Multidimensional turbulence spectra - Statistical analysis of turbulent vortices

Strong nonlinear or very fast phenomena such as mixing, coalescence and breakup in chemical engineering processes, are not correctly described using average turbulence properties. Since these phenomena are modeled by the interaction of fluid particles with single or paired vortices, distribution of the properties of individual turbulent vortices should be studied and understood. In this paper, statistical analysis of turbulent vortices was performed using a novel vortex tracking algorithm. The vortices were identified using the normalized Q-criterion with extended volumes calculated using the Biot Savart law in order to capture most of the coherent structure related to each vortex. This new and fast algorithm makes it possible to estimate the volume of all resolved vortices. Turbulence was modeled using large-eddy simulation with the dynamic Smagorinsky-Lilly subgrid scale model for different Reynolds numbers. Number density of turbulent vortices were quantified and compared with different models. It is concluded that the calculated number densities for vortices in the inertial subrange and also for the larger scales are in very good agreement with the models proposed by Batchelor and Martinez-Bazan. Moreover, the associated enstrophy within the same size of coherent structures is quantified and its distribution is compared to models for distribution of turbulent kinetic energy. The associated enstrophy within the same size of coherent structures has a wide distribution that is normal distributed in the logarithmic scale

Dimensioning Links for IP Telephony

Transmitting telephone calls over the Internet causes problems not present in current telephone technology such as packet loss and delay due to queueing in routers. In this undergraduate thesis we study how a Markov modulated Poisson process is applied as an arrival process to a multiplexer and we study the performance in terms of loss probability. The input consists of the superposition of independent voice sources. The predictions of the model is compared with results obtained with simulations of the multiplexer made with a network simulator. The buffer occupancy distribution is also studied and we see how this distribution changes as the load increases

Implementation of the Functional Mock-up Interface in Matlab and Simulink

New products on the market are likely to be simulated in a computer sometime during the development process. The environment for which the physical model of the product is developed may not always be the optimal for control simulations of the model. To be able to export models from one environment to another a common model definition must be defined. The Functional Mock-up Interface, FMI, provides such a model definition and makes it possible to incorporate models from different environments together. In this thesis we will witness a successful implementation of the interface for model exchange, FMI, into the well known MATLAB and Simulink environment. Simulink is widely used in industry to develop control systems but not that used for physical modelling. It is therefore of great interest to be able to simulate models created from other physical modelling environments in to Simulink. A block is developed in Simulink and a user interface in MATLAB such that models created according to the FMI standard can be simulated. The FMI is a standard for solving ODEs with events. The thesis discusses the most essential parts of the FMI standard. Events may be discontinuities that the ODE solver needs to take special care of and is therefore discussed in more detail. In Simulink an S-function block is used with a GUI developed for the user to easily configure the model. The MATLAB interface is developed using MEX functions and is discussed based on how MATLAB's ODE solvers can be used to simulate a model. MEX functions are MATLAB's way to incorporate C, C++ and Fortran code. The FMI standard models consists of DLL functions that enforce the use of MEX functions.The implementations are verified to be correct by comparison of simulation results from different environments such as Dymola and JModelica.org. A comparison of simulation times and the number of function evaluations are also done where we can see that the S-function and the MEX interface performs on a similar level as the other simulation environments

On the Sherwood number correction due to Stefan flow

This study contributes with insight and new models for mass transfer to spheres for three different conditions i.e. mass transfer without Stefan flow, mass transfer with even Stefan flow and mass transfer with uneven flux driven Stefan flow. High resolved computational fluid dynamic simulations of mass transfer without Stefan flow resulted in a new correlation that improves the prediction compared to the model suggested by Ranz-Marshall particularly for higher Sc numbers. A new model for mass transfer which accounts for the convective transport within the ‘film’ surrounding the particle, and an increase in the film thickness due to the Stefan flow was successfully derived. It is concluded that the new model provides improved accuracy compared to the Spalding and Abrahamzon models and that the relative error for the Sherwood number can be reduced by about half

Linnebjer—a south swedish oak forest and meadow area—revisited after half a century

An oak forest and three wet meadows/fens were reinvestigated after 50 years concerning tree vitality,biomass and productivity, and soil chemistry. Sulphur and nitrogen deposition has changeddramatically during these years, and the aim was to analyse the differences in both the oak forestand the open field ecosystems. Trees were re-measured and soil profiles were resampled. Importantvisible changes in the oak forest were stated concerning the vitality of oaks. Abovegroundthere was a decrease in tree biomass, production and litter fall, but a huge increase in standingdead logs. During the years, the deposition of sulphur had decreased drastically, but nitrogendeposition was still high. Soil acidification in the forest had decreased, reflected in an increasedbase saturation in the forest, in spite of slightly lowered pH-values. Strongly increased amounts ofexchangeable Ca and Mg now appeared in the forest soil, and a substantial transport of calciumand magnesium had obviously taken place from the forest soil to the meadow and fens during theyears. However, the most important soil change was the accumulation of organic matter. The increasedaccumulation of organic matter in turn meant increased amounts of colloid particles andmicrosites for ion exchange in the soil. This favoured 2-valence base cations, and especially Ca andMg that increased very much in all the studied ecosystems. Carbon as well as nitrogen had stronglyincreased in the forest, meadow and fen soils. This was interpreted as a natural result of increasedvegetation growth due to high nitrogen deposition, increased global annual temperature and increasedcarbon dioxide concentration in air. It was concluded that the decreased deposition ofsulphur had have a positive effect on soil chemistry, and that the deposition of nitrogen probablyhad stimulated vegetation growth in general, and contributed to increased amount of organicmatter in the soils. However, in this studied oak forest, the decreased vitality and many killedtrees were also suspected to be a result of high nitrogen deposition. Obviously increased treegrowth was counteracted by decreased stress resistance, and increased appearance of pathogensin the oak trees

Solid Oxide Fuel Cell Material Structure Grading in the Direction Normal to the Electrode/Electrolyte Interface using COMSOL Multiphysics

Fuel cells (FCs) are promising as an energy producing device, which at this stage of development will require extensive analysis and benefit from numerical modeling at different time- and length scales. COMSOL Multiphysics is used to describe an intermediate temperature solid oxide fuel cell (SOFC). Governing equations for heat, gas-phase species, electron, ion and momentum transport are implemented and coupled to kinetics describing electrochemical as well as internal reforming reactions. It is found, from the parameter study, that grading the electron tortuosity (decreased under the fuel and air channels), the electron conducting material fraction (increased under the fuel and air channels) and the pore tortuosity (decreased under the interconnect ribs), in the direction normal to the electrode/electrolyte interface increases the performance (average ion current density) slightly. On the other hand, is the performance slightly decreased from grading the porosity (increased under the interconnect ribs)

Three-Dimensional Design Optimization Of An Anode-Supported SOFC Using FEM

Abstract in UndeterminedSolid oxide fuel cells (SOFCs) are promising as energy producing devices, which at this stage of development will require extensive analysis and benefit from numerical modeling. A 3D model is developed based on the FEM for a single cell planar SOFC design optimization. Ion, electron, heat, gas-phase species and momentum transport equations are implemented and coupled to the kinetics of electrochemical reactions. High current density spots are identified, where the electron transport distance is short and the oxygen concentration is high. The relatively thin cathode results in a significant oxygen mole fraction gradient in the direction normal to the main flow direction. The electron transport especially within the cathode is found to be limiting for the electrochemical reactions at positions far from the channel walls (interconnect ribs). It is concluded that an increased pore size in the cathode support layer increases the current density more than an increased pore size in the anode support layer

SOFC Modeling Considering Electrochemical Reactions at the Active Three Phase Boundaries

Abstract in UndeterminedIt is expected that fuel cells will play a significant role in a future sustainable energy system, due to their high energy efficiency and the possibility to use renewable fuels. A fully coupled CFD model (COMSOL Multiphysics) is developed to describe an intermediate temperature SOFC single cell, including governing equations for heat, mass, momentum and charge transport as well as kinetics considering the internal reforming and the electrochemical reactions. The influences of the ion and electron transport resistance within the electrodes, as well as the impact of the operating temperature and the cooling effect by the surplus of air flow, are investigated. As revealed for the standard case in this study, 90% of the electrochemical reactions occur within 2.4 mu m in the cathode and 6.2 mu m in the anode away from the electrode/electrolyte interface. In spite of the thin electrochemical active zone, the difference to earlier models with the reactions defined at the electrode-electrolyte interfaces is significant. It is also found that 60% of the polarizations occur in the anode, 10% in the electrolyte and 30% in the cathode. It is predicted that the cell current density increases if the ionic transfer tortuosity in the electrodes is decreased, the air flow rate is decreased or the cell operating temperature is increased

VISÖ - Visualization of Infrastructure and Sustainable development in Öresund

Sustainable growth is a shared goal and challenge for policy-makers at local, regional, national as well as European levels of government. However, due to the complexity and interplay of the driving forces it is often difficult for decision-makers to predict the lasting effects of large infrastructure investments and other significant changes to the land use structure and transport system. In cross-border regions, the difficulties facing decision-makers are even more demanding due to shortage of joint planning tools and institutions. Substantial research literature confirms the hypothesis that analysis aimed at decision-support in general has limited impact on actual decision-making. One reason might be that effects and results often are evaluated based on complex system analyses and mathematically derived formulas which are often only understood by experts. This, combined with the lack of coordinated facts, maps and tools on which alternative development scenarios are formulated and long term planning decisions are ultimately based, constitutes a considerable difficulty for decision-makers at all levels, whether politicians or civil servants. Project VISÖ - Visualization of Infrastructure and Sustainable development in Öresund is a joint Swedish and Danish research collaboration to develop a coordinated planning tool and methodology – from which alternative development scenarios and strategic infrastructure choices and their effects can be better understood, communicated and visualized in pedagogic illustrations and purposeful dialogues in a cross- border region context. The new platform will ultimate enable planning authorities, politicians and affected communities to better understand and shape decisions regarding the development within a cross-border region in a positive, smarter and greener direction. Project VISÖ is financially supported by EU Interreg IVA. Partners are the Swedish Transport Administration, the Danish Road Directorate, Region Hovedstaden, Region Själland and Region Skåne