Combustion systems utilizing bluff bodies to stabilize the combustion processes can experience oscillatory heat release due to the alternate shedding of coherent, von Kármán vortices under certain operating conditions. This phenomenon needs to be understood in greater detail, since unsteady burning due to vortex shedding can lead to combustion instabilities and flame extinction in practical combustion systems. The primary objective of this study was to elucidate the influence of combustion process heat release upon the Bénard-von Kármán (BVK) instability in reacting bluff body wakes. For this purpose, spatial and temporal heat release distributions in bluff body-stabilized combustion of liquid Jet-A fuel with high-temperature, vitiated air were characterized over a wide range of operating conditions. Upon comparing the spatial and temporal heat release distributions, the fuel entrainment and subsequent heat release in the near-wake were found to strongly influence the onset and amplitude of the BVK instability. As the amount of heat release in the near-wake decreased, the BVK instability increased in amplitude. This was attributed to the corresponding decrease in the local density gradient across the reacting shear layers, which resulted in less damping of vorticity due to gas expansion. The experimental results were compared to the results of a parallel, linear stability analysis in order to further understand the influence of the combustion processes in the near-wake upon the wake instability characteristics. The results of this analysis support the postulate that oscillatory heat release due to BVK vortex shedding is the result of local absolute instability in the near-wake, which is eliminated only if the temperature rise across the reacting shear layers is sufficiently high. Furthermore, the results of this thesis demonstrate that non-uniform fuelling of the near-wake reaction zone increases the likelihood of absolutely unstable, BVK flame dynamics due to the possibility of near-unity products-to-reactants density ratios locally, especially when the reactants temperature is high.

The problem of bluff body flows and the drag associated with them has been the subject of numerous investigations in the literature. In the two-dimensional case, the flow past a circular cylinder has been most widely studied both experimentally and computationally. As a result, a well documented understanding of the gross features of the near-wake around a circular cylinder exists in the literature. In contrast, very little is understood on the general features of three-dimensional bluff body near-wakes, except that the vortex shedding is known to be less intense. Control or management of bluff body flows, both from the point of view of drag reduction as well as suppressing unsteady forces caused by vortex shedding, has been an area of considerable interest in engineering applications. The basic aim in the different control methods involves direct or indirect manipulation (or modification) of the near-wake structure leading to weakening or inhibition of vortex shedding. Many passive and energetic techniques (such as splitter plates, base and trailing edge modifications and base bleed) have been effective in the two-dimensional case in increasing the base pressure, leading to varying amounts of drag reduction; a large body of this work is centered around circular cylinders because of direct relevance in applications. The present work is an attempt to understand some of the major aspects of the near-wake structure of a sphere and to control the same for drag reduction employing a passive technique. Many of the passive control techniques found useful in two-dimensional flows are not appropriate in the context of a sphere. In this thesis, the effects of natural ventilation on the wake and drag of a sphere at low speeds have been studied experimentally in some detail. Natural bleed into the base is created when the stagnation and base regions of a sphere are connected through an internal duct. Although natural ventilation has features broadly similar to the well known base-bleed technique (both involve addition of mass, momentum and energy into the near-wake), there are many significant differences between the two methods; for example, in base bleed, the mass flow injected can be controlled independent of the outer flow, whereas in natural ventilation, it is determined by an interaction between the internal and the external flow around the body. Experiments have been conducted in both wind and water tunnels, which covered a wide range of Reynolds number (ReDj based on the diameter of the sphere) from of 1.7 x 103 to 8.5 x 105 with natural boundary layer transition. The ratio of the frontal vent area to the maximum cross sectional area of the sphere was varied from 1% to 2.25% and the effect of the internal duct geometry, including a convergent and a divergent duct was examined as well. After preliminary force measurements involving different duct geometries and vent areas, it was decided to make detailed measurements with a straight (parallel) duct with a vent area ratio of 2.25%. Extensive flow visualization studies involving dye-flow, hydrogen bubble, surface oil-flow and laser-light-sheet techniques were employed to gain insight into many aspects of the near-wake structure and the flow on the surface of the sphere. Measurements made included model static pressures, drag force using a strain gauge balance and velocity profiles in the near-wake and internal flow through the vent. In addition, wake vortex shedding frequency was measured using a hotwire. In the subcritical range of Reynolds numbers (ReD< 2 x 105), the near-wake of the sphere (without ventilation) was found to be vortex shedding, with laminar separation occurring around a value of0s = 80° (where 0s is the angle between the stagnation point and separation location). In contrast, there was little evidence of vortex shedding in the supercritical range (ReD> 4 x 105), consistent with many earlier observations in the literature; however, flow visualization studies in the near-wake clearly showed the existence of a three-dimensional vortex-like structure exhibiting random rotations about the streamwise axis. In this range of Reynolds numbers, surface flow visualization studies indicated the existence of a laminar separation bubble which was followed by a transitional/turbulent reattachment and an ultimate separation around 0S = 145°. All the above observations are broadly consistent with the results available in the literature. With ventilation at subcritical Reynolds numbers, the pressure distributions on the sphere including in the base region was only weakly altered, resulting in a marginal reduction in the total drag; because of the higher pressure difference between the stagnation and base regions, the mean velocity in the vent-flow was about 0.9 times the free-stream velocity. As may be expected, there was little change in the location of laminar separation on the sphere and the vortex shedding frequency was virtually unaltered due to ventilation. The relatively small effects on pressure distribution and drag suggest weak interaction between the vent-flow and the separated shear layer in the subcritical regime. The time-averaged near-wake flow revealed a stagnation point occurring between the vent-flow and the reverse flow in the near-wake, along with the formation of a torroidal vortex between the stagnation point and the near-wake closure; these features bear some resemblance to those observed with base bleed from a blunt base. With ventilation in the supercritical range of Reynolds numbers (ReD > 4 x 105), significant reduction in the total drag, of as much as 65%, was observed from force measurements. Pressure distributions showed higher pressures in the separated flow zone (consistent with reduced drag) as a result of which the internal mass and the mean velocity of the vent-flow were lower (0.69 times the free-stream velocity) compared to the value in the subcritical flow regime. Flow visualization studies clearly showed that the three-dimensional rotating structure (associated with the wake of the unvented sphere) was significantly modified by ventilation, leading to more symmetric and steady near-wake features. The larger effects on pressure distribution and drag suggest strong interaction between the vent-flow and the separated shear layer, promoted by their close proximity. The comparison of power spectral density of u1 signals in the near-wake showed significant reduction in the amplitude at all frequencies, consistent with observations from flow visualization studies. The time-averaged near-wake flow features a pair of counterrotating ring vortices which are trapped between the outer separated shear layer and the vent-flow shear layer; such a mean flow pattern is qualitatively similar to that behind an axisymmetric base with a central jet with unequal freestream velocities in the jet and outer flow. This study strongly suggests that natural ventilation can provide significant total drag reduction provided the vent-flow is in close proximity of the separated shear layer promoting a strong interaction between them. Drag reduction is associated with more symmetric and relatively steady near-wake features in contrast with the unvented sphere.

Com a crescente necessidade de projetos cada vez mais econômicos, bem como a valorização dos terrenos nos grandes centros urbanos, que leva a conseqüente necessidade de aumentar o aproveitamento destes, e com o desenvolvimento das técnicas construtivas e dos processos de análise estrutural, as edificações tornaram-se mais altas, leves, flexíveis, menos amortecidas, e, portanto, mais susceptíveis a problemas de vibrações, inclusive as induzidas pela ação do vento. Nessa situação, o amortecimento natural da edificação pode tornar-se insuficiente para reduzir os movimentos causados pela ação do vento, o que pode gerar desconforto aos usuários, quebra de vidros e até mesmo danos à estrutura. Para se atingir a redução destes deslocamentos, em alguns casos, requer-se um suplemento adicional de amortecimento, para evitar tais movimentações excessivas. Tal suplemento de amortecimento é alcançado pela instalação de um sistema de dissipação de energia na edificação. Os atenuadores dinâmicos sintonizados constituem-se em um destes sistemas de dissipação de energia, sendo utilizados para aumentar o amortecimento geral do sistema estrutural. Foram realizados testes com um modelo do edifício alto padrão “CAARC Standard Tall Building”, primeiramente sem nenhum atenuador e após com dois tipos de atenuadores com características diferentes instalados no modelo. No trabalho são apresentados e discutidos os resultados dos ensaios, realizados no Túnel de Vento Professor Joaquim Blessmann, da Universidade Federal do Rio Grande do Sul. Os atenuadores dinâmicos sintonizados (amortecedores de massa) mostraram-se eficazes na redução das vibrações transversais por desprendimento de vórtices, validando o túnel de vento como ferramenta de projeto para a prevenção e controle de fenômenos associados às vibrações induzidas pelo vento. / With the increasing need of more economic buildings, as well as the great valorization of the terrains in the center of big cities, which leads to a consequent need to improve the utilization of this terrains, and with the development of the construction techniques and of the structural analysis process, the buildings have become higher, lighter, more flexible and less damped, and, therefore, more susceptible to problems of vibrations, such as those induced by wind action. On this new situation, the natural damping of the building could become insufficient to reduce the motion caused by wind action, which can lead to discomfort to the users, break of glasses and even damage to the structure. In order to obtain a reduction on this displacements, in some cases, an additional supply of damping is needed, to avoid this excessive movements. This supply of damping is obtained by the installation of an energy dissipation system on the building. The tuned mass dampers constitute on one of this systems of energy dissipation, being used to improve the overall damping of the structural system. Tests with a model of the “CAARC Standard Tall Building” were conduced, first with no damper attached to it, and then with two different types of tuned mass dampers installed on the model in each time. The results obtained with these tests, conduced on the Túnel de Vento Professor Joaquim Blessmann, at the Universidade Federal do Rio Grande do Sul are presented and discussed. In the tests, the tuned mass dampers have shown good efficiency in the reduction of the transversal vibration caused by vortex shedding, which validates the wind tunnel as a design tool for the control and prevention of the phenomena of wind-induced vibrations.

Com a crescente necessidade de projetos cada vez mais econômicos, bem como a valorização dos terrenos nos grandes centros urbanos, que leva a conseqüente necessidade de aumentar o aproveitamento destes, e com o desenvolvimento das técnicas construtivas e dos processos de análise estrutural, as edificações tornaram-se mais altas, leves, flexíveis, menos amortecidas, e, portanto, mais susceptíveis a problemas de vibrações, inclusive as induzidas pela ação do vento. Nessa situação, o amortecimento natural da edificação pode tornar-se insuficiente para reduzir os movimentos causados pela ação do vento, o que pode gerar desconforto aos usuários, quebra de vidros e até mesmo danos à estrutura. Para se atingir a redução destes deslocamentos, em alguns casos, requer-se um suplemento adicional de amortecimento, para evitar tais movimentações excessivas. Tal suplemento de amortecimento é alcançado pela instalação de um sistema de dissipação de energia na edificação. Os atenuadores dinâmicos sintonizados constituem-se em um destes sistemas de dissipação de energia, sendo utilizados para aumentar o amortecimento geral do sistema estrutural. Foram realizados testes com um modelo do edifício alto padrão “CAARC Standard Tall Building”, primeiramente sem nenhum atenuador e após com dois tipos de atenuadores com características diferentes instalados no modelo. No trabalho são apresentados e discutidos os resultados dos ensaios, realizados no Túnel de Vento Professor Joaquim Blessmann, da Universidade Federal do Rio Grande do Sul. Os atenuadores dinâmicos sintonizados (amortecedores de massa) mostraram-se eficazes na redução das vibrações transversais por desprendimento de vórtices, validando o túnel de vento como ferramenta de projeto para a prevenção e controle de fenômenos associados às vibrações induzidas pelo vento. / With the increasing need of more economic buildings, as well as the great valorization of the terrains in the center of big cities, which leads to a consequent need to improve the utilization of this terrains, and with the development of the construction techniques and of the structural analysis process, the buildings have become higher, lighter, more flexible and less damped, and, therefore, more susceptible to problems of vibrations, such as those induced by wind action. On this new situation, the natural damping of the building could become insufficient to reduce the motion caused by wind action, which can lead to discomfort to the users, break of glasses and even damage to the structure. In order to obtain a reduction on this displacements, in some cases, an additional supply of damping is needed, to avoid this excessive movements. This supply of damping is obtained by the installation of an energy dissipation system on the building. The tuned mass dampers constitute on one of this systems of energy dissipation, being used to improve the overall damping of the structural system. Tests with a model of the “CAARC Standard Tall Building” were conduced, first with no damper attached to it, and then with two different types of tuned mass dampers installed on the model in each time. The results obtained with these tests, conduced on the Túnel de Vento Professor Joaquim Blessmann, at the Universidade Federal do Rio Grande do Sul are presented and discussed. In the tests, the tuned mass dampers have shown good efficiency in the reduction of the transversal vibration caused by vortex shedding, which validates the wind tunnel as a design tool for the control and prevention of the phenomena of wind-induced vibrations.

Com a crescente necessidade de projetos cada vez mais econômicos, bem como a valorização dos terrenos nos grandes centros urbanos, que leva a conseqüente necessidade de aumentar o aproveitamento destes, e com o desenvolvimento das técnicas construtivas e dos processos de análise estrutural, as edificações tornaram-se mais altas, leves, flexíveis, menos amortecidas, e, portanto, mais susceptíveis a problemas de vibrações, inclusive as induzidas pela ação do vento. Nessa situação, o amortecimento natural da edificação pode tornar-se insuficiente para reduzir os movimentos causados pela ação do vento, o que pode gerar desconforto aos usuários, quebra de vidros e até mesmo danos à estrutura. Para se atingir a redução destes deslocamentos, em alguns casos, requer-se um suplemento adicional de amortecimento, para evitar tais movimentações excessivas. Tal suplemento de amortecimento é alcançado pela instalação de um sistema de dissipação de energia na edificação. Os atenuadores dinâmicos sintonizados constituem-se em um destes sistemas de dissipação de energia, sendo utilizados para aumentar o amortecimento geral do sistema estrutural. Foram realizados testes com um modelo do edifício alto padrão “CAARC Standard Tall Building”, primeiramente sem nenhum atenuador e após com dois tipos de atenuadores com características diferentes instalados no modelo. No trabalho são apresentados e discutidos os resultados dos ensaios, realizados no Túnel de Vento Professor Joaquim Blessmann, da Universidade Federal do Rio Grande do Sul. Os atenuadores dinâmicos sintonizados (amortecedores de massa) mostraram-se eficazes na redução das vibrações transversais por desprendimento de vórtices, validando o túnel de vento como ferramenta de projeto para a prevenção e controle de fenômenos associados às vibrações induzidas pelo vento. / With the increasing need of more economic buildings, as well as the great valorization of the terrains in the center of big cities, which leads to a consequent need to improve the utilization of this terrains, and with the development of the construction techniques and of the structural analysis process, the buildings have become higher, lighter, more flexible and less damped, and, therefore, more susceptible to problems of vibrations, such as those induced by wind action. On this new situation, the natural damping of the building could become insufficient to reduce the motion caused by wind action, which can lead to discomfort to the users, break of glasses and even damage to the structure. In order to obtain a reduction on this displacements, in some cases, an additional supply of damping is needed, to avoid this excessive movements. This supply of damping is obtained by the installation of an energy dissipation system on the building. The tuned mass dampers constitute on one of this systems of energy dissipation, being used to improve the overall damping of the structural system. Tests with a model of the “CAARC Standard Tall Building” were conduced, first with no damper attached to it, and then with two different types of tuned mass dampers installed on the model in each time. The results obtained with these tests, conduced on the Túnel de Vento Professor Joaquim Blessmann, at the Universidade Federal do Rio Grande do Sul are presented and discussed. In the tests, the tuned mass dampers have shown good efficiency in the reduction of the transversal vibration caused by vortex shedding, which validates the wind tunnel as a design tool for the control and prevention of the phenomena of wind-induced vibrations.

Neste trabalho simulou-se o escoamento do fluxo de ar em um tubo, com um anteparo de formato triangular com arestas cortantes, posicionado no centro do tubo. O objetivo do estudo é a análise do comportamento do número de Strouhal em função do número de Reynolds. Para isto, foi utilizada a técnica da análise tempo-freqüência, baseada na transformada de Fourier e na transformada de Gabor. Os ensaios foram realizados com o fluxo com velocidades médias de escoamento de 3 a 10 m/s, sendo utilizado um sensor de pressão tipo piezo-resistivo para a detecção da flutuação de pressão ocasionada pelo desprendimento e formação dos vórtices. Os ensaios foram realizados em cinco etapas com o objetivo de se verificar a influência dos seguintes parâmetros na coleta de sinais e no fenômeno: ruído da rede elétrica; influência do anteparo e do ruído proveniente do escoamento do fluxo de ar; número de pontos da amostragem na coleta dos dados; do comprimento da tubulação; e posicionamento do sensor. Pode-se observar, a sensibilidade do sistema de medição através do ensaio realizado sem o anteparo, sendo verificada a influência do ruído do escoamento de ar pelo tubo; pode-se observar também uma pequena interferência do ruído da rede elétrica predominantemente para velocidades abaixo de 3 m/s. Apesar das influências citadas, e utilizando a transformada de Gabor para análise dos sinais, observou-se um sinal mais intenso na freqüência dos vórtices para as velocidades de escoamento, podendo-se comprovar que o número de Strouhal permanece quase constante e é independente do número de Reynolds, devendo-se ressaltar que esta conclusão é valida para números de Reynolds compreendidos na faixa de 3000 a 100000. No experimento obteve-se um fator de sensibilidade (freqüência vórtices/velocidade média) de 8,2 Hz/m/s, e número de Strouhal médio de 0,196. / This work concerns the simulation of an air flux through a pipe with a triangular bluff body positioned inside it. In order to study the behavior of the Strouhal number in function of the Reynolds number. For this, the time-frequency analysis technique was used, based on Fourier transform and the Gabor transform. The experiments were carried out with an air flux velocity ranging from 3 to 10 m/s and using a piezoresistive pressure sensor to detect pressure fluctuations caused by the shedding and vortex formation. The experimental procedures were divided in five stages to make it possible to verify the influence of the following parameters in the signal data acquisition: electric network noise, the bluff body presence and the noise generated due to its presence, number of sampling data points, tubing length and sensor positioning. The sensitivity of the experiment could be observed testing the air flowing with no bluff body inside the pipe. Thus, it was possible to investigate the influence of the noise generated due to this flux limiting body. It could be also observed, mainly at 3 m/s or less, the noise generated due to the electric network. Despite the listed influences, and with the use of the Gabor transform, a more intense signal on the vortex frequency for the flow velocity was observed, showing that the Strouhal number remains almost constant and is independent of the Reynolds number. It is important to recall that this conclusion is valid for Reynolds numbers between 3000 and 100000. In the experiments the factor of sensitivity (vortex frequency/mean velocity) obtained was 8,2 Hz/m/s and the mean Strouhal number 0,196.

The limitations lying behind the applications of EN-1991-1-4, Eurocode1, actions on structures-general actions-wind load-part 1-4, lead the structural designers to a great confusion. This may be due to the fact that EC1 only provides the guidance for bridges whose fundamental modes of vibration have a constant sign (e.g. simply supported structures) or a simple linear sign (e.g. cantilever structures) and these modes are the governing modes of vibration of the structure. EC1 analyzes only the along-wind response of the structure and does not deal with the cross wind response. The simplified methods that are recommended in this code can be used to analyze structures with simple geometrical configurations. In this report, the analytical methods which are used to describe the fluctuating wind behavior and predict the relative static and dynamic response of the structure are studied and presented. The criteria used to judge the acceptability of the wind load and the corresponding structural responses along with the serviceability considerations are also presented. Then based on the given methods the wind forces acting on a continuous bridge whose main span is larger than the 50 meters (i.e. &gt; 50 meter requires dynamic assessment) is studied and compared with the results which could be obtained from the simplified methods recommended in the EC1.

[pt] A supervisão da vazão da água de resfriamento em equipamentos de produção de energia de usinas hidrelétricas tem sido uma questão de preocupação devido à possibilidade de obstrução dos equipamentos de medição pela água sem tratamento. Outrossim, por razões econômicas, um instrumento barato e confiável deve ser escolhido para os vários pontos de monitoramento. Neste trabalho, um dispositivo não intrusivo foi desenvolvido, tendo seu desempenho avaliado para a medição da vazão da água na faixa de 0,7 a 7 m3/h. O princípio básico de operação é a variação da freqüência de formação de vórtices como função da vazão. Um acelerômetro colocado na parede externa de uma tubulação mede a freqüência de vibração induzida pelos vórtices. Vários testes mostraram que o instrumento é sensível a ruídos, que devem ser filtrados para a redução da incerteza de medição. O número de Strouhal foi avaliado como função do número de Reynolds do escoamento, mostrando um comportamento assintótico para números de Reynolds elevados. O instrumento diferencia nitidamente a existência ou não de escoamento. Presentemente, um esforço para condicionamento do sinal está sendo feito para a redução da incerteza de medição da vazão, que é estimada nesta dissertação. / [en] The supervision of the cooling water flow rate in power producing equipments of hydro-electric plants has been an issue of concern due to the possibility of clogging up measurement instruments by the used non treated flowing water. Furthermore, for economic reasons, a cheap and reliable instrument must be chosen for each of the many monitoring points. In this work, a non-intrusive device was developed and its performance analized for measuring water flow rate in the 0,7 to 7 m3/h range. The basic operating principle is the variation of the vortex shedding frequency with flow rate. An accelerometer placed outside the pipe wall measures the vortex induced vibration frequency. Several tests showed that the instrument is noise sensitive, which must be filtered to reduce the uncertainty of measurement. The Strouhal number was plotted as a function of the flow Reynolds number, showing an asymptotic trend towards an approximately constant value at high Reynolds numbers. The instrument sharply differenciates between flow and non-flow situations. Presently, a signal conditioning effort is being conducted to reduce the uncertainty of measurement of the flow rate, which is estimated in this dissertation.

Em uma aeronave, dispositivos de hiper-sustentação à altos ângulos de ataque promovem a formação de esteiras. Tais esteiras influenciam o desempenho aerodinâmico. A esteira do eslate, por exemplo, afeta o ponto de transição do elemento principal da asa e é a fonte de ruído mais importante na estrutura do avião. Devido a geração de sustentação estas esteiras são assimétricas. Corpos imersos em escoamento cisalhante também produzem esteiras assimétricas, com importantes aplicações para a indústria petrolífera. Existem aspectos de tais escoamentos que precisam de investigação. Por exemplo, não há consenso sobre se pode ou não a assimetria suprimir desprendimento de vórtice ou como afeta o desprendimento da frequência. Na verdade, existe uma aparente discrepância entre os resultados encontrados na literatura. O objetivo do presente trabalho é contribuir para esta questão. A idéia foi investigar a influência da assimetria em um perfil de esteira bidimensional sob desenvolvimento temporal. O perfil da esteira assimétrica foi obtido por uma combinação entre um perfil de uma esteira gaussiana e um perfil tangente hiperbólico da camada de mistura. Foi desenvolvido uma análise bidimensional (2D) da teoria de estabilidade linear para o perfil. O trabalho também incluiu simulações numéricas diretas (DNS) bidimensionais da evolução da perturbação usando formulação característica das equações compressíveis de Navier-Stokes na forma não-conservativa. Os resultados mostraram que a assimetria reduz o frequência de desprendimento para a faixa investigada do parâmetro de assimetria 0 \'< OU =\' K \'< OU =\' 0,25. Para pequenos valores deste parâmetro os resultados mostraram que a assimetria promove a estabilidade. Efeitos do número de Mach na esteira assimétrica também foram investigados. Os resultados mostraram que o aumento do Ma reduziu a máxima taxa de amplificação e a banda instável. Além disso, os resultados da literatura foram reconciliados, levando-se em conta a variação do coeficiente de arrasto com a assimetria. Portanto, uma possível explicação para a controvérsia foi oferecido. / In an aircraft, high-lift devices operating at high angle of attack promote the formation wakes. Such wakes influence the aerodynamic performance. The slat wake, for example, affects the transition point of the wing main element and is the most important source of noise in the airframe. Owing to the generation of lift these wakes are asymmetries. Bodies immersed in a shear flow also produce asymmetric wakes, with important applications to the oil industry. There are aspects of such flows that need investigation. For instance, there is no consensus about whether or not the asymmetry can suppress vortex shedding or how it affects the shedding frequency. Indeed there is an apparent discrepancy between results found in literature. The aim of the present work was to contribute to this issue. The idea was to investigate the influence of asymmetry on a two-dimensional wake profile under temporal development. The asymmetric wake profile was obtained by a combination between a Gaussian wake profile and a hyperbolic tangent mixing layer profile. The bidimensional (2D) linear stability theory analysis of the profile was performed. The work also included 2D Direct Numerical Simulation (DNS) using the characteristic formulation of the compressible Navier-Stokes equations in non-conservative form. The results showed that the asymmetry reduces the shedding frequency for the range of 0 \'< OU =\' K \'< OU =\' 0,25 investigated. For small values of the asymmetric parameter the results showed that the asymmetry promotes stability. Effects of the number of Mach in asymmetric wake also were investigated. The results showed that the increase of Ma reduced the maximum rate amplification and the band unstable. Moreover, the results from literature were reconciled by taking into account the variation of the drag coefficient with the asymmetry. Therefore, a possible explanation for the controversy was offered.

In nature, many swimming and flying creatures use the principle of oscillatory lift-based propulsion. Often the flapping element is flexible, totally or partially. The flow dynamics because of a flexible flap is thus of considerable interest. We are interested especially in lunate fish propulsion. The present work investigates the effect of trailing edge flexibility on the flow field created by an oscillating airfoil in an attempt to mimic the flow around the flexible tails often found in fish. A flexible flap with negligible mass and stiffness is attached at the trailing edge of NACA0015 airfoil. The flap length is 75% of the rigid chord length. The airfoil oscillates about a hinge point at 30% chord from the leading edge and at the same time it moves in a circular path in stationary water. The parameters varied are frequency, amplitude of oscillation and forward speed. For a given combination of amplitude and frequency of oscillation, the forward speed is chosen such that the Strouhal number comes around 0.3, which falls in the gamut of Strouhal numbers for maximum propulsive efficiency. We visualize the flow with dye and particles and measure velocities using Particle Image Velocimetry (PIV). We use shadow technique and image processing to study the flap dynamics. We do a qualitative and quantitative comparison of the wake flow generated by two airfoil models, one with rigid trailing edge (model -B) and the other with flexible trailing edge (model -A) i.e. with a flexible flap fixed to the trailing edge. We study the flap dynamics, the flow around the flap, evolution of vortices, wake width, circulations around airfoil and vortices, momentum and energy in the wake (which is measure of propulsion efficiency), vortex geometry in the wake in terms of vortex spacing, etc. We also conduct a parametric study for both the models. Flap dynamics plays a prominent role in defining the signature of the wake. The observed flap deflections are quite large and the flap exhibits more than one mode of deflection; this affects the vortex-shedding pattern. The flap tip also executes a near sinusoidal motion with a phase difference between the trailing edge and the flap tip. The dye visualization studies show that a flexible trailing edge induces multiple vortices while in case of a rigid trailing edge, large vortical structures are shed. In case of flexible trailing edge (model -A), the vortices are shed away from the mean path of motion and are arranged in a ‘reverse Karman vortex street’ pattern producing an undulating jet representing a thrust on the airfoil. For the same Strouhal number, in case of rigid trailing edge (model -B), the vortices are shed nearly along the mean path of motion indicating a momentumless wake. The wake structures, particularly in case of model -A, are nearly insensitive to variations in amplitude and frequency. The wake of model -B shows some variable flow patterns for different amplitudes of oscillation. Although the total chord of model -A is 1.75 times more than the chord of model -B, the wake width is nearly the same for the two models when the amplitude of oscillation is same. The addition of the flap to the airfoil keeps the wake flow two-dimensional or symmetric about the center plane for longer times and longer downstream distances in comparison with the wake flow generated by rigid trailing edge. For 15o and 20o amplitudes of oscillations, the flow separates over the airfoil itself; the interaction of the separated flow with the flexible flap is quite interesting, which needs further investigations. The wake generated by the airfoil with flexible flap at the trailing edge has some common features with the wakes generated by the flow over a flapping filament (which is the one-dimensional representation of a fluttering flag), an accelerating mullet fish (a carangiform swimmer) and a steadily swimming eel (an anguilliform swimmer).