Le décrochage est un phénomène aérodynamique instationnaire susceptible d'apparaître sur de nombreux profils aérodynamiques. Il résulte d'un décollement important de l'écoulement vis-à-vis de la paroi de l'aile et dégrade considérablement les performances de vol. Sur certains profils de pales d'hélicoptères, d'éoliennes ou de rotors, ce phénomène se produit dans des conditions d'utilisation normales et justifie la recherche de méthodes de modélisation accessibles industriellement. Le décrochage est initié au bord d'attaque par l'apparition d'une petite région de recirculation de fluide appelée bulbe de décollement laminaire où l'écoulement transitionne de l'état laminaire vers l'état turbulent. Ce phénomène encore mal connu met en jeu transition et écoulements hors équilibre auxquels les outils de modélisation RANS habituellement employés ne sont pas adaptés. Dans cette étude, un bulbe transitionnel typique d'un écoulement de bord d'attaque de pale d'hélicoptère (profil OA209 à un nombre de Reynolds Rec∞=1.8x106 et 15° d'incidence) est isolé sur une plaque plane. Une simulation DNS de cet écoulement est réalisée à l'aide du logiciel FUNk de l'ONERA afin de servir de base de données pour l'amélioration des modèles RANS. L'évolution des bilans de l'équation de transport de l'énergie cinétique turbulente ainsi que les principales hypothèses RANS (isotropie de la turbulence, Boussinesq, équilibre production/dissipation) sont analysées. Une étude des principaux modèles RANS développés dans le logiciel elsA de l'ONERA est ensuite réalisée en pondérant les grandeurs turbulentes par une fonction de transition reproduisant l'intermittence de la turbulence. Le modèle k-ω de Wilcox couplé à une fonction de transition optimisée a donné les résultats les plus proches de la DNS et a donc été l'objet d'une analyse plus approfondie, notamment une évaluation des principales équations bilans et une application de ce modèle et de sa méthode de transition à un cas de transition naturelle de plaque plane. / The stall is an unsteady aerodynamic phenomenon that may occur on many aerodynamic profiles. It consists in a large separation of the flow from the wall of the wing and significantly deteriorates the flight performances. On some blade profiles such as helicopters, turbines or rotors, this phenomenon occurs under normal conditions of use and justifies the research of industrially accessible modeling methods. The stall is initiated at the leading edge by the appearance of a small region of fluid recirculation called a “laminar separation bubble” where the flow transitions from the laminar to the turbulent state. This still poorly understood phenomenon involves transition and non-equilibrium flows for which commonly used RANS modeling tools are not suitable. In this study, a transitional bubble typical of an helicopter leading edge flow (OA209 profile at a Reynolds number Rec∞=1.8x106 and 15° of incidence) is reproduced on a flat plate. A DNS simulation of this flow is performed using the ONERA FUNk software to serve as a database for RANS models improvements. The evolution of turbulent kinetic energy budgets as well as the main RANS assumptions (isotropy of turbulence, Boussinesq hypothesis, production/dissipation balance) are analyzed. The main RANS models developed in the ONERA elsA software are then studied by weighting the turbulent quantities with a transition function reproducing the intermittency of the turbulence. The k-ω Wilcox model coupled with an optimized transition function gave the best results and was therefore kept for a more in-depth analysis, including an assessment of the main budgets and an application of this model and its transition method to a natural transition test case on a flat plate.

Generating an adverse pressure gradient (APG), using a rotating cylinder in the proximity of a plane wall under a laminar freestream flow, is studied numerically in this work. The magnitude of the generated APG is a function of the gap, G, between the cylinder and the wall, and the rotational speed of the cylinder, Ω. The flow in such a configuration is characterized by periodic transient vortex shedding at high Reynolds number. A numerical model for the computation of the transient flow for this configuration is developed using the ANSYS CFD simulation tool. The model is validated against published experimental and numerical data for similar flow configurations and excellent agreement is observed. A parametric study is carried out for different combinations of G and Ω for two different Reynolds numbers of 200 and 1000 to examine the development of the resulting separation bubble due to the generated APG. The mechanism of the boundary layer separation over the plane wall and the corresponding wake dynamics is investigated. Results are presented in terms of the distribution of the pressure coefficient as well as skin friction coefficient along the wall and flow patterns around and downstream of the cylinder in the proximity of the wall. The results of these computations confirm that using a rotating cylinder over a plane wall in a freestream flow is an effective technique to generate a controlled range of adverse pressure gradients.

O presente trabalho apresenta uma metodologia de simulação numérica de perfis aerodinâmicos bidimensionais com foco na utilização para o projeto e otimização de pás e rotores de pequenas turbinas eólicas de eixo horizontal, bem como o emprego desses métodos em simulações nas quais efeitos de alta solidez do rotor e baixos números de Reynolds são avaliados. Essa metodologia inclui geração de malhas, seleção de métodos numéricos e validação, tendo as escolhas sido guiadas pelas práticas mais bem sucedidas na simulação de perfis aerodinâmicos, e foi aplicada na simulação dos aerofólios NACA 0012, S809 e SD7062. O código comercial ANSYS Fluent foi utilizado em todas as simulações. Na simulação de aerofólios isolados a altos números de Reynolds dos perfis NACA 0012 e S809, o modelo Transition SST (γ-Reθ) apresentou resultados mais próximos a dados experimentais do que aqueles apresentados pelo modelo k-ω SST para CL e CD, além de produzir resultados para CP que mostraram boa precisão quando comparados aos mesmos dados experimentais. Resultados de CL, CD, CF e CP são apresentados para 20 diferentes condições de operação às quais o perfil SD7062 foi submetido, com números de Reynolds variando entre 25.000 e 125.000. As distribuições dos dois últimos coeficientes sobre os dorsos do aerofólio evidenciam com clareza a presença e magnitude da bolha de separação laminar. Os coeficientes de sustentação e arrasto mostram o impacto negativo da presença da bolha nessa faixa de números de Reynolds. Além disso, nos casos simulados, o arrasto aumenta em função da diminuição do Re. Um design de pá produzido com o auxílio do código de otimização SWRDC, baseado em algoritmos genéticos, é apresentado. Três seções ao longo da envergadura dessa pá foram simuladas em uma bateria de 45 simulações, sob diversas condições de operação em função de solidez, ângulo de ataque e razão de velocidade de ponta de pá. Esses resultados mostram que a bolha de separação laminar se move na direção do bordo de ataque com o aumento da solidez, do ângulo de ataque e da TSR. Além disso, distribuições do CP mostram aumento de pressão em ambos os dorsos do perfil quando submetido aos efeitos da solidez, embora esses efeitos tenham sido responsáveis por um aumento na relação CL/CD nos casos estudados. / This thesis presents a methodology of two-dimensional airfoil simulation focusing on its application on the design and optimization of blades and rotors of small horizontal axis wind turbines, and its application in a set of numerical simulations involving high rotor solidity and low-Re effects. This methodology includes grid generation, selection of numerical methods and validation, reflecting the most successful practices in airfoil simulation, and was applied in the simulation of the NACA 0012, S809 and SD7062 airfoils. The ANSYS Fluent commercial code was used in all simulations. Results for the isolated NACA 0012 and S809 airfoils at high Reynolds numbers show that the Transition SST (γ-Reθ) turbulence model produces results closer to experimental data than those yielded by the SST k-ω model for CL and CD, having also produced CP plots that show good agreement to the same experimental data. Plots of CL, CD, CF and CP for the SD7062 airfoil are presented, for simulations at 20 different operating conditions. The CF and CP distributions evidence the negative impact of the laminar separation bubble in the range of Reynolds numbers evaluated. Results show that, for Re between 25,000 and 125,000, drag increases with decreasing Re. A blade design generated using the SWRDC optimization code, based on genetic algorithms, is presented. Three sections of the resulting blade shape were selected and were tested in a set of 45 simulations, under an array of operating conditions defined by solidity, angle of attack and TSR. Results show that the laminar separation bubble moves towards the leading edge with increasing solidity, angle of attack and TSR. Furthermore, CP plots show an increase in pressure on both surfaces when the airfoil is subject to solidity effects, although these effects show an increase in the lift-to-drag ratio at the conditions evaluated.

This is an experimental and theoretical study of a laminar separation bubble and the associated transition dynamics in its early stages. The separation of a laminar boundary layer from a solid surface is prevalent in very many flow situations such as over gas turbine blades (especially in the low-pressure turbine stage) and the wings of micro-aero-vehicles (MAVs) that operate at fairly low Reynolds numbers. Flow separation occurs in such cases due to the presence of an adverse pressure gradient. The separated shear layer becomes unstable due to the presence of an inflection point and presumably transitions to turbulence rapidly. Eventually, there is reattachment back to the solid surface further downstream, if conditions are right. The region enclosed by the shear layer is called a laminar separation bubble and has been a subject of many studies in the past. The present experiments have been conducted in a closed-circuit wind tunnel. A separation bubble was obtained on the upper surface of a flat plate by appropriately contouring the top wall of the tunnel. Four different techniques were used for qualitative and quantitative study viz. surface flow visualisation, smoke flow visualisation, surface pressure measurements and hotwire anemometry. Response of the bubble to both natural as well as artificial (impulsive excitation) disturbance environment has been studied. Linear stability analyses (both Orr-Sommerfeld and Rayleigh calculations), in the spatial framework, have been performed for the mean velocity profiles starting from an attached adverse pressure gradient boundary layer all the way up to the front portion of the separation bubble region (i.e. up to the end of the dead-air region where linear evolution of disturbances could be expected). The measured velocity profiles (both attached and separated) were fitted with analytical model profiles for doing stability calculations. A separation bubble consists of aspects of both wall-bounded and wall-free shear layers and therefore both viscous and inviscid mechanisms are expected to be at play. Most of the studies in the literature point to the inviscid instability associated with the shear layer to be the main mechanism. The main aim of the present work is to understand the exact origin of the primary instability mechanism responsible for the amplification of disturbances. We argue that at least up to the front portion of the bubble, the instability mechanism is due to the inflectional mode associated with the mean velocity profile. However, the seeds of this inviscid inflectional instability could be traced back to the attached boundary layer upstream of separation. In other words, the inviscid inflectional instability of the separated shear layer should be logically seen as an extension of the instability of the upstream attached adverse-pressure-gradient boundary layer. This modifies the traditional view that pegs the origin of the instability in a separation bubble to the free shear layer outside the bubble with its associated Kelvin-Helmholtz mechanism. Our contention is that only when the separated shear layer has moved considerably away from the wall (and this happens near the maximum height of the mean bubble) that a description by Kelvin-Helmholtz instability paradigm with its associated scaling principles could become relevant. We also propose a new scaling for the most amplified frequency for a wall-bounded shear layer in terms of the inflection point height and the vorticity thickness, and show its universality. Next, we theoretically investigate the role played by the re-circulating region of the separation bubble in the linear instability regime. In the re-circulating region near the wall, associated with the so-called wall mode, the production of disturbance kinetic energy is found to be negative. This is a very interesting observation which has been cursorily noted in earlier studies. Here we show that the near-wall negative production region exerts a stabilising influence on the downstream travelling disturbances. A theoretical support for such a mechanism to exist close to the wall is presented. It is shown that the stabilising wall-proximity effect is not a peripheral aspect but has a significant effect on the overall stability especially for the waves close to the upper neutral branch. We demonstrate the appropriateness of inviscid analysis for the stability of the separated flow velocity profile away from the wall, by comparing the numerical solutions of Rayleigh and Orr-Sommerfeld equations. Following this, the analytical consequences of the Rayleigh equation such as the inflection point criterion and the Fjortoft criterion are derived for the wall-bounded inflectional velocity profiles. Furthermore, we also discuss the relevance of the negative production region towards flow control and management for the wall-bounded flows. It appears fruitful to divide the separation bubble region into two parts with respect to the nature of disturbance dynamics: one outside the mean dividing streamline (which behaves as an amplifier) and the other inside the bubble corresponding to the re-circulating region (having oscillator type characteristics). To explore the oscillator-like behaviour of the bubble further, we have carried out spatio-temporal stability analysis of the reversed flow velocity profiles and determined the conditions for the onset of absolute instability. We contend that the presence of the negative production region for the upstream travelling waves has a restraining effect arresting the tendency of the flow (both wall-free and wall-bounded) to become absolutely unstable and thereby requiring a particular threshold of the backflow velocity to be crossed for its realisation. Moreover, the delay in the onset of absolute instability for a wall-bounded profile as compared to a free shear layer is attributed to a certain ‘negative-drag’ effect of the wall on the overall flow which increases the group velocities for the wall-bounded flows. A related theme in the literature regarding the dynamics of laminar separation bubbles is the so-called ‘bursting’ of the bubble wherein there is a sudden increase in the length and height of the bubble as some critical conditions are reached. Bubbles before bursting are termed as ‘short’ bubbles and those after bursting as ‘long’ bubbles. In this work, we provide a criterion to predict bursting which is a refinement over the existing criteria. The proposed criterion takes into account not just the length of the bubble but also the maximum height and it is shown to be more universal in differentiating short bubbles from the long ones, as compared to the other criteria. We also present a hypothesis regarding the sequence of events leading to bubble bursting by relating its onset to the instability of the re-circulating region. For this we observe that as the amount of backflow velocity is increased for a reversed flow velocity profile, the inflection point moves inside the mean dividing streamline and this happens before the onset of absolute instability. This causes a vorticity maximum to develop inside the re-circulating region which could lead to the instability of the closed streamlines with respect to two-dimensional cylindrical disturbances. The actual bursting process may be expected to involve non-linear interactions of the disturbances and the long bubble could be a nonlinearly saturated state of the instability of the re-circulating region. In order to explore the three-dimensionality associated with the bubble, extensive surface flow visualisation experiments have been performed. The surface streamline pattern is obtained for the entire span of the plate for three different freestream velocities. The patterns have been interpreted using topological ideas and various critical points have been identified. It is shown that the arrangement of critical points satisfies the ‘index theorem’ which is a topological necessity and the streamline patterns are ‘structurally stable’. An interesting observation from these patterns is the presence of three-dimensionality upstream of the separation line close to the wall even though the oncoming flow is nominally two-dimensional. Using the critical point theory, we propose a hypothesis which could be used to construct a semi-empirical model wherein the critical points are assigned with a quantity called ‘strength’ for determining the extent of upstream influence of a given separation line. Finally, we derive a necessary condition for the existence of inviscid spatial instability in plane parallel flows. It states that for spatial instability the curvature of the velocity profile should be positive in some region of the profile. This includes Rayleigh’s inflection point theorem (which was proposed and proved by Rayleigh for temporal instability) as a special case. It thus provides a rigorous basis for applying the inflection point criterion to the flows in the framework of spatial stability theory (which we have used extensively in the present thesis). Moreover, the condition derived here is more general as it also includes velocity profiles with the curvature positive everywhere which are excluded by Rayleigh’s theorem in the temporal framework. An example of such a profile is presented (Couette-Poiseuille flow with adverse pressure gradient) and it is shown that this flow is an exceptional case which is temporally stable but spatially unstable. Eigenvalue calculations as well as energy considerations suggest that the mechanism governing instability of this flow is inviscid and non-inflectional in character. This is a new result which could have important implications in understanding the instability dynamics of parallel flows.

O presente trabalho apresenta uma metodologia de simulação numérica de perfis aerodinâmicos bidimensionais com foco na utilização para o projeto e otimização de pás e rotores de pequenas turbinas eólicas de eixo horizontal, bem como o emprego desses métodos em simulações nas quais efeitos de alta solidez do rotor e baixos números de Reynolds são avaliados. Essa metodologia inclui geração de malhas, seleção de métodos numéricos e validação, tendo as escolhas sido guiadas pelas práticas mais bem sucedidas na simulação de perfis aerodinâmicos, e foi aplicada na simulação dos aerofólios NACA 0012, S809 e SD7062. O código comercial ANSYS Fluent foi utilizado em todas as simulações. Na simulação de aerofólios isolados a altos números de Reynolds dos perfis NACA 0012 e S809, o modelo Transition SST (γ-Reθ) apresentou resultados mais próximos a dados experimentais do que aqueles apresentados pelo modelo k-ω SST para CL e CD, além de produzir resultados para CP que mostraram boa precisão quando comparados aos mesmos dados experimentais. Resultados de CL, CD, CF e CP são apresentados para 20 diferentes condições de operação às quais o perfil SD7062 foi submetido, com números de Reynolds variando entre 25.000 e 125.000. As distribuições dos dois últimos coeficientes sobre os dorsos do aerofólio evidenciam com clareza a presença e magnitude da bolha de separação laminar. Os coeficientes de sustentação e arrasto mostram o impacto negativo da presença da bolha nessa faixa de números de Reynolds. Além disso, nos casos simulados, o arrasto aumenta em função da diminuição do Re. Um design de pá produzido com o auxílio do código de otimização SWRDC, baseado em algoritmos genéticos, é apresentado. Três seções ao longo da envergadura dessa pá foram simuladas em uma bateria de 45 simulações, sob diversas condições de operação em função de solidez, ângulo de ataque e razão de velocidade de ponta de pá. Esses resultados mostram que a bolha de separação laminar se move na direção do bordo de ataque com o aumento da solidez, do ângulo de ataque e da TSR. Além disso, distribuições do CP mostram aumento de pressão em ambos os dorsos do perfil quando submetido aos efeitos da solidez, embora esses efeitos tenham sido responsáveis por um aumento na relação CL/CD nos casos estudados. / This thesis presents a methodology of two-dimensional airfoil simulation focusing on its application on the design and optimization of blades and rotors of small horizontal axis wind turbines, and its application in a set of numerical simulations involving high rotor solidity and low-Re effects. This methodology includes grid generation, selection of numerical methods and validation, reflecting the most successful practices in airfoil simulation, and was applied in the simulation of the NACA 0012, S809 and SD7062 airfoils. The ANSYS Fluent commercial code was used in all simulations. Results for the isolated NACA 0012 and S809 airfoils at high Reynolds numbers show that the Transition SST (γ-Reθ) turbulence model produces results closer to experimental data than those yielded by the SST k-ω model for CL and CD, having also produced CP plots that show good agreement to the same experimental data. Plots of CL, CD, CF and CP for the SD7062 airfoil are presented, for simulations at 20 different operating conditions. The CF and CP distributions evidence the negative impact of the laminar separation bubble in the range of Reynolds numbers evaluated. Results show that, for Re between 25,000 and 125,000, drag increases with decreasing Re. A blade design generated using the SWRDC optimization code, based on genetic algorithms, is presented. Three sections of the resulting blade shape were selected and were tested in a set of 45 simulations, under an array of operating conditions defined by solidity, angle of attack and TSR. Results show that the laminar separation bubble moves towards the leading edge with increasing solidity, angle of attack and TSR. Furthermore, CP plots show an increase in pressure on both surfaces when the airfoil is subject to solidity effects, although these effects show an increase in the lift-to-drag ratio at the conditions evaluated.

O presente trabalho apresenta uma metodologia de simulação numérica de perfis aerodinâmicos bidimensionais com foco na utilização para o projeto e otimização de pás e rotores de pequenas turbinas eólicas de eixo horizontal, bem como o emprego desses métodos em simulações nas quais efeitos de alta solidez do rotor e baixos números de Reynolds são avaliados. Essa metodologia inclui geração de malhas, seleção de métodos numéricos e validação, tendo as escolhas sido guiadas pelas práticas mais bem sucedidas na simulação de perfis aerodinâmicos, e foi aplicada na simulação dos aerofólios NACA 0012, S809 e SD7062. O código comercial ANSYS Fluent foi utilizado em todas as simulações. Na simulação de aerofólios isolados a altos números de Reynolds dos perfis NACA 0012 e S809, o modelo Transition SST (γ-Reθ) apresentou resultados mais próximos a dados experimentais do que aqueles apresentados pelo modelo k-ω SST para CL e CD, além de produzir resultados para CP que mostraram boa precisão quando comparados aos mesmos dados experimentais. Resultados de CL, CD, CF e CP são apresentados para 20 diferentes condições de operação às quais o perfil SD7062 foi submetido, com números de Reynolds variando entre 25.000 e 125.000. As distribuições dos dois últimos coeficientes sobre os dorsos do aerofólio evidenciam com clareza a presença e magnitude da bolha de separação laminar. Os coeficientes de sustentação e arrasto mostram o impacto negativo da presença da bolha nessa faixa de números de Reynolds. Além disso, nos casos simulados, o arrasto aumenta em função da diminuição do Re. Um design de pá produzido com o auxílio do código de otimização SWRDC, baseado em algoritmos genéticos, é apresentado. Três seções ao longo da envergadura dessa pá foram simuladas em uma bateria de 45 simulações, sob diversas condições de operação em função de solidez, ângulo de ataque e razão de velocidade de ponta de pá. Esses resultados mostram que a bolha de separação laminar se move na direção do bordo de ataque com o aumento da solidez, do ângulo de ataque e da TSR. Além disso, distribuições do CP mostram aumento de pressão em ambos os dorsos do perfil quando submetido aos efeitos da solidez, embora esses efeitos tenham sido responsáveis por um aumento na relação CL/CD nos casos estudados. / This thesis presents a methodology of two-dimensional airfoil simulation focusing on its application on the design and optimization of blades and rotors of small horizontal axis wind turbines, and its application in a set of numerical simulations involving high rotor solidity and low-Re effects. This methodology includes grid generation, selection of numerical methods and validation, reflecting the most successful practices in airfoil simulation, and was applied in the simulation of the NACA 0012, S809 and SD7062 airfoils. The ANSYS Fluent commercial code was used in all simulations. Results for the isolated NACA 0012 and S809 airfoils at high Reynolds numbers show that the Transition SST (γ-Reθ) turbulence model produces results closer to experimental data than those yielded by the SST k-ω model for CL and CD, having also produced CP plots that show good agreement to the same experimental data. Plots of CL, CD, CF and CP for the SD7062 airfoil are presented, for simulations at 20 different operating conditions. The CF and CP distributions evidence the negative impact of the laminar separation bubble in the range of Reynolds numbers evaluated. Results show that, for Re between 25,000 and 125,000, drag increases with decreasing Re. A blade design generated using the SWRDC optimization code, based on genetic algorithms, is presented. Three sections of the resulting blade shape were selected and were tested in a set of 45 simulations, under an array of operating conditions defined by solidity, angle of attack and TSR. Results show that the laminar separation bubble moves towards the leading edge with increasing solidity, angle of attack and TSR. Furthermore, CP plots show an increase in pressure on both surfaces when the airfoil is subject to solidity effects, although these effects show an increase in the lift-to-drag ratio at the conditions evaluated.

This research focuses on measurements of a convective shear-layer instability seen naturally in quiet hypersonic flow. Experiments were carried out in the Boeing/AFOSR Mach 6 Quiet Tunnel (BAM6QT) at Purdue University. The BAM6QT provides low-disturbance hypersonic flow with freestream noise levels similar to what would be experienced by a flight vehicle. To obtain high-speed, off-the-surface measurements of the instability, a modified focused laser differential interferometer (FLDI) was first designed to work with the contoured Plexiglas windows available in the tunnel.<div><br>A cone-cylinder-flare geometry was then selected to study the instabilities related to an axisymmetric separation bubble at Mach 6. The sharp cone had a 5-degree half-angle, while flare angles of 10 degrees and 3.5 degrees were tested to compare axisymmetric compression with and without separation, respectively. Under quiet flow, laminar separation and reattachment was confirmed by schlieren and surface pressure-fluctuation measurements. Coherent traveling waves were observed. These were attributed to both the second-mode instability, as well as a shear-generated instability from the separation bubble. The symmetry of the bubble was found to be highly sensitive to angle of attack. Additionally, by introducing controlled disturbances on the cone upstream of the separation, larger-amplitude shear-generated waves were measured while the second-mode amplitudes remained unchanged. Therefore, the shear-generated waves were amplified moving through the shear layer, while the second mode remained neutrally stable. These appear to be the first measurements of traveling waves that are generated in the shear layer of a separation bubble in hypersonic flow. <br></div>

Lifting surfaces of unmanned aerial vehicles (UAV) are often operated in low Reynolds number (Re) ranges, wherein the transition of boundary layer from laminar-to-turbulent plays a more significant role than in high-Re aerodynamics applications. This poses a challenge for traditional computational fluid dynamics (CFD) simulations, since typical modeling approaches assume either fully laminar or fully turbulent flow. In particular, the boundary layer state must be accurately predicted to successfully determine the separation behavior which significantly influences the aerodynamic characteristics of the airfoil. Reynolds-averaged Navier-Stokes (RANS) based CFD simulations of an elliptic airfoil are performed for time-varying angles of attack, and results are used to elucidate relevant flow physics and aerodynamic data for an elliptic airfoil under realistic operating conditions. Results are also used to evaluate the performance of several different RANS-based turbulence modeling approaches for this class of flowfield.

In order to study three-dimensional unsteady turbulent flow fields such as the wakes of bluff bodies, a Digital Particle Image Velocimetry (DPIV) system was developed. This system allows non-intrusive two-dimensional and time varying velocity measurements. Software and hardware modifications necessary to enhance the capabilities of the system were preformed, resulting in increased frequency resolution. However, due to hardware limitations and limitations inherited from the implementation of the method, space resolution is reduced. Subsequently, digital image processing tools to improve the space resolutions were developed. The advantages and limitations of the method for the study of turbulent flows are presented in detail. The developed system is employed in the documentation of time-varying turbulent flow fields. Initially we study the spanwise variation of the near wake of a low-aspect ratio, surface-mounted, circular cylinder piercing a free surface. The asymmetry of the end conditions combined with the natural unsteadiness of the vortex shedding generates a very complex flow filed which is difficult to study with conventional methods. By employing the aforementioned system we are able to reveal a departure of the two-dimensional character of the flow in the form of oblique vortex shedding. The effect of free surface on the vortex formation length and on the vortex reconnection process is documented. Near the free surface the alternate mode of vortex shedding is suppressed, leading to simultaneous shedding of vortices in the wake. Indications of vortex dislocations and change of the vortex axis in order to reconnect to the free surface are observed. Finally, a novel approach of reconstructing the three-dimensional, time -varying volume of the flow field by obtaining simultaneous measurements of Laser Doppler Velocimetry and Particle Image Velocimetry planes is presented. The same field is investigated with focus on the streamwise structures. Three-dimensional streamwise vortical structures are known to exist due to instabilities of plane shear layers. Similar streamwise vortices, also known as braid vortices have been observed in the past in the wake of circular cylinders with symmetric boundary conditions. The present spatio-temporal analysis demonstrated coexistence of two types of streamwise vortices in the wake, bilge and braid type of vortices. These may be due to the three dimensionality introduced by the free surface. In addition, the sufficient time resolution allowed the detection of the primary Von-Karman vortex through a plane of interrogation normal to the free stream, thus revealing the spanwise variation of the vortex shedding and its evolution at different downstream stations. The combination of the effect of the asymmetric boundary conditions with a free surface is investigated by adding one more source of three-dimensionality in terms of inclination of the cylinder axis. Hydrogen-bubble and particle-flow visualizations are preformed in combination with Laser-Doppler Velocimetry measurements. From both qualitative and quantitative results the effects of inclination and Froude number are documented. It is proved that the vortex shedding is suppressed for high values of the Froude number, however the inclination counteracts the vortex suppression and favors the vortex shedding mechanism. In addition, in the region of the no-slip boundary condition the flow is dominated by the effect of the horseshoe vortex. The case of a three-dimensional separated flow over a surface-mounted prism is investigated using a modified version of the system. The character of the separated from the leading edge corner shear layer and the formed separation bubble are documented in space and time along the mid-plane of symmetry of the body. Three different flows corresponding to different Reynolds numbers are studied. The unsteadiness of the flow is presented indicating a pseudo-periodic character. Large-scale, low-frequency oscillations of the shear layer that have been observed in the past using point measurement methods are now confirmed by means of a whole field velocity measurement, technique allowing a holistic view of the flow. In addition, the unsteadiness of the point of reattachment is associated with the flapping of the shear layer and the shedding of vorticity in the wake. Finally, it is demonstrated that the apparent vortex shedding mechanism of such flows is dependent on the interaction of the primary vortex of the separation bubble with a secondary vortex formed by the separation of the reverse flow boundary layer. By performing measurements with such time and space resolution the inadequacy of time averaged or point measurement methods for the treatment of such complex and unsteady flow fields becomes evident. In final case we employ Particle-Image Velocimetry to show the effect of unsteady excitation on two-dimensional separated flow over a sharp edged airfoil. It is proved that such an approach can be used to effectively control and organize the character of the flow, potentially leading to lift increase and drug reduction of bluff bodies / Ph. D.

<p dir="ltr">Pressure transducer, infrared heat transfer, and schlieren imaging data for a flared cone and cone-cylinder-flare in Mach 6 quiet flow are presented. Flared cone pressure transducer results show second-mode RMS values comparable to that found in prior experimental work. Second-mode frequency is found to linearly increase with increasing freestream unit Reynolds number, and frequency varies little between sensors for a given freestream unit Reynolds number. Turbulent intermittency begins to increase at a freestream unit Reynolds number 2x10⁶/m greater than the unit Reynolds number corresponding to peak second-mode RMS. peak RMS. High-speed schlieren imaging on the downstream section of the flared cone shows the second-mode disturbance following trends in power which correlate with PCB RMS. Infrared heat transfer results contain the azimuthal heating streak pattern observed for the flared cone in prior research, but the hot-cold-hot streak pattern is not seen due to limited model length. Streak heating occurs downstream of second-mode peak RMS over the freestream unit Reynolds number range of 6.4x10⁶/m to 10.4x10⁶/m. The heat transfer of streaks is found to vary significantly from streak to streak, while mean streak heating variation with freestream unit Reynolds number is small.</p><p dir="ltr">PCB results of the cone-cylinder-flare show intermittent turbulence at a freestream unit Reynolds number of 16.0x10⁶/m. Examination of shear-layer and second-mode instabilities show significant increases in RMS moving downstream along the flare and with increasing freestream unit Reynolds number. High-speed schlieren imaging of the shear-layer reattachment region on the flare show the presence of the shear-layer and second-mode instabilities when the model is configured with a sharp nose tip. The instabilities are not present with a blunt 5 mm radius nose tip. Heat transfer is observed to increase along the downstream portion of the flare. The sharp nose tip configuration has higher heat transfer rates than the 5 mm radius nose tip configuration.</p>