No description available.

The increased fatigue loads and decreased power output of a wind turbine placed in the wake of another turbine is a well-known problem when building new wind-power farms and a subject of intensive research. These problems are caused by the velocity gradients and high turbulence levels present in the wake of a turbine. In order to better estimate the total power output and life time of a wind-power farm, knowledge about the development and stability of wind-turbine wakes is crucial. In the present thesis, the flow field around small-scale model wind turbines has been investigated experimentally in two wind tunnels. The flow velocity was measured with both hot-wire anemometry and particle image velocimetry. To monitor the turbine performance, the rotational frequency, the power output and the total drag force on the turbine were also measured. The power and thrust coefficients for different tip-speed ratios were calculated and compared to the blade element momentum method, with a reasonable agreement. The same method was also used to design and manufacture new turbine blades, which gave an estimate of the distribution of the lift and drag forces along the blades. The influence of the inlet conditions on the turbine and the wake properties was studied by subjecting the turbine to both uniform in flow and different types of boundary layer in flows. In order to study the stability and development of the tip vortices shed from the turbine blades, a new experimental setup for phase-locked measurements was constructed. The setup made it possible to introduce perturbations of different frequencies and amplitudes, located in the rear part of the nacelle. With a newly developed method, it was possible to characterize the vortices and follow their development downstream, using only the streamwise velocity component. Measurements were also performed on porous discs placed in different configurations. The results highlighted the importance of turbine spacings. Both the measurements on the turbine and the discs were also used to compare with large eddy simulations using the actuator disc method. The simulations managed to predict the mean velocity fairly well in both cases, while larger discrepancies were seen in the turbulence intensity. / <p>QC 20140424</p>

An isolated wing-tip vortex from a square-tipped NACA 0012 wing at an angle of attack of 5 degrees was studied in a water tunnel at a chord based Reynolds number of approximately 24000. Measurements were taken using stereo particle image velocimetry at three measurement planes downstream of the wing under each of three freestream turbulence conditions. The amplitude of wandering of the vortex axis increased with increasing distance downstream of the wing and with increasing freestream turbulence intensity. The magnitude of the peak azimuthal velocity decreased with increasing distance from the wing as well as with increases in the freestream turbulence intensity. The streamwise velocity in the vortex core was less than the freestream velocity in all cases. Time resolved histories of the instantaneous waveform shape and location of the vortex axis were determined from sequences of images of fluorescent dye released from the wing.

Liquid water formation and flooding in PEM fuel cell gas distribution channels can significantly degrade fuel cell performance by causing substantial pressure drop in the channels and by inhibiting the transport of reactants to the reaction sites at the catalyst layer. A better understanding of the mechanisms of discrete water droplet transport by air flow in such small channels may be developed through the application of quantitative flow visualization techniques. This improved knowledge could contribute to improved gas channel design and higher fuel cell efficiencies. An experimental investigation was undertaken to gain better understanding of the relationships between air velocity in the channel, secondary rotational flows inside a droplet, droplet deformation, and threshold shear, drag, and pressure forces required for droplet removal. Micro-digital-particle-image-velocimetry (micro-DPIV) techniques were used to provide quantitative visualizations of the flow inside the liquid phase for the case of air flow around a droplet adhered to the wall of a 1 mm x 3 mm rectangular gas channel model. The sidewall against which the droplet was adhered was composed of PTFE treated carbon paper to simulate the porous GDL surface of a fuel cell gas channel. Visualization of droplet shape, internal flow patterns and Velocity measurements at the central cross-sectional plane of symmetry in the droplet were obtained for different air flow rates. A variety of rotational secondary flow patterns within the droplet were observed. The nature of these flows depended primarily on the air flow rate. The peak velocities of these secondary flow fields were observed to be around two orders of magnitude below the calculated channel-averaged driving air velocities. The resulting flow fields show in particular that the velocity at the air-droplet interface is finite. The experimental data collected from this study may be used for validation of numerical simulations of such droplet flows. Further study of such flow scenarios using the techniques developed in this experiment, including the general optical distortion correction algorithm developed as part of this work, may provide insight into an improved force balance model for a droplet exposed to an air flow in a gas channel.

Liquid water formation and flooding in PEM fuel cell gas distribution channels can significantly degrade fuel cell performance by causing substantial pressure drop in the channels and by inhibiting the transport of reactants to the reaction sites at the catalyst layer. A better understanding of the mechanisms of discrete water droplet transport by air flow in such small channels may be developed through the application of quantitative flow visualization techniques. This improved knowledge could contribute to improved gas channel design and higher fuel cell efficiencies. An experimental investigation was undertaken to gain better understanding of the relationships between air velocity in the channel, secondary rotational flows inside a droplet, droplet deformation, and threshold shear, drag, and pressure forces required for droplet removal. Micro-digital-particle-image-velocimetry (micro-DPIV) techniques were used to provide quantitative visualizations of the flow inside the liquid phase for the case of air flow around a droplet adhered to the wall of a 1 mm x 3 mm rectangular gas channel model. The sidewall against which the droplet was adhered was composed of PTFE treated carbon paper to simulate the porous GDL surface of a fuel cell gas channel. Visualization of droplet shape, internal flow patterns and Velocity measurements at the central cross-sectional plane of symmetry in the droplet were obtained for different air flow rates. A variety of rotational secondary flow patterns within the droplet were observed. The nature of these flows depended primarily on the air flow rate. The peak velocities of these secondary flow fields were observed to be around two orders of magnitude below the calculated channel-averaged driving air velocities. The resulting flow fields show in particular that the velocity at the air-droplet interface is finite. The experimental data collected from this study may be used for validation of numerical simulations of such droplet flows. Further study of such flow scenarios using the techniques developed in this experiment, including the general optical distortion correction algorithm developed as part of this work, may provide insight into an improved force balance model for a droplet exposed to an air flow in a gas channel.

[pt] O presente trabalho consiste no projeto, construção e qualificação de um aparato para o estudo experimental de bolhas de separação laminar sobre uma placa plana. A previsão do comportamento das bolhas de separação é importante para o projeto de aeronaves, turbinas e geradores eólicos, pois o desprendimento dessas bolhas tem grande impacto na performance de aerofólios. A dinâmica das bolhas não é bem compreendida, além de ser difícil de ser capturada por simulações numéricas que utilizam modelagem da turbulência. Por isso, ensaios experimentais são muito utilizados para a calibração dos modelos utilizados em simulações de engenharia. Neste trabalho, foram criados equipamentos para simular experimentalmente bolhas de separação laminar sobre uma placa plana. Os dispositivos foram projetados para o canal de água do Laboratório de Engenharia de Fluidos da PUC-Rio. O gradiente de pressão necessário para induzir a separação da camada limite e consequente formação da bolha foi ajustado com uma parede falsa, de modo a criar um canal convergente-divergente com a placa plana. Foi projetado um mecanismo de sucção da camada limite na parede falsa para evitar a separação do escoamento nessa superfície. A localização e as vazões de cada ponto de sução foram determinadas com o auxílio de simulações numéricas. Também foi projetado e testado um sistema de sopro e sucção para a excitação de ondas de instabilidade do tipo Tollmien-Schlichting na camada limite da placa plana. O funcionamento de cada dispositivo foi avaliado através da medição da velocidade do escoamento. Para isso, foram empregadas técnicas de medição por velocimetría laser doppler e velocimetría por imagem de partículas. Os resultados obtidos validam o projeto e qualificam o aparato para o estudo de bolhas de separação laminar. / [en] The present work involves the design, construction and performance test of an apparatus for the investigation of laminar separation bubbles in a flat plate boundary layer. Laminar separation bubbles are relevant for many engineering applications and the dynamic of such bubbles has a strong impact on the performance of aircrafts and turbines. The separated boundary layer reattaches to the surface due to the laminar-turbulent transition in the bubble region. This dynamic process is highly challenging for flow simulation tools used for engineering purposes. Thus, there is a demand for experimental studies that can be used for calibration of models present in those simulation tools. To this end, an apparatus was designed and built for the water channel of the Laboratory of Fluid Engineering at PUC-Rio. The boundary layer separation on the flat plate was induced by imposing a constant adverse pressure gradient to the flow. To this end a false wall was built, in order to form a converging-diverging channel with the flat plate. Flow separation on the false wall was avoided using a suction mechanism that was designed to reduce locally the boundary layer thickness. Location of suction and suction flow rates were determined with aid of numerical simulations. In addition, it was designed and built a disturbance source to generate Tollmien-Schlichting waves in the boundary layer of the flat plate. This device was used to trigger the boundary layer transition in a controlled manner. All equipment were tested and their designs were validated against experimental measurements. Laser Doppler anemometry and Particle Image Velocimetry techniques were adopted for assessment of each equipment. Results validate the design and show that separation bubbles can be investigated in detail using this apparatus.

To quench a thermal runaway reaction in a chemical rector, an efficient approach is the introduction of a small quantity of a liquid inhibiting agent, named a “killer”, into the mixing vessel. In this thesis, an experimental approach has been coupled tightly with numerical modelling using Computational Fluid Dynamics (CFD). The first part of this thesis is devoted to a study of the hydrodynamics of partially-baffled mixing vessels, including the free-surface deformation caused by the central vortex. The use of an inhomogeneous, multiphase approach allowed simulation of the free-surface deformation. The capability of this novel method was demonstrated by very good agreement between the numerical predictions and experimental data. In the second part, liquid jet injection at the free-surface was coupled with the vessel hydrodynamics. Numerical results, obtained using an Eulerian-Lagrangian approach, have again shown good agreement with experimental data. These results allowed the jet trajectory to be modelled and its penetration into the agitated vessel was quantified. New mixing criteria were introduced that are specific to this application. Finally, the numerical methods validated at the pilot scale were applied at the industrial scale and allowed the proposal of practical improvements to the safety of the synthesis reactors studied

No description available.

Cette étude est consacrée à l'élaboration d'une méthodologie de détermination des vitessesfondamentales des flammes laminaires, en utilisant un diagnostic de Vélocimétrie par Imagerie deParticules (PIV). Ce dernier est appliqué aux écoulements réactifs avec point de stagnation, permettant lastabilisation de flammes planes, stationnaires et en conditions quasi adiabatiques. Les effets d’étirementssubits par la flamme sont également quantifiables et parfaitement maîtrisés. L’approche ici développée atout d’abord été appliquée aux mélanges méthane/air pour validation. Une comparaison exhaustive desrésultats obtenus avec les données de la littérature est effectuée. Les codes de combustion 1D (PREMIX,OPPDIF) et 2D (Fluent©) ont été utilisés afin de confirmer la fiabilité et la précision de l’approche proposée.Une attention particulière a été accordée à la caractérisation du mouvement des particules ensemencéesdans les écoulements réactifs divergents, avec notamment la prise en considération de la force dethermophorèse. La méthode développée a ensuite été appliquée à la détermination des vitesses deflammes laminaires de divers mélanges de syngas (H2+CO). Une étude comparative sur ces mélanges aété conduite en utilisant des approches expérimentales multiples comprenant : les flammes à contrecourant,les flammes à propagation sphérique ainsi que les flammes stabilisées coniques. Les résultatsobtenus pour chaque approche ont été confrontés et la sensibilité à l’étirement des flammes de syngas aété caractérisée pour une large gamme de richesses (E.R.=0.4 to 5.0) et de compositions de mélanges(5/95 to 50/50 % H2/CO). / In the context of CO2 emission reduction, the present study is devoted to the development of alaminar flame speed measurement methodology, using the Digital Particle Image Velocimetry (DPIV)diagnostic. The latter is applied to stagnation flow flames, seen to have considerable assets for suchstudies. Indeed, flames stabilized in these diverging flows are planar, steady and in near-adiabaticconditions, while subtraction of strain effects on flame is intrinsically allowed. The methodology developedherein has been applied to the well-characterized methane/air mixtures for validation. An extensivecomparison with the literature datasets has been provided. Both 1D (PREMIX, OPPDIF) as well as 2D(Fluent©) numerical tools have been used to confirm the reliability and accuracy of the developed approach.A particular attention has been given to the characterization of the seeding particle motion within thediverging flow, with consideration of the often-neglected thermophoretic force. Fundamental flame velocitiesof various syngas (H2+CO) mixtures have been investigated using multiple experimental approachesincluding the aforementioned counterflow methodology as well as spherical and conical flameconfigurations. Performed measurements from the different approaches have been confronted and flamesensitivities to stretch have been characterized for a wide range of equivalence ratios (E.R.=0.4 to 5.0) andmixture compositions (5/95 to 50/50 % H2/CO).

La propagation d'une onde acoustique en guide est associée, pour de forts niveaux, à un certain nombre de phénomènes de l'acoustique non linéaire. Parmi ces phénomènes, les écoulements redressés (ou vent acoustique), l'effet d'une discontinuité et la transition à la turbulence, à l'étude dans ce mémoire, sont associés à la génération d'écoulements induits. L'étude expérimentale de ces phénomènes repose sur l'adaptation des méthodes de vélocimétrie Laser : Vélocimétrie Laser par effet Doppler (VLD) et Vélocimétrie par Images de Particules (PIV) à la mesure des différents écoulements. Ainsi, des mesures PIV en sortie de convergent, viennent compléter des mesures VLD réalisées antérieurement. Dans l'espoir de mieux appréhender les spécificités de la transition à la turbulence en guide d'onde acoustique, l'évolution de la couche limite de Stokes est étudiée pour des amplitudes de vitesse acoustique croissantes. Une étude expérimentale des écoulements redressés dans un guide d'onde à section carrée est proposée et les spécificités liées à cette géométrie sont recherchées. En outre, l'évolution des tourbillons du vent acoustique en guide d'onde cylindrique est analysée lorsque le vent devient rapide et certains facteurs pouvant être à l'origine de cette évolution sont modifiés. La répartition harmonique dans le guide est ainsi modifiée, puis l'influence des conditions thermiques est abordée en couplant les mesures de vitesses à des mesures de température moyenne dans le guide et en paroi. Une comparaison avec des résultats issus de simulations numériques permet de conforter l'évolution des écoulements redressés observée. / High amplitude acoustic propagation in a guide is associated with several non linear phenomena including acoustic streaming, discontinuity effects and transition to turbulence. Those phenomena are studied in this work and are all associated with acoustically induced flows. The present experimental study therefore is based on velocimetry laser techniques: Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV), wich are fitted to the measurement of the different flow velocity components. Firstly, PIV measurements at the exit of a convergent enable to complement previous LDV measurements. Then, in order to a better understanding of the specificity of transition to turbulence in acoustics, the evolution of the Stokes boundary layer is studied for increasing acoustic velocity amplitudes. Then an experimental study of acoustic streaming in a square channel is reported, and the influence of the geometry is examined. Moreover, the evolution of acoustic streaming vortices in a cylindrical waveguide is analyzed for fast streaming and some parameters that could control such evolution are modified. The harmonicdistribution inside the guide is changed and then the influence of thermal conditions is studied by coupling velocity measurements and mean temperature measurements inside the waveguide and along the wall. Some comparisons between measured streaming velocities and numerical simulation results are presented.