral analysis. Consequently, an anti-aliasing operation prior to sampling is necessary to eliminate such errors from the acoustic source terms. In the present study, an anti-aliasing filter based on the compact finite difference formulation was designed within a time-domain, compact filter scheme. This filter was directly applied to the Navier-Stokes solver prior to sampling for CAA analysis. A cavity flow case was simulated to validate this mitigation strategy. The results indicate that the artificial spectral peak induced by aliasing error is removed without affecting other signature peaks. The anti-aliasing filter was also applied to more complex cases for predicting the acoustic field of a vortex whistle. The acoustic field around the vortex whistle, with both constant and variable inlet flow rates, was simulated, and the aliasing peak was successfully removed. Although the peak magnitudes decreased slightly due to the filter, the signature frequencies remained unchanged. Thus, the simulation with anti-aliasing operation can predict acoustic features without introducing aliasing errors, even if the time step is not sufficiently small, thereby significantly reducing simulation time.</p><p dir="ltr">In engineering applications, once noise sources are identified, the subsequent concern is noise reduction. An effective strategy for noise reduction involves acoustical absorbing materials to minimize noise emissions from components. Traditionally, experiments in engineering applications have focused on surface treatments to explore noise control techniques. However, the CFD-CAA method commonly assumes smooth and purely reflective wall surfaces. Consequently, there is growing interest in incorporating impedance boundary conditions into the CFD-CAA method. Since impedance boundary conditions are defined in the frequency domain, while CFD-CAA simulations operate in the time domain, direct implementation is not feasible. To address this issue, several methods have been proposed to define time-domain impedance boundary conditions in simulations. In the present study, a wall softness model was implemented in the CFD-CAA method and to examine a vortex whistle featuring an acoustically permeable surface. In simulations, an impedance boundary condition representing the properties of melamine foam was defined over the surface of a cylindrical cavity. The simulation results were validated against experimental data obtained from a vortex whistle with melamine foam. The findings revealed that the impedance of the melamine foam contributed to noise reduction at high frequencies. Additionally, at low airflow rates, the impedance boundary condition enhanced the signal-to-noise ratio for the low-frequency peak, which is advantageous in clinical applications.</p>

<p dir="ltr">Nature has been shown to build environments to drive specific reactivity across boundaries; multiphase systems, for example, have been shown to drive reactions that would otherwise not occur in bulk, continuous phases. Within this work, we show how multiphase environments are essential in driving specific reactivity at phase boundaries and offer unique physicochemical and electrochemical opportunities that are usually inaccessible in continuous phases alone. Here, we present several diverse approaches toward harnessing observed interfacial phenomena to study and take advantage of three-phase systems. Firstly, we demonstrate precise manipulation of nucleation at the water|1,2-dichloroethane (DCE)|electrode interface through electrode geometry adjustment, resulting in selective precipitation of ferrocenemethanol (FcMeOH). Cyclic voltammetry and numerical simulations elucidate this phenomenon's physico-chemical foundations, enabling localized precipitation and reactivity control. Secondly, we introduce a novel mechanism for emulsion formation driven by interfacial solute flux induced via phase transfer agents. Systematically exploring phase combinations and ion interactions, we elucidate the microscopic mechanisms governing droplet formation and propose design principles for tailored emulsion synthesis. Furthermore, leveraging current-driven ion flux, we achieve emulsification across oil|water interfaces, offering control over droplet size and charge. This low-energy, robust method presents an efficient alternative to traditional emulsification techniques. Additionally, we demonstrate facile electrodeposition of gold nanorings at water|oil interfaces, enabled by spontaneous emulsification facilitated by quaternary ammonium salts. We further demonstrate deposition parameters for control over nanoring array characteristics, offering a streamlined approach to nanoring fabrication. Finally, we introduce biphasic electrodeposition as a versatile method for fabricating ultra-high aspect ratio gold nanowires. By manipulating antagonistic metal salt interactions at liquid|liquid interfaces, we achieve precise control over nanowire geometry and positioning, opening new avenues for nanowire synthesis with enhanced simplicity and versatility.</p>