Rendimento volumétrico de um motor de pistões opostos a quatro tempos

Este trabalho tem como objectivo avaliar o rendimento volumétrico de um motor de pistões opostos a quatro tempos, no seu processo de lavagem dos gases de escape. Um motor nesta configuração, normalmente funciona a dois tempos, mas neste estudo transformou-se num motor a quatro tempos. O motor em estudo é da marca Robin America,Inc. Modelo EY15, que na sua forma original, não funciona numa configuração de pistões opostos. Para a realização do estudo teórico considerou-se que o cabeçote de dois motores Robin EY15 idênticos são removidos de forma a permitir a união de um com o outro. A junção fará com que os pistões estejam em direcções opostas. Nesta configuração o colector de admissão de cada um dos motores encontra-se numa posição frontal ao colector de escape do outro motor. Visto que será um motor a quatro tempos considerou-se a existência de válvulas nessas condutas em vez das tradicionais janelas dos motores a dois tempos. Esta transformação não será realizada na prática pois nesse caso seria necessário construir cambotas, sistemas de admissão, sistemas de injecção e combustível, etc. Portanto foi utilizado um modelo matemático de turbulência (K-ε), aplicado no programa Ansys Fluent (software de CFD - Computacional Fluid Dynamics), que possibilitou a avaliação teórica do rendimento volumétrico deste motor em três dimensões para diferentes velocidades de rotação. Com este estudo, pretende-se então considerar esta experiência credível de ser executada na realidade, com vista à sua utilização no meio aeronáutico. Possibilita-se assim a construção de um motor a partir de outro já existente reduzindo desta maneira o seu custo de fabrico. Além disso, pretende-se ter um motor leve, mas com grande potência e realizando o mínimo de poluição possível.his thesis aims to evaluate the volumetric efficiency of a four-stroke engine with opposite pistons in its washing process of the exhaust gases. An engine in this configuration usually works in two strokes, but in this study has become a four-stroke engine. The engine in question is a Robin America’s, Inc. EY15 model, which in its original form, does not work in a setting of opposite pistons. To carry out this theoretical study, there was considered the removal of the head in two identical Robin EY15’s to allow its union. The junction will result in the pistons being in opposite directions. In this configuration the inlet manifold of each engine is in a forward position in relation to the exhaust manifold of the other engine. Since it is a four-stroke engine, the existence of valves in these pipes was taken into consideration instead of the traditional windows in two-stroke engines. This transformation will not be conducted manually because you would have to build crankshafts, intake systems, fuel injection systems, etc. Therefore, a mathematical model of turbulence (K-ε) was used, applied in the Ansys Fluent software (CFD - Computational Fluid Dynamics) programme, which made the theoretical evaluation of the volumetric efficiency of this engine in three dimensions for different rotational speeds possible. This study intends to show that this experience is credible and that its implementation is possible in the aeronautical field. Thus enabling the construction of an engine from an existing one as well as reducing the cost of manufacture. In addition, its intention is to build a lightweight, more powerful engine along with causing the least possible environmental damage

Аэродинамические характеристики замкнутого параболического крыла

Работа посвящена разработке методики определения аэродинамических характеристик замкнутого параболического крыла на основе аналитических, численных и экспериментальных исследований. В работе выполнен анализ компоновок самолетов с замкнутыми крыльями, рассмотрены особенности их аэродинамики, определена научная новизна исследований, цели и задачи диссертационной работы. Предложена аэродинамическая компоновка замкнутого параболического крыла и винтового движителя, позволяющая в сравнении с крылом классической схемы реализовать существенно большие максимальные несущие свойства. На основании анализа особенностей аэродинамики замкнутых несущих поверхностей выполнена оценка применимости методов экспериментальной и вычислительной аэродинамики к решению постановленной задачи. В работе выполнен анализ результатов испытаний модели замкнутой несущей поверхности в аэродинамической трубе Т-5 ХАИ, выполненных с целью оценки общей эффективности предлагаемой авторами компоновки. Выполнено сравнение результатов, полученных в аэродинамической трубе Т-5 с результатами, полученными с помощью панельно-вихревого метода и метода конечных элементов. С использованием принятых методов математического моделирования выполнена расчетная оценка влияния струи движителя на аэродинамические характеристики замкнутого параболического крыла, на основании анализа полученных результатов спроектирована летающая экспериментальная модель для дальнейших исследований

Large-Eddy Simulations of Turbulent Flows, from Desktop to Supercomputer

In this paper, a general intro ductio nto the large-eddy simulatio n (LES) technique will be given

Large eddy simulation of separated boundary layer transition under free-stream turbulence

Physics of laminar-to-turbulent transition in a separated-reattached flow subjected to two free-stream turbulence levels have been explored using Large-Eddy Simulation (LES). Separation of the laminar boundary layer occurs at a curvature change over a flat plate with a semi-circular leading edge. A numerical trip has been used to generate the targeted free-stream turbulence levels. A dynamic Sub-grid-scale (SGS) model has been employed and excellent agreement has been achieved between the LES results and the experimental data. Detailed investigation of the LES data has been carried out to explore the primary instability mechanism at low (< 0.2%) and high free-stream turbulence (5.6%). The flow visualisations and spectral analysis of the separated shear layer reveal that the two-dimensional Kelvin-Helmholtz instability mode, well known to occur at low free-stream turbulence levels, is bypassed at a higher level leading to earlier breakdown to turbulence. The whole transition process leading to breakdown to turbulence has been revealed clearly by the flow visualisations and the differences between the low and high free-stream turbulence cases are clearly evident. Coherent structures are also visualised using iso-surfaces of the Q-criterion and for the high free-stream turbulence case the spanwise oriented two-dimensional rolls, which are clearly apparent in the low free-stream turbulence case, are not visible anymore. Detailed quantitative comparisons between the present LES results against experimental data and the previous LES results at low free-stream turbulence using a staggered grid have been done and a good agreement has been obtained, indicating that the current LES using a co-located grid with pressure smoothing can predict transitional flows accurately. Comprehensive spectral analysis of the separated shear layer at two free-stream turbulence levels has been performed. Under very low free-stream turbulence condition, a distinct regular vortex shedding and trace of the low-frequency flapping phenomena were detected. Under the higher free-stream turbulence however, a mild high-frequency activity was observed. No low frequency oscillations could be detected

Large Eddy Simulation and Analysis of Shear Flows in Complex Geometries

In the present work, large eddy simulation is used to numerically investigate two types of shear flows in complex geometries, (i) a novel momentum driven countercurrent shear flow in dump geometry and (ii) a film cooling flow (inclined jet in crossflow). Verification of subgrid scale model is done through comparisons with measurements for a turbulent flow over back step, present cases of counter current shear and film cooling flow. In the first part, a three dimensional stability analysis is conducted for countercurrent shear flow using Dynamic mode decomposition and spectral analysis. Kelvin-Helmholtz is identified as primary instability mechanism and observed as global mode at a specific parameter. Mechanism of global mode synchronization over distinct spatial location is studied. In the second part, the flow physics of film cooling flows is analysed. The origin, evolution of various coherent flow structures and their role in film cooling heat transfer is studied based on detailed flow visualization. Further, the contribution of coherent structures in film cooling heat transfer and mixing is studied through modal analysis. Low frequency modes are found to have large contribution in cooling surface adiabatic temperature fluctuation while high frequency modes play larger role in bulk mixing. Finally, a new contoured crater shape is developed and shown to have improved performance at shallow depth compared to earlier designs

Synthetic turbulence generation for LES on unstructured Cartesian grids

A parallel CFD code to solve incompressible fluid flow on unstructured Cartesian meshes has been developed almost from ground up. Turbulence statistics have been computed using the Large Eddy Simulation technique. The new code was subjected to some validation where results are compared to available reference data. An analysis on the iteration and discretisation errors was carried out. This code was then applied to predict the lid driven cubical cavity flow in at a bulk Reynolds number of 10,000. Three different mesh sizes were used to investigate independence of results on grid size. Amongst others, turbulence statistics were checked against Kolmogorov -5/3 law. A detailed study of synthetic turbulence methods was carried out and applied to the prediction of flow in a duct with square cross section using an inlet and outflow boundaries. Three different turbulence generation methods were investigated namely the artificial turbulence generation method, random perturbation method and a novel hybrid particle-wave method also termed as the enhanced vortex particle method in this study. The mean and instantaneous field variables together with the turbulence statistics from each method were compared and analysed. Finally, the code was used to solve turbulent flow over arrays of wall-mounted obstacles with mesh densities comparable to previous studies. The velocity profiles and vector fields at various locations in the domain were compared to data obtained from recent LES simulations. The artificial turbulence generation case was applied for the first time to produce turbulence at the inlet. The turbulence kinetic energy spectrum distribution agrees well with reference data. Important findings from this study are clarified and some suggestions for future work are given in the conclusions section.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Computational modelling and analysis of the flow and performance in hydrocyclones

Hydrocyclones have been widely used to separate particles by size in many industries. Their flows are complicated and involve multiple phases: liquid, gas, and particles of different sizes and densities. A two fluid model, facilitated with the mixture model, has been used to study the flow in hydrocyclones under wide range of conditions and used here to study the effects of geometrical configuration and material properties of cyclones operated at different feed solids concentrations. The variables considered include geometrical configurations such as dimensions and shape of body, cone and vortex finder as well as particle density. The outcome shows a smaller cyclone results in an increased cut size, decreased pressure drop, sharper separation and higher water split. Both large and small spigot diameters lead to poor separation performances. Accordingly, an optimum spigot diameter can be identified depending on feed solids concentration. It is also shown that for all considered hydrocyclones, a better separation performance can be achieved by the operation at lower feed solid concentration. Further research shows that cyclone performance is sensitive to both length and shape of conical section. A longer conical section leads to decreased inlet pressure drop, d50, and Ep, and an increased water split. When cone shape varies from concave to convex, a compromise optimum performance for the cyclone with a convex cone is observed with a minimum Ep and relatively small pressure drop and water split. A new hydrocyclone featured with a long convex cone is then proposed which can improve the performance of the conventional cyclone. The keycharacteristics of flow in a hydrocyclone are then investigated when vortex finder geometry including diameter length and shape varies. It has been shown that a compromise optimum performance can be identified with relatively small inlet pressure drop, Ep, and water split. Discussion is then extended to flow behaviour analysis under the effect of different density fractions. The origin of flow pattern and the motion of coal particles have been predicted and discussed. The effect of coal density variation on operational conditions and performance of the large diameter hydrocyclones are also studied in this work

Turbulent Transport in the Atmospheric Boundary Layer with Application to Wind Farm Dynamics

With the recent push for renewable energy sources, wind energy has emerged as a candidate to replace some of the power produced by traditional fossil fuels. Recent studies, however, have indicated that wind farms may have a direct effect on local meteorology by transporting water vapor away from the Earth\u27s surface. Such turbulent transport could result in an increased drying of soil, and, in turn, negatively affect the productivity of land in the wind farm\u27s immediate vicinity. This numerical study will analyze four scenarios with the goal of understanding turbulence transport in the wake of a turbine: the neutrally-stratified boundary layer with system rotation, the unstably-stratified atmospheric boundary layer, and wind turbine simulations of these previous two cases. For this work, the Ekman layer is used as an approximation of the atmospheric boundary layer and the governing equations are solved using a fully-parallelized direct numerical simulation (DNS). The in-depth studies of the neutrally and unstably-stratified boundary layers without introducing wind farm effects will act to provide a concrete background for the final study concerning turbulent transport due to turbine wakes. Although neutral stratification rarely occurs in the atmospheric boundary layer, it is useful to study the turbulent Ekman layer under such conditions as it provides a limiting case when unstable or stable stratification are weak. In this work, a thorough analysis was completed including turbulent statistics, velocity and pressure autocorrelations, and a calculation of the full turbulent energy budget. The unstably-stratified atmospheric boundary layer was studied under two levels of heating: moderate and vigorous. Under moderate stratification, both buoyancy and shearing contribute significantly to the turbulent dynamics. As the level of stratification increases, the role of shearing is shown to diminish and is confined to the near-wall region only. A recent, multi-equation closure model, used to model the third and fourth velocity-temperature moments, performed well for the unstable cases. Optimal model coefficients found for the DNS data are shown to agree with atmospheric observations as well as LES data. Finally, the effects of top-down diffusion (entrainment-induced flux at the temperature inversion) and bottom-up diffusion (non-zero surface flux) were studied and improvements to correlation functions are suggested. This thesis concludes by analyzing the neutral and unstable cases under the effects of wind turbine wakes. A unique means of converting a periodic simulation into a spatially evolving flow in the wake of a turbine is demonstrated; present results under neutral stratification are shown to agree with wind tunnel experiments under similar conditions. By introducing a scalar (humidity) into the flow field, the effect of a turbine wake on scalar transport in a wind farm is uncovered. The results show a clear drying effect under both neutral and unstable stratification given a wet surface. An investigation of energy and flux budgets gives guidance as to why such a phenomena occurs

A study of subgrid scale modelling and inflow boundary conditions for large eddy simulation of wall-bounded flows

The complicated turbulence structures in wall-bounded flows require accurate subgrid scale, SGS, modelling and realistic inlet boundary conditions for Large Eddy Simulation, LES. The present study focused on the investigation and development of transport equation SGS models and on the development of inlet conditions generation algorithms specialised for LES of wall-bounded flows. The investigation of SGS models has been carried out in two stages. In the first stage, models based on resolved scales and models based on subgrid scales were tested on a series of channel flow cases. Among the second group of models, there was a new SGS model whose development was based on the concept of dissipation calculated from the energy spectrum. The results indicated the superiority of the models based on subgrid scales, with the new model providing the most accurate flow field in general. (Continues...).EThOS - Electronic Theses Online ServiceGBUnited Kingdo