Cette étude est consacrée à l’analyse des écoulements réactifs supersoniques cisailléset, plus particulièrement, des couches de mélange compressibles pouvant se développerdans les moteurs ramjet et scramjet. Des méthodes numériques appropriées ont été implémentéeset vérifiées pour aboutir au développement d’un code de calcul numériquemassivement parallèle, appelé CREAMS (compressible reactive multi-species solver). Cedernier a été spécialement conçu pour conduire des simulations numériques haute précision(simulations numériques directes ou DNS) de ce type d’écoulements. Une attentionparticulière a été portée à la description des termes de transport moléculaire et des termessources chimiques de façon à considérer la description physique la plus fidèle possible desmélanges des gaz réactifs à haute vitesse, au sein desquelles les temps caractéristiqueschimiques et de mélange aux petites échelles sont susceptibles d’être du même ordre degrandeur. Les simulations des couches de mélange bidimensionnelles et tridimensionnelles,inertes et réactives, confirment l’importance des effets associés à la compressibilité et autaux de dégagement de chaleur. Les résultats ainsi obtenus diffèrent en certains points deceux issus d’autres simulations qui introduisaient certaines hypothèses simplificatrices :développement temporel, emploi d’une chimie globale ou encore lois de transport simplifiées.En revanche, ils reproduisent certains tendances déjà observées dans un certainnombre d’études expérimentales conduites dans des conditions similaires. / This study is devoted to the analysis of supersonic reactive shear flows and, in particular,compressible mixing layers that can develop inside the ramjet and scramjet engines.Appropriate numerical methods have been implemented and tested to achieve the developmentof a massively parallel numerical solver, called CREAMS (compressible reactivemulti-species solver). This tool was designed to conduct high-precision numerical simulations(direct numerical simulations or DNS) of such flows. Particular attention waspaid to the description of the molecular transport terms and chemical source terms toconsider the most accurate physical description of reactive gas mixtures at high velocity,in which the chemical and mixing time scales, corresponding to the smallest scalesof the flow, are susceptible to be of the same order of magnitude. Simulations of twoandthree-dimensional, inert and reactive, mixing layers confirm the importance of theeffects associated with compressibility and rate of heat release. The results obtained differin some points from other simulations which introduced simplifying assumptions such astemporal development, use of a global chemistry or a simplified description of the moleculartransport terms. Nevertheless, they reproduce some trends already observed in severalexperimental studies conducted under similar conditions.

Ce travail porte sur l’évaluation de la méthode ZDES pour la simulation de jets propulsifs. L’analyse se concentre sur le positionnement des cellules de chocs et le développement des couches de mélange d’une tuyère double-flux avec plug externe, typique des moteurs d’avions modernes. Les champs statistiques sont comparés aux résultats expérimentaux et discutés en termes de grandeurs moyennes, fluctuantes et dans le domaine fréquentiel. L’intérêt d’utiliser un schéma spatial peu dissipatif ainsi qu’une échelle de longueur sous-maille basée sur la vorticité locale est mis en évidence, notamment pour le dévelopement de la couche de mélange interne, et le mode 2 ("automatique") de la ZDES a démontré un comportement similaire au mode 1 ("manuel") dans les couches de mélange. Par ailleurs, la technique Random Flow Generation (RFG) mise en oeuvre afin de reproduire la turbulence amont existant au coeur des jets primaire et secondaire a permis d’accélérer la transition RANS-LES dans les deux couches de mélanges, plus conformément à l’expérience. La transition est d’autant plus rapide que le taux de turbulence est élevé et l’échelle de la turbulence injectée est petite. Le positionnement des cellules de choc est également amélioré, soulignant l’importance de prendre en compte la turbulence amont dans les simulations de jets. / In this thesis, the ZDES method is assessed for the simulation of propulsive jets. This work focuses on the shock-cell positioning and the mixing layer development of a dual-stream nozzle configuration with an external plug, typical of modern aircraft engines. Reynolds averaged data are discussed in terms of mean and fluctuating quantities as well as in the frequency domain and compared with experimental data. First, the advantage of using a low dissipative spatial scheme as well as a subgrid length scale based on the local vorticity is demonstrated, especially for the development of the core mixing layer. Besides, the "automatic" mode of ZDES (mode 2) is found to provide similar mixing layers as the user defined mode.Then, the use of the Random Flow Generation (RFG) technique at the inlet boundaries of the core and fan channels in order to reproduce the turbulence rate at the center of the nozzle ducts is shown to accelerate the RANS-to-LES transition in both external and internal mixing layers, which is in better agreement with the experimental results. The transition length is further reduced when the injected turbulent ratio is higher, but also when the injected turbulent length scale is smaller. Of interest, the shock-cell positioning in the fan jet is also improved using RFG, which emphasizes the importance of accounting for upstream turbulence for this type of simulations.

<p>Turbulent mixing layers have been researched for many years. Currently, research is focused on studying compressible mixing layers because of their widespread applications in high-speed flight systems. While the effect of compressibility on the shear layer growth rate is well established, there is a lack of consensus over its effect on the turbulent stresses and hence warrants additional research in this area. Computational studies on compressible shear layers could provide a deep cognizance of the dynamics of fluid structures present in these flow fields which in turn would be viable for understanding the effects of compressibility on such flows. However, performing a Direct Numerical Simulation (DNS) of a highly compressible shear layer with experimental flow conditions is extremely expensive, especially when resolving the boundary layers that lead into the mixing section. The attractive alternative is to use Large Eddy Simulation (LES), as it possesses the potential to resolve the flow physics at a reasonable computational cost. Therefore the current work deals with developing a methodology to perform LES of a compressible mixing layer with experimental flow conditions, with resolving the boundary layers that lead into the mixing section through a wall model. The wall model approach, as opposed to a wall resolved simulation, greatly reduces the computational cost associated with the boundary layer regions, especially when using an explicit time-stepping scheme. An in house LES solver which has been used previously for performing simulations of jets, has been chosen for this purpose. The solver is first verified and validated for mixing layer flows by performing simulations of laminar and incompressible turbulent mixing layer flows and comparing the results with the literature. Following this, LES of a compressible mixing layer at a convective Mach number of 0.53 is performed. The inflow profiles for the LES are taken from a precursor RANS solution based on the k-ε and RSM turbulence models. The results of the LES present good agreement with the reference experiment for the upstream boundary layer properties, the mean velocity profile of the shear layer and the shear layer growth rate. The turbulent stresses, however, have been found to be underpredicted. The anisotropy of the normal Reynolds stresses have been found to be in good agreement with the literature. Based on the present results, suggestions for future work are also discussed.</p>

Better understanding of the role of vegetation in the transport of fluid and pollutants requires improved knowledge of the detailed flow structure within the vegetation. Instead of spatial averaging, this study uses discrete measurements at multiple locations within the canopy to develop velocity and turbulence intensity profiles and observe the changes in the flow characteristics as water travels through a vegetation array simulated by rigid dowels. Velocity data were collected with a one dimensional laser Doppler velocimeter (LDV) under single layer emergent and submerged flow conditions, and through two layers of vegetation. The effects of dowel arrangement, density, and roughness are also examined under the single layer experiments. The results show that the velocity within the vegetation array is constant with depth and the velocity profile is logarithmic above it. The region immediately behind a dowel, where the vorticity and turbulence intensity are highest, is characterized by a velocity spike near the bed and an inflection point near the top of the dowel arrays. With two dowel layers, the velocity profile in the region behind a tall dowel exhibits multiple inflection points and the highest turbulence intensities are found there. / Master of Science

Tanto os motores turbo-jato quanto os turbo-fan, são os maiores responsáveis pela geração de ruído durante a decolagem, segmento de subida e de aceleração de uma aeronave. Devido a isto, o problema de ruído em jatos vem sendo intensamente investigado ao longo dos últimos anos. Já na fase do pouso, o slat é uma das fontes de ruído mais importantes. Para este caso, na maioria das aplicações práticas, existe o descolamento da camada limite no intradorso do slat a partir de onde se desenvolve uma camada de mistura. Ainda assim, existem inúmeros aspectos de tais escoamentos que precisam de investigação. Uma abordagem frequentemente feita para o estudo da instabilidade hidrodinâmica e ruído em jatos é o estudo de metade do jato. A estratégia consiste em estudar os fenômenos na camada de mistura, o que é uma aproximação razoável quando o jato tem diâmetro muito grande comparado à espessura da camada cisalhante que se desenvolve nas bordas do jato. Assim, alguns aspectos do ruído gerado pelos modos axi-simétricos de instabilidade são em grande parte reproduzidos. Um aspecto aparentemente jamais estudado antes é o efeito do emparelhamento de vórtices de diferentes geometrias na camada de mistura. Caso o efeito da modulação dos vórtices produzisse um padrão de ruído com características diferentes no emparelhamento, um controle ativo de escoamento por excitação periódica poderia ser usado para reduzir o ruído em jatos. O objetivo do presente trabalho é investigar tal efeito. A idéia é investigar este emparelhamento de vórtices na camada de mistura em desenvolvimento temporal bi-dimensional. Com isto foi possível visualizar um emparelhamento isolado de outros emparelhamentos e sem o efeito Doppler (presente na camada de mistura em desenvolvimento espacial). O método adotado foi a simulação numérica direta (DNS) das equações de Navier Stokes compressíveis na forma não-conservativa escritas na formulação característica. Os resultados mostram que a modulação dos vórtices não produz alteração significativa do ruído gerado no emparelhamento. / Turbo-fan and turbo-jet engines are the most important noise sources during the aircraft take off, climb and acceleration segments. Owing to this fact, the jet flow noise has been studied in the past years. For the landing stage, the slat is an important sound source. In this case, the slat leading edge frequently experiences a boundary layer deattachment causing the development of a mixing layer inside the slot. Nevertheless, there are many aspects of such phenomenon that have not been studied yet. Mixing layers constitutes an usual approach for jet flow instability in aeroacoustics studies. The stategy is to study the mixing layer in order to understand the jet-flow. This strategy becomes better as the ratio between the jet diameter and mixing layer thickness becomes larger. This approach is only reazonable for the jet flow axi-symetric unstable modes. The effect of vortex modulation on the vortex pairing sound production has not been found in the literature. If such effect could cause a significant change in the sound generation patterns, an active flow control system could be developed in order to enhance the jet noise performance. The purpose of the present work was to investigate such effect. It was also possible to observe a single vortex pairing inside a wide domain without the Doppler effect. The strategy was to study the vortex pairing in a bi-dimensional mixing layer under temporal development. The method used was the direct numerical simulation (DNS) of the compressible bidimensional (2D) Navier Stokes equations written in a nonconservative form of the characteristics formulation. The results showed that the vortex modulation did not produce a significant change on the vortex pairing sound.

A poluição sonora é um problema central de uma grande diversidade de aplicações industriais. Na engenharia, podemos citar diversos casos que geram ruído, como exemplos os trens, automóveis, rotores de helicópteros e o ruído aerodinâmico das aeronaves, o qual se divide em ruído gerado pelos motores a jato e a estruturas da aeronave. No presente momento o ruído dos motores aeronáuticos, principalmente os jatos, atingiu níveis de ruídos semelhantes às estruturas da aeronave, como por exemplo, eslates, flaps e trens de pouso. Desta forma, as autoridades de transporte aéreo estão exigindo também redução no ruído das estruturas. O presente trabalho apresenta a verificação das potencialidades e limitações do software CFD++, programa este adquirido pela EMBRAER e inserido como parte do projeto Aeronave Silenciosa, para assim poder compreender de uma melhor maneira o fenômeno da aeroacústica, e deste modo, poder contribuir para a redução do ruído externo das aeronaves. Para verificar as potencialidades e limitações do CFD++, foi proposto investigar o mecanismo de som do eslate. Tal fenômeno é devido ao deslocamento da camada limite no intradorso do eslate a partir de onde se desenvolve a camada de mistura, foco do presente trabalho. / Noise pollution is a central problem of a wide variety of industrial applications. In engineering, cite several cases that generate noise, as examples trains, automobiles, rotors of helicopters and the noise generated by aircraft, which is divided into noise generated by jet engines and airframe. At present the noise of aircraft engines, largely the jets reached noise levels similar structures, such as slat, flaps and landing gear. Thus, the air transport authorities are also demanding a reduction in noise of the structures of airframe. This report presents the verification of potentialities and limitations of CFD++, a program acquired by EMBRAER and inserted as part of the Silent Aircraft, so they can understand better how the phenomenon of aeroacoustics, and thus able to reduce contribute external noise from aircraft. To check the potentialities and limitations of CFD++, was proposed to investigate the mechanism of sound generated by the slat. This phenomenon is due to the displacement of the boundary layer on the lower surface of the slat from which the mixed layer develops. The mixing layer is the focus of this work.

No description available.

Tanto os motores turbo-jato quanto os turbo-fan, são os maiores responsáveis pela geração de ruído durante a decolagem, segmento de subida e de aceleração de uma aeronave. Devido a isto, o problema de ruído em jatos vem sendo intensamente investigado ao longo dos últimos anos. Já na fase do pouso, o slat é uma das fontes de ruído mais importantes. Para este caso, na maioria das aplicações práticas, existe o descolamento da camada limite no intradorso do slat a partir de onde se desenvolve uma camada de mistura. Ainda assim, existem inúmeros aspectos de tais escoamentos que precisam de investigação. Uma abordagem frequentemente feita para o estudo da instabilidade hidrodinâmica e ruído em jatos é o estudo de metade do jato. A estratégia consiste em estudar os fenômenos na camada de mistura, o que é uma aproximação razoável quando o jato tem diâmetro muito grande comparado à espessura da camada cisalhante que se desenvolve nas bordas do jato. Assim, alguns aspectos do ruído gerado pelos modos axi-simétricos de instabilidade são em grande parte reproduzidos. Um aspecto aparentemente jamais estudado antes é o efeito do emparelhamento de vórtices de diferentes geometrias na camada de mistura. Caso o efeito da modulação dos vórtices produzisse um padrão de ruído com características diferentes no emparelhamento, um controle ativo de escoamento por excitação periódica poderia ser usado para reduzir o ruído em jatos. O objetivo do presente trabalho é investigar tal efeito. A idéia é investigar este emparelhamento de vórtices na camada de mistura em desenvolvimento temporal bi-dimensional. Com isto foi possível visualizar um emparelhamento isolado de outros emparelhamentos e sem o efeito Doppler (presente na camada de mistura em desenvolvimento espacial). O método adotado foi a simulação numérica direta (DNS) das equações de Navier Stokes compressíveis na forma não-conservativa escritas na formulação característica. Os resultados mostram que a modulação dos vórtices não produz alteração significativa do ruído gerado no emparelhamento. / Turbo-fan and turbo-jet engines are the most important noise sources during the aircraft take off, climb and acceleration segments. Owing to this fact, the jet flow noise has been studied in the past years. For the landing stage, the slat is an important sound source. In this case, the slat leading edge frequently experiences a boundary layer deattachment causing the development of a mixing layer inside the slot. Nevertheless, there are many aspects of such phenomenon that have not been studied yet. Mixing layers constitutes an usual approach for jet flow instability in aeroacoustics studies. The stategy is to study the mixing layer in order to understand the jet-flow. This strategy becomes better as the ratio between the jet diameter and mixing layer thickness becomes larger. This approach is only reazonable for the jet flow axi-symetric unstable modes. The effect of vortex modulation on the vortex pairing sound production has not been found in the literature. If such effect could cause a significant change in the sound generation patterns, an active flow control system could be developed in order to enhance the jet noise performance. The purpose of the present work was to investigate such effect. It was also possible to observe a single vortex pairing inside a wide domain without the Doppler effect. The strategy was to study the vortex pairing in a bi-dimensional mixing layer under temporal development. The method used was the direct numerical simulation (DNS) of the compressible bidimensional (2D) Navier Stokes equations written in a nonconservative form of the characteristics formulation. The results showed that the vortex modulation did not produce a significant change on the vortex pairing sound.

A poluição sonora é um problema central de uma grande diversidade de aplicações industriais. Na engenharia, podemos citar diversos casos que geram ruído, como exemplos os trens, automóveis, rotores de helicópteros e o ruído aerodinâmico das aeronaves, o qual se divide em ruído gerado pelos motores a jato e a estruturas da aeronave. No presente momento o ruído dos motores aeronáuticos, principalmente os jatos, atingiu níveis de ruídos semelhantes às estruturas da aeronave, como por exemplo, eslates, flaps e trens de pouso. Desta forma, as autoridades de transporte aéreo estão exigindo também redução no ruído das estruturas. O presente trabalho apresenta a verificação das potencialidades e limitações do software CFD++, programa este adquirido pela EMBRAER e inserido como parte do projeto Aeronave Silenciosa, para assim poder compreender de uma melhor maneira o fenômeno da aeroacústica, e deste modo, poder contribuir para a redução do ruído externo das aeronaves. Para verificar as potencialidades e limitações do CFD++, foi proposto investigar o mecanismo de som do eslate. Tal fenômeno é devido ao deslocamento da camada limite no intradorso do eslate a partir de onde se desenvolve a camada de mistura, foco do presente trabalho. / Noise pollution is a central problem of a wide variety of industrial applications. In engineering, cite several cases that generate noise, as examples trains, automobiles, rotors of helicopters and the noise generated by aircraft, which is divided into noise generated by jet engines and airframe. At present the noise of aircraft engines, largely the jets reached noise levels similar structures, such as slat, flaps and landing gear. Thus, the air transport authorities are also demanding a reduction in noise of the structures of airframe. This report presents the verification of potentialities and limitations of CFD++, a program acquired by EMBRAER and inserted as part of the Silent Aircraft, so they can understand better how the phenomenon of aeroacoustics, and thus able to reduce contribute external noise from aircraft. To check the potentialities and limitations of CFD++, was proposed to investigate the mechanism of sound generated by the slat. This phenomenon is due to the displacement of the boundary layer on the lower surface of the slat from which the mixed layer develops. The mixing layer is the focus of this work.

Trailing edge noise control is an important problem associated mainly with wind turbines. As turbulence in the air flows over a wind turbine blade, it impacts the trailing edge and scatters, producing noise. Traditional methods of noise control involve modifying the physical trailing edge, or the scattering efficiency. Recently, inspired by the downy covering of owl feathers, researchers developed treatments that can be applied to the trailing edge to significantly reduce trailing edge noise. It was hypothesized that the noise reduction was due to manipulating the incoming turbulence, rather than the physical trailing edge itself, representing a new method of noise control. However, only acoustic measurements were reported, meaning the associated flow physics were still unknown. This thesis describes a comprehensive wall jet experiment to measure the flow effects near the bio-inspired treatments, termed “finlets” and “rails,” and relate those flow effects to the noise reduction. This was done using far-field microphones, a single hot-wire probe, and surface pressure fluctuation microphones. The far-field noise results showed that each treatment successfully reduced the noise, by up to 7 dB in some cases. The surface pressure measurements showed that the spanwise coherence was slightly reduced when the treatments were applied to the trailing edge. The velocity measurements clearly established the presence of a shear layer near the top of the treatments. As a whole, the dataset led to the shear-sheltering hypothesis: the bio-inspired treatments are effective based on reducing the spanwise pressure correlation and by sheltering the trailing edge from turbulent structures with the shear layer they create. / Master of Science / This thesis describes a project aimed at developing a technology inspired by the silent flight of owls, with the end goal of using this technology to reduce the noise generated by wind turbines. Specifically, the phenomenon known as "trailing edge noise" is the primary source of wind turbine noise, and is the noise source of interest here. It occurs when air turbulence (which can be thought of as unsteady air fluctuations) crashes into the rear (trailing) edge of wind turbine blades, scattering and producing noise. Typically, methods of reducing this noise source involve changing the shape of the trailing edge; this may not always be practical for existing wind turbines. Recently, inspired by the downy covering of owl feathers, researchers developed treatments that can be applied directly to the trailing edge, significantly reducing trailing edge noise. This bio-inspired concept was verified with numerous acoustic measurements. Based on those measurements, researchers hypothesized that the noise reduction was achieved by manipulating the incoming turbulence before it scattered off the trailing edge, rather than by changing the existing wind turbine blade, representing a new method of trailing edge noise control. However, as only acoustic measurements (not flow measurements) were reported, the changes in turbulence could not be examined. With the above motivation in mind, this thesis describes a comprehensive wind tunnel experiment to measure the changes in the aerodynamics and turbulence near the bio-inspired treatments, and relate those changes to the reduction in trailing edge noise. This was done using a hot-wire probe to measure the aerodynamics, as well as microphones to measure the radiated noise and surface pressure fluctuations. As a whole, the experimental results led to the shear-sheltering hypothesis: the bio-inspired treatments are effective based on the creation of a shear layer (a thin region between areas with different air speeds) which shelters the trailing edge from some turbulence, as well as by de-correlating surface pressure fluctuations along the trailing edge.