The aerodynamics of modern rotorcraft is highly complex and has proven to be an arduous challenge for computational fluid dynamics (CFD). Flow features such as massively separated boundary layers or transition to turbulence are common in engineering applications and need to be accurately captured in order to predict the vehicle performance. The recent advances in numerical methods and turbulence modeling have resolved each of these issues independent of the other. First, state-of-the-art hybrid RANS-LES turbulence closures have shown great promise in capturing the unsteady flow details and integrated performance quantities for stalled flows. Similarly, the correlation-based transition model of Langtry and Menter has been successfully applied to a wide range of applications involving attached or mildly separated flows. However, there still lacks a unified approach that can tackle massively separated flows in the transitional flow region. In this effort, the two approaches have been combined and expended to yield a methodology capable of accurately predicting the features in these highly complex unsteady turbulent flows at a reasonable computational cost. Comparisons are evaluated on several cases, including a transitional flat plate, circular cylinder in crossflow and NACA 63-415 wing. Cost and accuracy correlations with URANS and prior hybrid URANS-LES approaches with and without transition modeling indicate that this new method can capture both separation and transition more accurately and cost effectively. This new turbulence approach has been applied to the study of wings in the reverse flow regime. The flight envelope of modern helicopters has increased significantly over the last few decades, with design concepts now reaching advance ratios up to μ = 1. In these extreme conditions, the freestream velocity exceeds the rotational speed of the blades, and a large region of the retreating side of the rotor disk experiences reverse flow. For a conventional airfoil with a sharp trailing edge, the reverse flow regime is generally characterized by massive boundary layer separation and bluff body vortex shedding. This complex aerodynamic environment has been utilized to evaluate the new hybrid transitional approach. The assessment has proven the efficiency of the new hybrid model, and it has provided a transformative advancement to the modeling of dynamic stall.

A new approach to large-eddy simulation (LES) based on the use of explicit spatial filtering combined with backscatter forcing is presented. The forcing uses a discrete dynamical system (DDS) called the compressible ``poor man's'' Navier--Stokes (CPMNS) equations. This DDS is derived from the governing equations and is shown to exhibit good spectral and dynamical properties for use in a turbulence model. An overview and critique of existing turbulence theory and turbulence models is given. A comprehensive theoretical case is presented arguing that traditional LES equations contain unresolved scales in terms generally thought to be resolved, and that this can only be solved with explicit filtering. The CPMNS equations are then incorporated into a simple forcing in the OVERFLOW compressible flow code, and tests are done on homogeneous, isotropic, decaying turbulence, a Mach 3 compression ramp, and a Mach 0.8 open cavity. The numerical results validate the general filter-forcing approach, although they also reveal inadequacies in OVERFLOW and that the current approach is likely too simple to be universally applicable. Two new proposals for constructing better forcing models are presented at the end of the work.

The fow around bluff bodies is an essential topic in fuid dynamics. This fow is characterized by large vortical fow regions separating from the surface of the bluff body, and they cause oscillating drag and lift forces on the structure. The fow around an infinite long cylinder is a well-known case being studied in the literature. However, a cylinder with low aspect-ratio piercing the free surface has not been studied much although such an arrangement can be found in many floating offshore structures. In this thesis the results of CFD calculations are presented for a fixed, free surface piercing cylinder with an aspect-ratio L/D equals to 2. The Reynolds number was equal to 4:3 x 104 indicating that the flow is in the sub-critical turbulent regime. An extensive methodology of verication and validation is followed to investigate the reliability of the results. To investigate the effect of the free surface on the calculated hydrodynamic loads, two approaches have been considered: a double-body symmetry condition and an interface capturing Volume-of-Fluid (VoF) method. Additionally, two turbulence models are investigated: a two-equation turbulence model; a non-linear Explicit Algebraic Reynolds Stress Model (EARSM); and the Improved Delayed Detached Eddy Simulation (IDDES) turbulence model. The results are presented in terms of integral results (drag and lift coefficients) and flow visualizations. Based on the results of the cases in which the free surface was modeled as a double body symmetry boundary condition, it is concluded that the model is not suitable for this type of flow as the model damps out the flow dynamics due to over-production of eddy-viscosity. Hence, the characteristic oscillating lift forces are not captured using this turbulence model. However, this turbulence model showed good agreements regarding the flow fields in comparison with experimental PIV measurements. Results of the case modeled with EARSM turbulence model shows better agreement with the experimental results compared with the turbulence model. In the cases where the free-surface is considered, results with the EARSM turbulence model show similar results for the drag forces whereas the lift uctuations were one order of magnitude smaller, compared with the double body case. Lastly, the results using the IDDES turbulence model and free-surface VoF modeling are shown to produce the best comparison with the experimental results, regarding both, integral values and flow field results. / O escoamento ao redor de corpos rombudos é um tópico essencial na dinâmica de fluidos. O escoamento é caracterizado por regiões com grande vorticidade que se separam do corpo e causam oscilações das forças de arrasto e sustentação sobre a estrutura. O escoamento ao redor de cilindros longos é um tema que tem sido extensivamente estudado com muitos trabalhos encontrados na literatura. Entretanto, o cilindro com baixa razão de aspecto perfurante à superfície livre é um caso pouco estudado, apesar desta estrutura ser encontrada em várias estruturas oceânicas flutuantes. Esta dissertação apresenta cálculos numéricos para o escoamento ao redor de um cilindro fixo, que trespassa a superfície livre com razão de aspecto L/D igual a 2. O problema é estudado em um regime subcrítico de turbulência, com número de Reynolds igual a 4:3 x 104. Uma vasta metodologia de verificação e validação foi seguida para avaliar a confiabilidade dos resultados obtidos numericamente. Para investigar os efeitos da superfície livre nas cargas hidrodinâmicas, duas abordagens s~ao consideradas: condição de simetria de duplo corpo e um método de captura de interface Volume of Fluid. Além disso, dois modelos de turbulência foram investigados: o modelo não linear Explicit Algebraic Reynolds Stress Model (EARSM), e o modelo de turbulência Improved Delayed Detached Eddy Simulation (IDDES). Os resultados relacionados aos coeficientes de arrasto e sustentação são apresentados a partir de análise estatística, complementados através de ilustrações que permitem visualizar os campos de escoamento e pressão. Com base nos resultados de casos em que a superfície livre é modelada com uma condição de contorno de simetria, conclui-se que o modelo de turbulência não é adequado para este tipo de escoamento, pois o modelo amortece a dinâmica do escoamento devido à superprodução de viscosidade turbulenta. Consequentemente, as oscilações na força de sustentação não são capturadas usando este modelo. Entretanto, resultados dos campos médios do escoamento mostram-se concordantes com imagens experimentais obtidas com técnicas de PIV - Particle Image Velocimetry. Resultados do caso modelado com o modelo de turbulência EARSM mostram melhores concordâncias na comparação dos parâmetros estatísticos com experimentos do que o modelado com o modelo EARSM. Nos casos em que a superfície livre é modelada com o método VoF, o modelo de turbulência EARSM mostra resultados semelhantes para o arrasto, enquanto as flutuações da sustentação apresentam-se uma ordem de grandeza menores, quando comparadas ao caso de duplo corpo. Resultados usando o modelo de turbulência IDDES e VoF apresentam melhores comparações aos resultados experimentais, tanto para os parâmetros estatísticos quando para as visualizações do escoamento.

To quantify primary side aerosol retention, an Eulerian/Lagrangian approach was used to investigate aerosol transport in a compressible, turbulent, adiabatic, internal, wall-bounded flow. The ARTIST experimental project (Phase I) served as the physical model replicated for numerical simulation. Realizable k-ε and standard k-ω turbulence models were selected from the computational fluid dynamics (CFD) code, FLUENT, to provide the Eulerian description of the gaseous phase. Flow field simulation results exhibited: a) onset of weak secondary flow accelerated at bend entrance towards the inner wall; b) flow separation zone development on the convex wall that persisted from the point of onset; c) centrifugal force concentrated high velocity flow in the direction of the concave wall; d) formation of vortices throughout the flow domain resulted from rotational (Dean-type) flow; e) weakened secondary flow assisted the formation of twin vortices in the outflow cross section; and f) perturbations induced by the bend influenced flow recovery several pipe diameters upstream of the bend. These observations were consistent with those of previous investigators. The Lagrangian discrete random walk model, with and without turbulent dispersion, simulated the dispersed phase behavior, incorrectly. Accurate deposition predictions in wall-bounded flow require modification of the Eddy Impaction Model (EIM). Thus, to circumvent shortcomings of the EIM, the Lagrangian time scale was changed to a wall function and the root-mean-square (RMS) fluctuating velocities were modified to account for the strong anisotropic nature of flow in the immediate vicinity of the wall (boundary layer). Subsequent computed trajectories suggest a precision that ranges from 0.1% to 0.7%, statistical sampling error. The aerodynamic mass median diameter (AMMD) at the inlet (5.5 μm) was consistent with the ARTIST experimental findings. The geometric standard deviation (GSD) varied depending on the scenario evaluated but ranged from 1.61 to 3.2. At the outlet, the computed AMMD (1.9 μm) had GSD between 1.12 and 2.76. Decontamination factors (DF), computed based on deposition from trajectory calculations, were just over 3.5 for the bend and 4.4 at the outlet. Computed DFs were consistent with expert elicitation cited in NUREG-1150 for aerosol retention in steam generators.

In this study, the behavior of pressure in the Rapid Distortion Limit, along with its concomitant modeling, are addressed. In the first part of the work, the role of pressure in the initiation, propagation and suppression of flow instabilities for quadratic flows is analyzed. The paradigm of analysis considers the Reynolds stress transport equations to govern the evolution of a dynamical system, in a state space composed of the Reynolds stress tensor components. This dynamical system is scrutinized via the identification of the invariant sets and the bifurcation analysis. The changing role of pressure in quadratic flows, viz. hyperbolic, shear and elliptic, is established mathematically and the underlying physics is explained. Along the maxim of "understanding before prediction", this allows for a deeper insight into the behavior of pressure, thus aiding in its modeling. The second part of this work deals with Rapid Pressure Strain Correlation modeling in earnest. Based on the comprehension developed in the preceding section, the classical pressure strain correlation modeling approaches are revisited. Their shortcomings, along with their successes, are articulated and explained, mathematically and from the viewpoint of the governing physics. Some of the salient issues addressed include, but are not limited to, the requisite nature of the model, viz. a linear or a nonlinear structure, the success of the extant models for hyperbolic flows, their inability to capture elliptic flows and the use of RDT simulations to validate models. Through this analysis, the schism between mathematical and physical guidelines and the engineering approach, at present, is substantiated. Subsequently, a model is developed that adheres to the classical modeling framework and shows excellent agreement with the RDT simulations. The performance of this model is compared to that of other nominations prevalent in engineering simulations. The work concludes with a summary, pertinent observations and recommendations for future research in the germane field.

Hydrodynamic and thermal analyses performed during the conceptual design of an unmanned underwater vehicle are presented in this study. The hull shape is determined by considering alternative shapes and the dimensions are determined from the internal arrangement of components. Preliminary thermal analyses of the watertight section are performed with a commercial software called FLUENT to check the risk of over-heating due to the heat dissipation of devices. Performance of the proposed hull design is analyzed by FLUENT. Before simulations of the vehicle, validation studies are performed. Models 4159, 4158 and 4154 of Series 58 are chosen as the experimental reference. Their total resistance coefficients are compared with the results of the validations analyses. Mesh densities, turbulence models, near wall modeling approaches and inlet turbulence intensities are varied to understand their effects on the accuracy of predictions. A suitable turbulence modeling approach is chosen to analyze forward and vertical motions of the vehicle to check whether speed requirements are fulfilled. Hull configurations with and without appendages are used to observe their effects on total drag. It is observed that the proposed design satisfies speed requirements of the vehicle and no overheating is expected in the watertight section.

In a novel application of the temporal numerical simulation, an investigation ofturbulence modeling techniques is carried for the turbulent wall jet over aconvex surface (Coanda wall jet.) The simultaneous presence of multipleinstability mechanisms and the interaction with the turbulence dynamics at thesolid boundary produces a unique combination of different large turbulentcoherent structures that constitutes both a consistent challenge for numericalsimulations and an ideal test bed for turbulence models.The Temporal Direct Numerical Simulation (TDNS) of the Coanda wall jetrestricts the focus from the global turbulent Coanda wall jet to a smaller, localportion of the flow and offers a wide array of advantages to the present work. Inparticular, the size of the computational domain can be arbitrarily chosen inboth the spanwise and the streamwise directions. This allows to either suppressor enhance individual physical mechanisms and, consequently, to selectivelyreproduce different large coherent structures within the local flow. In the firstpart, temporal numerical simulations are employed to reproduce four differentflow scenarios of the local Coanda wall jet with a level of numerical resolutionthat, because of the reduced size of the computational domain, cannot be matchedby standard DNS of the entire physical flow (spatial DNS, or SDNS.)The TDNS of these four flow scenarios are then used in the second part for ana--posteriori analysis of different turbulence models in order to addresscommon shortcomings shown by Hybrid Turbulence Models (HTM). For each flowscenario, the turbulent flow field is deliberately decomposed in resolved andunresolved flows by the application of different filters in space correspondingto different grid resolution. The behavior of turbulence models can be reproducedfrom the resolved flow and compared to the turbulent stress tensor directlycalculated from the unresolved part of the flow field. Starting from the RANSlimit, turbulence models with different levels of complexity are studied.Successively, the performance of these models is analyzed at intermediatenumerical resolutions between RANS, LES, and DNS. Finally, an improvedformulation of the Flow Simulation Methodology (FSM) is proposed.

This thesis presents the implementation of several two-equation turbulence models into a finite difference, two- and three-dimensional Navier-Stokes Solver. Theories of turbulence modeling and the historical development of these theories are briefly investigated. Turbulence models that are defined by two partial differential equations, based on k-&amp / #969 / and k-&amp / #949 / models, having different correlations, constants and boundary conditions are selected to be adapted into the base solver. The basic equations regarding the base Navier-Stokes solver to which the turbulence models are implemented presented by briefly explaining the outputs obtained from the solver. Numerical work regarding the implementation of turbulence models into the base solver is given in steps of non-dimensionalization, transformation of equations into generalized coordinate system, numerical scheme, discretization, boundary and initial conditions and limitations. These sections of implementation are investigated and presented in detail with providing every steps of work accomplished. Certain trial problems are solved and outputs are compared with experimental data. Solutions for fluid flow over flat plate, in free shear, over cylinder and airfoil are demonstrated. Airfoil validation test cases are analyzed in detail. For three dimensional applications, computation of flow over a wing is accomplished and pressure distributions from certain sections are compared with experimental data.

Les premières simulations numériques d'écoulements réactifs sur des configurationscomplexes ont été réalisées à l'aide d'approches RANS (Reynolds Averaged Navier Stokes). Ces dernières, bien adaptées aux écoulements de type couches limites attachées et relativement peu coûteuses en temps de calcul, ne donnent accès qu'à des résultats stationnaires qui s'éloignent parfois de la réalité. Pour réaliser des simulations instationnaires d'écoulements, les méthodes de type LES (Large Eddy Simulation) -- plus précises mais plus coûteuses -- sont de plus en plus utilisées. Cependant, ces méthodes sont mal adaptées à la simulation de la dynamique pariétale, car elles nécessitent un effort de maillage souvent prohibitif près de la paroi. Cette thèse est consacrée au développement dans le code CEDRE (code de simulation d'écoulements réactifs complexes de l'Onera) d'une méthode hybride RANS/LES, appelée Delayed Detached Eddy Simulation (DDES), et à son application à des écoulements réactifs massivement décollés. Après une étape de validation sur des couches limites attachées, la DDES a été appliquée à la simulation des écoulements inerte et réactif dans une chambre de combustion en forme de marche descendante (A3C) et comparée aux résultats des approches RANS et LES classiques, ainsi qu'aux résultats expérimentaux. Cette méthode a ensuite permis de réaliser l'étude de la dynamique de l'écoulement réactif décollé dans la tuyère ATAC montée sur le banc cryotechnique MASCOTTE de l'Onera. / The first numerical simulations of reactive flows on complex configurations were performed using RANS (Reynolds Averaged Navier Stokes) approaches. These methods, which are well adapted to attached boundary layer flows and relatively not expensive in computation time, provide only steady results, which may not correctly reproduce reality. For unsteady flow simulations, LES (Large Eddy Simulation) method -- more accurate but more expensive – are increasingly employed. However, these approaches are poorly suited to simulate wall turbulence since they often require a prohibitive meshing effort close to the wall. This PhD thesis is devoted to the development in the CEDRE code (Onera code for the simulation of complex reactive flows) of a hybrid RANS/LES approach, called Delayed Detached Eddy Simulation (DDES) and to its application to massively separated reactive flows. After a step of validation on attached boundary layers, the DDES is applied to the simulation of the inert and reactive flows in a backward facing step combustor (A3C) and compared to the results obtained by RANS and LES approaches, as well as to experimental results. The DDES has then been used to study the dynamics of the separated reactive flow in the ATAC nozzle mounted on the cryotechnic MASCOTTE facility of Onera.

Au sein d’un échangeur à air, les trompes à air permettent de créer l’écoulement d’air froid nécessaireà son bon fonctionnement. Ces dispositifs, qui peuvent ^etre assimilés à des jets subsoniques confinésen conduit, peuvent contribuer au bruit rayonné par les avions lors des phases au sol. Nous proposonsdans cette thèse de développer un outil numérique prédictif de l’acoustique rayonnée par ces dispositifsafin de pouvoir proposer des solutions de réduction de bruit appropriées. Cet outil est adapté au contexteindustriel de Liebherr Aerospace. Une méthode stochastique permet, à partir d’un calcul stationnaireRANS, de générer un champ de vitesse turbulente qui autorise la formation d’un terme de forçage dansles équations d’Euler linéarisées qui sont alors utilisées comme un opérateur de propagation. Un nouveaumodèle stochastique basé sur l’hypothèse de sweeping est développé. Ce dernier permet de produiredes champs instationnaires respectant certaines propriétés aérodynamiques statistiques dans le cadre dejets libres subsoniques. Cette méthode est couplée avec le solveur Euler de l’Onera sAbrinA_v0 et l’outilrésultant est appliqué sur le cas d’un jet libre subsonique à M=0.72. Moyennant une calibration duterme source, la méthodologie permet de reproduire les spectres acoustiques en champ lointain, exceptépour les angles faibles. L’outil numérique est ensuite couplé avec un solveur FW-H pour étudier le casconcret de la trompe à air. Les résultats aérodynamiques et acoustiques sont validés par comparaison àune base de données aérodynamique et acoustique constituée au préalable à partir d’une campagne d’essaiscomprenant des mesures par anémométrie laser Doppler à l’intérieur du conduit et des microphonesacoustiques en champ lointain. / In air exchangers, the cool air flow can be produced by jet pumps. These devices, which can be consideredas subsonic jets confined in ducts, could contribute directly to ramp noise. A predictive numerical toolof the acoustic radiated by jet pumps is therefore developped in order to be able to propose appropriatenoise reduction solutions. This tool is adapted to Liebherr Aerospace’s industrial context. A stochasticmethod allows, starting from a steady RANS computation, to synthetise a turbulent velocity fields andto enforce source terms in the linearized Euler equations therefore used as a wave propagator. A newstochastic model relying on the sweeping hypothesis is developped. Unsteady fields reproducing someaerodynamics features of a free subsonic jet flow can be generated. This method is then coupled withOnera’s Euler solver sAbrinA_v0 and the resulting tool is applied on a free subsonic jet configuration atMach 0.72. Assuming a cabration of the source terms, this methodology models properly the far fieldacoustic spectra except for small angles. The numerical tool is then coupled with a FW-H solver to studya realistic jet pump. Aerodynamic and acoustic results are validated by comparison with a data baseobtained from an experimental campaign including laser Doppler anemometry measures inside the ductand pressure recording in the far-field.