Lean flame blowout is investigated experimentally within a high-speed combustor to analyze the temporal extinction dynamics of turbulent premixed bluff body stabilized flames. The lean blowout process is induced through fuel flow reduction and captured temporally using simultaneous high-speed particle imaging velocimetry (PIV) and CH* chemiluminescence. The evolution of the flame structure, flow field, and the resulting strain rate along the flame are analyzed throughout extinction to distinguish the physical mechanisms of blowout. Flame-vortex dynamics are found to be the main driving mechanism of flame extinction; namely, a reduction of flame-generated vorticity coupled with an increase of downstream shear layer vorticity. The vorticity dynamics are linked to hydrodynamic instabilities that vary as a function of the decreasing equivalence ratio. Frequency analysis is performed to characterize the dynamical changes of the hydrodynamic instability modes during flame extinction. Additionally, various bluff body inflow velocity regimes are investigated to further characterize the extinction instability modes. Both equivalence ratio and flow-driven instabilities are captured through a universal definition of the Strouhal number for the reacting bluff body flow. Finally, a Karlovitz number-based criterion is developed to consistently predict the onset of global extinction for different inflow velocity regimes.

[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.

Submitted by Maicon Juliano Schmidt (maicons) on 2015-03-20T19:50:03Z No. of bitstreams: 1 000002F2.pdf: 1101205 bytes, checksum: 35ea0ac880e5841836ff1b5e64d2f9ff (MD5) / Made available in DSpace on 2015-03-20T19:50:04Z (GMT). No. of bitstreams: 1 000002F2.pdf: 1101205 bytes, checksum: 35ea0ac880e5841836ff1b5e64d2f9ff (MD5) Previous issue date: 2012-11-27 / CNPQ – Conselho Nacional de Desenvolvimento Científico e Tecnológico / Componentes eletrônicos estão cada vez mais potentes, necessitando de dissipações térmicas maiores. Os ventiladores atuais, conhecidos comercialmente como "coolers", estão se tornando ineficientes para esta evolução por dependerem de uma maior vazão para atender a demanda de calor dissipado, o que também causa aumento no seu ruído. Como uma alternativa para aprimorar a troca de calor, estudam-se (micro) jatos sintéticos. Estes são produzidos através de uma cavidade selada por uma membrana oscilatória e uma placa com um orifício. A movimentação periódica da membrana produz um jato com valor positivo de quantidade de movimento, que pode ser direcionado para o resfriamento de um dispositivo eletrônico.Para análise térmica, um modelo numérico do dispositivo de refrigeração foi construído em ANSYS CFX 12.0. Variações nos números de Reynolds e Strouhal dos jatos sintéticos e posição da região aquecida na superfície de interesse foram realizadas e seu efeito no desempenho térmico analisado. Os resultados foram comparados a um escoamento convencional de mesma geometria em regime permanente e submetido à mesma vazão mássica média induzida por cada jato sintético. Para a configuração testada, observou-se que os (micro) jatos sintéticos podem fornecer um fluxo de ar mais direcionado para os "hotspots" com maior necessidade de resfriamento. Os resultados encontrados indicam um aumento de número de Nusselt até 122% em jatos sintéticos comparados aos escoamentos contínuos. Logo, confirmam o maior desempenho térmico do jato sintético em relação ao método convencional equivalente e justificam a necessidade de investigações adicionais nesta área. Isto indica que os jatos sintéticos podem ser personalizados ou direcionados especificamente para atender a demanda de resfriamento do problema de interesse. / The rising power consumption of electronic components requires higher and higher thermal dissipation. Current fan systems, commercially known as "coolers", are becoming ineffective to cope with this demand since their performance is dependent on the volumetric flow rate of the driving fan, which becomes more wasteful and noisy. An alternative to improve the heat exchange of current systems is the application of (micro) synthetic jets. These are produced by the oscillations in a cavity bounded by a membrane and a plate with an orifice. Membrane actuation produces a net forward momentum jet through the orifice, which can be applied to cool an electronic device. For this analysis, a numerical model of the cooling device was built on ANSYS CFX 12.0. Variations in jet Reynolds and Strouhal numbers and positioning of the heated region of interest were made and their effect on thermal performance analyzed. Results were compared to a conventional flow with the same geometry but subjected to a single-fan providing steady flow with the same average mass flow rate induced by each synthetic jet. For the configurations tested, it was found that (micro) synthetic jets may provide more directed air flow for "hotspots" with the greatest need of cooling. The results indicate a thermal performance up to 122% higher compared to their equivalent conventional cooling case. This confirmation of the higher thermal performance of synthetic jets relative to a convencional method and justifies the need for the current and additional investigations in this area. Results also indicate that synthetic jets can be customized and specifically directed to meet the cooling demand of the problem in question.

Submitted by Silvana Teresinha Dornelles Studzinski (sstudzinski) on 2016-02-04T15:21:28Z No. of bitstreams: 1 Marino Trisch_.pdf: 3535397 bytes, checksum: 4cc7a6dc219d9c91a6de57725e4515d1 (MD5) / Made available in DSpace on 2016-02-04T15:21:28Z (GMT). No. of bitstreams: 1 Marino Trisch_.pdf: 3535397 bytes, checksum: 4cc7a6dc219d9c91a6de57725e4515d1 (MD5) Previous issue date: 2015-06-22 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Este trabalho apresenta um estudo experimental relacionado ao resfriamento de dispositivos eletrônicos utilizando jatos sintéticos direcionados de modo que o jato flua tangencialmente à superfície aquecida, utilizando para isso uma bancada experimental especialmente desenvolvida. Para o desenvolvimento deste trabalho foram analisados outros estudos relacionados ao assunto, simulados e experimentais, utilizando neste caso um alto-falante como membrana montada em conjunto com a estrutura da bancada para formar a câmara e consequentemente o gerador de jatos sintéticos. O jato sintético gerado irá resfriar um elemento de aquecimento que simula o funcionamento de um dispositivo eletrônico, posicionado tangencialmente em diversas posições de distância em relação à saída do jato. Os procedimentos de teste de resfriamento foram realizados na bancada experimental em diversos modos de funcionamento do elemento de aquecimento, utilizando temperatura média de 80 °C semelhante à temperatura máxima de trabalho de dispositivos eletrônicos. Para a geração do jato sintético foram aplicados sinais senoidais em frequências de pulsação entre 20 e 120 Hz e com amplitude de aproximadamente 7,52 V_p, que resulta em 20 Wrms de potência no gerador de jatos sintéticos. Nos testes utilizando potência fixa do elemento de aquecimento, a temperatura no elemento de aquecimento é monitorada. Em outro modo de teste, foi mantida uma temperatura constante e monitorada a potência máxima correspondente dissipada no elemento de aquecimento. Por fim, também foi realizado comparativo entre resfriamento eletrônico utilizando jatos sintéticos e método tradicional com a utilização de ventiladores, onde são utilizados três diferentes tamanhos de coolers acoplados à bancada experimental e arrefecendo o mesmo elemento de aquecimento, verificando e comparando velocidades e rendimento entre os métodos de resfriamento. / This paper presents an experimental study related to the cooling of electronic devices using synthetic jets directed so that the jet flows tangentially to the heated surface. A custom-built experimental test bench especially developed based on other studies related to the subject. In this case, a speaker was used as a membrane and installed in a cavity in the test bench to form the synthetic jet generator. The synthetic jet cools a heating element that simulates the operation of an electronic device, positioned tangentially at various distance in relation to the exit plane of the synthetic jet. Cooling test procedures were performed in the custom-built experimental test bench in various operation modes of the heating element, using an average temperature of 80 ° C which is similar to the operating temperature of electronic devices. To generate the synthetic jet, sinusoidal input signals were applied with frequencies between 20 and 120 Hz and with amplitude of approximately 7.52 Vp which resulted in 20 Wrms power consumed by generator. In tests using a fixed power dissipated by the heating element, the temperature drop is monitored in the heating element. In the other test mode, the temperature on the heating element was set at a constant value the maximum power dissipated in the heating element was measured. Finally, comparisons were also performed between the cooling performance of synthetic jets and the conventional method with the use of three different coolers sizes. The same tests were performed on the same heating element and the corresponding velocities and cooling performance between the two methods were compared.

The flow around a finite square prism has not been studied extensively when compared with an “infinite” (or two-dimensional) square prism. In the present study, the effect of a wake-mounted splitter plate on the flow around a surface-mounted square prism of finite height was investigated experimentally using a low-speed wind tunnel. Of specific interest were the combined effects of the splitter plate length and the prism’s aspect ratio on the vortex shedding, mean drag force coefficient, and the mean wake. Four square prisms of aspect ratios AR = 9, 7, 5 and 3 were tested at a Reynolds number of Re = 7.4×104 and a boundary layer thickness of /D = 1.5. Splitter plate lengths of L/D = 1, 1.5, 2, 3, 5, and 7, were tested, with all plates having the same height as the prism. Measurements of the mean drag force were obtained with a force balance, and measurements of the vortex shedding frequency were obtained with a single-component hot-wire probe. A seven-hole pressure probe was used to measure the time-averaged wake velocity at a Reynolds number of Re = 3.7×104 for AR = 9 and 5 with splitter plates of lengths L/D = 1, 3, 5, and 7. These measurements were carried out to allow for a better understanding of how the splitter plate affects the mean wake of the finite prism. The results show that the splitter plate is a less effective drag-reduction, but more effective vortex-shedding-suppression, device for finite square prisms than it is for infinite square prisms. Significant reduction in the mean drag coefficient was realized only for short prisms (of AR ≤ 5) when long splitter plates (of L/D ≥ 5) were used. A splitter plate of length L/D = 3 was able to suppress vortex shedding for all aspect ratios tested. However, for square prisms of aspect ratios AR ≤ 7, the splitter plate is a less effective vortex-shedding-suppression device when compared to its use with finite circular cylinders, i.e. longer splitter plates are needed for vortex shedding suppression with square prisms. Wake measurements showed distinct wake velocity fields for the two prisms tested. For the prism of AR = 9, a strong downwash flow in the upper part of the wake became weaker towards the ground plane. For the prism of AR = 5, the downwash remained strong close to the ground plane. With splitter plates installed, the downwash became weaker for both prisms. The splitter plate was found to narrow the wake width, especially close to the ground plane, and led to the stretching of the streamwise vortex structures in the vertical direction, and increased entrainment towards the wake centreline in the cross-stream direction.

Les écoulements rampants, recirculants et instationnaires d'un fluide viscoplastique autour d'un cylindre ont été étudiés.Numériquement, les morphologies des écoulements, la localisation des zones rigides, les champs de contraintes et pression autour du cylindre ainsi que le coefficient de traînée, ont été déterminés sur un large domaine des nombres de Reynolds et d'Oldroyd.Expérimentalement, les fluides étudiés sont des gels de polymère Carbopol®. Le comportement élastoviscoplastique de ces gels a été modélisé par une loi d'Herschel-Bulkley adaptée. Le montage expérimental conçu et réalisé a été validé par l'étude de l'écoulement d'un fluide newtonien autour d'un cylindre et la mise en place d'une procédure adaptée pour les fluides à seuil.On a pu constater l'influence des conditions d'interface avec l'apparition d'une morphologie de lâchers de tourbillons simultanés et symétriques.

Thesis is concerned with modeling fluid dynamics and computing wind load on thin-walled structure of cooling tower. Two models for computational fluid dynamics are presented – one with singleton cooling tower and second with group of four cooling tower. Thesis includes also a structural model of cooling tower and methodology of wind load transfer is presented.

The acoustic emissions from supersonic jets represent an area of significant research needs; not only in the field of aero-acoustics, but in industry as well where high pressure let down processes have been known to cause acoustically induced vibrations. A common method to reduce the acoustic emissions of such processes involves dividing the single larger supersonic flow into several smaller ones. Though this is common practice, there is not yet a current model which describes the reduction of acoustic emissions from an array of smaller supersonic jets. Current research which studies supersonic jet arrays are mainly focused on the effects of screech. Though screech is important, due to its high amplitude acoustic pressure, this research focuses on the overall acoustic emissions radiated from supersonic jet arrays which can cause severe acoustic loadings. This research investigated the acoustic emissions and shock formations from several eight by eight arrays of axisymmetric jet experimentally. The array nozzle diameters investigated ranged from 1/8 inch to 1/4 inch and the spacing over diameter ratio ranged from 1.44 to 3. The net pressure ratios investigated ranged from 2 to 24. Results revealed a strong correlation between the acoustic emissions and the shock formations of the flow. Up until a critical net pressure ratio, the overall sound pressure levels were comparable to that of a single jet within an array. At net pressure ratios beyond the critical the overall sound pressure levels transitioned to higher decibel levels; equivalent to a single jet with an equivalent exit area of an entire array. Also, the characteristic acoustic frequency emitted from a nozzle array remained ultrasonic (above 20 kHz) at lower net pressure ratios and then shifted to audible levels (between 20 Hz to 20 kHz) at net pressure ratios beyond the critical. Also, before the critical net pressure ratio the shock cells from the jets within the array remained unmerged, but at net pressure ratios beyond the critical the shock cells merged and formed lattices of weak oblique shocks at first and then strong oblique shocks as the net pressure ratio continued to increase. The critical net pressure ratio was investigated by non-dimensional analysis. The non-dimensional analysis revealed that the critical net pressure ratio was a strong linear function of the spacing over diameter ratio. A linear model was derived which is able to predict the critical net pressure ratio, and in turn, predict a critical shift in the acoustic emissions of a nozzle array.

There is currently a great deal of interest within the aviation community in the design of small, slow-flying but manoeuvrable uninhabited vehicles for reconnaissance, surveillance, and search and rescue operations in urban environments. Inspired by observation of birds, insects, fish and cetaceans, flapping wings are being actively studied in the hope that they may provide greater propulsive efficiencies than propellers and rotors at low Reynolds numbers for such Micro-Air Vehicles (MAVs). Researchers have posited the Strouhal number (combining flapping frequency, amplitude and forward speed) as the parameter controlling flapping wing aerodynamics in cruising flight, although there is conflicting evidence. This thesis explores the effect of flapping frequency and amplitude on forces and wake structures, as well as physical mechanisms leading to optimum propulsive efficiency. Two-dimensional rigid airfoils are considered at Reynolds number 2,000 ??? 40,000. A compressible Navier-Stokes simulation is combined with numerical and analytical potential flow techniques to isolate and evaluate the effect of viscosity, leading and trailing edge vortex separation, and wake vortex dynamics. The wake structures of a plunging airfoil are shown to be sensitive to the flapping frequency independent of the Strouhal number. For a given frequency, the wake of the airfoil exhibits ???vortex lock-in??? as the amplitude of motion is increased, in a manner analogous to an oscillating circular cylinder. This is caused by interaction between the flapping frequency and the ???bluff-body??? vortex shedding frequency apparent even for streamlined airfoils at low Reynolds number. The thrust and propulsive efficiency of a plunging airfoil are also shown to be sensitive to the flapping frequency independent of Strouhal number. This dependence is the result of vortex shedding from the leading edge, and an interaction between the flapping frequency and the time for vortex formation, separation and convection over the airfoil surface. The observed propulsive efficiency peak for a pitching and plunging airfoil is shown to be the result of leading edge vortex shedding at low flapping frequencies (low Strouhal numbers), and high power requirements at large flapping amplitudes (high Strouhal numbers). The efficiency peak is governed by flapping frequency and amplitude separately, rather than the Strouhal number directly.

There is currently a great deal of interest within the aviation community in the design of small, slow-flying but manoeuvrable uninhabited vehicles for reconnaissance, surveillance, and search and rescue operations in urban environments. Inspired by observation of birds, insects, fish and cetaceans, flapping wings are being actively studied in the hope that they may provide greater propulsive efficiencies than propellers and rotors at low Reynolds numbers for such Micro-Air Vehicles (MAVs). Researchers have posited the Strouhal number (combining flapping frequency, amplitude and forward speed) as the parameter controlling flapping wing aerodynamics in cruising flight, although there is conflicting evidence. This thesis explores the effect of flapping frequency and amplitude on forces and wake structures, as well as physical mechanisms leading to optimum propulsive efficiency. Two-dimensional rigid airfoils are considered at Reynolds number 2,000 ??? 40,000. A compressible Navier-Stokes simulation is combined with numerical and analytical potential flow techniques to isolate and evaluate the effect of viscosity, leading and trailing edge vortex separation, and wake vortex dynamics. The wake structures of a plunging airfoil are shown to be sensitive to the flapping frequency independent of the Strouhal number. For a given frequency, the wake of the airfoil exhibits ???vortex lock-in??? as the amplitude of motion is increased, in a manner analogous to an oscillating circular cylinder. This is caused by interaction between the flapping frequency and the ???bluff-body??? vortex shedding frequency apparent even for streamlined airfoils at low Reynolds number. The thrust and propulsive efficiency of a plunging airfoil are also shown to be sensitive to the flapping frequency independent of Strouhal number. This dependence is the result of vortex shedding from the leading edge, and an interaction between the flapping frequency and the time for vortex formation, separation and convection over the airfoil surface. The observed propulsive efficiency peak for a pitching and plunging airfoil is shown to be the result of leading edge vortex shedding at low flapping frequencies (low Strouhal numbers), and high power requirements at large flapping amplitudes (high Strouhal numbers). The efficiency peak is governed by flapping frequency and amplitude separately, rather than the Strouhal number directly.