Solar energy is one of a very few low-carbon energy technologies with the enormous potential to grow to a large scale. Currently, solar power is generated via the photovoltaic (PV) and concentrating solar power (CSP) technologies. The ability of CSPs to scale up renewable energy at the utility level, as well as to store energy for electrical power generation even under circumstances when the sun is not available (after sunset or on a cloudy day), makes this technology an attractive option for sustainable clean energy. The levelised electricity cost (LEC) of CSP with thermal storage was about 0.16-0.196 Euro/kWh in 2013 (Kost et al., 2013). However, lowering LEC and harvesting more solar energy from CSPs in future motivate researchers to work harder towards the optimisation of such plants. The situation tempts people and governments to invest more in this ultimate clean source of energy while shifting the energy consumption statistics of their societies from fossil fuels to solar energy. Usually, researchers just concentrate on the optimisation of technical aspects of CSP plants (thermal and/or optical optimisation). However, the technical optimisation of a plant while disregarding economic goals cannot produce a fruitful design and in some cases may lead to an increase in the expenses of the plant, which could result in an increase in the generated electrical power price. The study focused on a comprehensive optimisation of one of the main CSP technology types, the linear Fresnel collector (LFC). In the study, the entire LFC solar domain was considered in an optimisation process to maximise the harvested solar heat flux throughout an imaginary summer day (optical goal), and to minimise cavity receiver heat losses (thermal goal) as well as minimising the manufacturing cost of the plant (economic goal). To illustrate the optimisation process, an LFC was considered with 12 design parameters influencing three objectives, and a unique combination of the parameters was found, which optimised the performance. In this regard, different engineering tools and approaches were introduced in the study, e.g., for the calculation of thermal goals, Computational Fluid Dynamics (CFD) and view area approaches were suggested, and for tackling optical goals, CFD and Monte-Carlo based ray-tracing approaches were introduced. The applicability of the introduced methods for the optimisation process was discussed through case study simulations. The study showed that for the intensive optimisation process of an LFC plant, using the Monte Carlo-based ray-tracing as high fidelity approach for the optical optimisation objective, and view area as a low fidelity approach for the thermal optimisation objective, made more sense due to the saving in computational cost without sacrificing accuracy, in comparison with other combinations of the suggested approaches. The study approaches can be developed for the optimisation of other CSP technologies after some modification and manipulation. The techniques provide alternative options for future researchers to choose the best approach in tackling the optimisation of a CSP plant regarding the nature of optimisation, computational cost and accuracy of the process. / Thesis (PhD)--University of Pretoria, 2017. / Mechanical and Aeronautical Engineering / PhD / Unrestricted

Migrating freshwater fish population has been significantly declined, mostly by the obstruction of their migratory routes by hydropower dams. To diminish such impacts, fish ladders have been developed to facilitate the passage of migrating fish. However, fish ladders are associated with mortality and migratory failure, resulting in an ecological problem which has been a concern for years. The paper contained in the following pages focuses on studying the viability of an innovative modular design for a fish ladder developed by Fiskvägsteknik AB. The design is based in a vertical slot fishway (VSF) to which some roughness elements are attached to modify the flow. The aim of this bypass system is to restore the original biological continuity and diversity in the Swedish rivers, therefore solving a long-lasting ecological problem that hydroelectric production has caused over the years. Through a computational analysis, the flow has been evaluated as a function of the parameters that present an influence over its behaviour. First, a study of the changes in flow velocity influenced by four different slopes: 5 %, 10 %, 15 % and 20 %, has been performed with the objective to find an appropriate inclination according to the capabilities of the fish species considered. The lowest slopes have resulted to be the most appropriate ones, coinciding with low values of turbulent kinetic energy and lower flow velocities. The second step has been checking the effect of roughness elements on the bottom part of the structure tilted a 5%. The flow velocity has shown a considerable decrease, falling in the range of the fish swimming capabilities established. As for the streamlines of the flow, they were altered compared to the cases without the roughness elements.

Three-dimensional turbulent flow in a cylindrical hydrocyclone is considered and studied by means of computational fluid dynamics using software packages CFX and Fluent. The aim has been to identify the methods that can be used for accurate simulation of the flow in three-dimensional configurations in hydrocyclones at high swirl numbers. As a starting point, swirling pipe flows created by tangential inlets, where detailed experimental data were available in literature, were considered. It was found that the velocity profiles for the flow with a swirl number of 2.67 could be predicted accurately using a Reynolds stress model and an accurate numerical discretization on a fine-enough mesh. At a higher swirl number, 7.84, under-prediction in the tangential velocity profiles was observed; however the prediction of the axial velocity profiles was satisfactory. The validated methods were then used to simulate the flow in a cylindrical hydrocyclone at a swirl number as large as 21. The calculated tangential velocity profiles were compared against experimental data measured with a pitometer. Acceptable agreements were recorded except near the geometric axis of the cyclone. Due to the lack of the aircore in the numerical model, disagreements near the axis of the cyclone could be expected to some extent. Numerical experiments performed in the present work indicated that the RNG k-ε model is not likely to be capable to predict highly swirling flows accurately and a Reynolds stress model is required. For three-dimensional models, where the computing capacity and the available memory set strong restrictions on the computational mesh, optimizing the maximum mesh resolution available play an important role on the accuracy and stability of the solution procedure. The most stable results in the present study were found using the Reynolds stress model proposed by Launder et al. on an as regular and structured mesh as possible using a higher order discretization scheme in Fluent. Therefore, the meshing capabilities of the pre-processor, the available turbulence models and the accuracy of the numerical methods must be considered in parallel. Acceptable results were also generated using the Baseline Reynolds stress model implemented in CFX, however, only with a transient procedure which was likely to be more time-consuming. Present simulations present a complex flow structure in the cylindrical cyclone with a double axial flow reversal. The effect of such a flow pattern on the fractionation of the fibres with small differences in density needs to be investigated in future studies. / QC 20101207

To make sure that a dam is safe it is important to have good knowledge about the energy dissipation in the spillway. Physical hydraulic model tests are reliable when investigating how the water flow behaves on its way through the spillway. The problem with physical model testing is that it is both expensive and time consuming, therefore computational fluid dynamics, CFD, is a more feasible option. This projects focuses on a spillway located in Sweden that Vattenfall R&D built a physical model of to simulate the water discharge and evaluate the energy dissipation in order to rebuild the actual spillway. The main purpose of this project is to evaluate if the physical hydraulic test results can be reproduced by using CFD, and obtain detailed results about the flow that could not be obtained by physical testing. There are several steps that need to be completed to create a CFD-model. The first step is to create a geometry, then the geometry needs to be meshed. After the meshing the boundary conditions need to be set and the different models, multiphase model and the viscous model, need to be defined. Next step is to set the operating conditions and decide which solution method that will be used. Then the simulation can be run and the results can get extracted. In this project two CFD simulations were performed. The first simulation was to be compared with the physical hydraulic model test results and the second CFD simulation was of the rebuilt spillway. The results proved that the physical model test results could be recreated by using CFD. It also gave a better understanding of how the energy dissipation was in the spillway and indicates that the reconstruction of the actual spillway was successful since the new spillway both had a higher water discharge capacity and better energy dissipation.

The main objective of this study was to evaluate the performance of three Large Eddy Simulation (LES) subgrid scale (SGS) models on a strongly heated, low Mach number upward gas flow in a vertical pipe with forced convection. The models chosen for this study were the Smagorinsky-Lilly Dynamic model (SLD), the Kinetic Energy Transport model (KET), and the Wall-Adaptive Local-Eddy viscosity model (WALE). The used heating rate was sufficiently large to cause properties to vary significantly in both the radial and streamwise directions. All simulations were carried out using the commercial software FLUENT. The effect of inlet turbulence generation techniques was considered as well. Three inlet turbulence generation techniques were compared, namely, the Spectral Synthesizer Method (SSM), the Vortex Method (VM), and the Generator (GEN) technique. A user-defined function (UDF) was written to implement the GEN technique into the solver; the SSM and VM techniques were already build-in. All simulation and solver settings were validated by performing computational simulations of isothermal fully developed pipe flow and results were compared to available experimental and Direct Numerical Simulation (DNS) data. For isothermal boundary conditions, among the three inlet turbulence generation techniques, the GEN technique produced results which best matched the experimental and DNS results. All three LES SGS models performed equally well when coupled with the GEN technique for the study of isothermal pipe flow. However, all models incorrectly predicted the behavior of radial and circumferential velocity fluctuations near the wall and the GEN technique proved to be the most computationally expensive. For simulations with longer computational domain, the effect of the inlet turbulence generation technique diminishes. However, results suggest that both the SLD and KET models need shorter computational domains to recover proper LES behavior when coupled with the VM technique in comparison to the WALE SGS model with the same turbulence inlet generation technique. For high heat flux simulations all SGS models were coupled with the VM technique to decrease the computational effort to obtain statistically steady-state solution. For comparative purposes, one simulation was carried out using the WALE and GEN techniques. All simulations equally significantly underpredicted the streamwise temperature distribution along the pipe wall as well as in the radial directions at various streamwise locations. These effects are attributed to the overpredicted streamwise velocity components and incorrect behavior of both the radial and circumferential velocity components in the near wall region for all subgrid scale models.

Dam spillways and downstream areas are used to guide large flows of water during for example heavy rainfall. The large flows give way to turbulent pattern sand velocities that may damage the river banks or the dam structure. Investigation of these water patterns at certain flows are therefore done to examine at risk areas. In this study CFD simulations were performed for different flows with different boundary conditions for varying surface roughness level. Results were then compared to a previous model study from 2009. The ANSYS ecosystem was used in production of the 3D model, construction of mesh and running of simulations.The flow for the maximum discharge capacity of the sluices was simulated as well as the design flow which is the highest flow the dam is supposed to be able to withstand. In this report the flow has been modeled using RANS with the SST kω-model in a VOF transient setup. Results showed that for both the design flow and the maximum discharge capacity flow the energy conversion is functioning poorly and that a considerable backward circulation exists on the right riverside. This behavior could possibly injure the right dam structure by moving debris upwards against the stream.

Air insulation, especially its efficiency, is often underestimated and most often it is recommended as combination of several methods by our experts. It is known that flowing air used to eliminate excessive humidity in lower construction is insignificantly promoted. Already our ancestors in ancient Rome had experience with implementation of air insulations. However, nowadays such design is connected with many doubts and it forces us to use sense, especially knowledge of construction physics. So it urges higher technical precision when designing functional ventilation system. Each building is actually a unique object, with regard to its position and used properties, that is why certain natural conditions must be accepted when designing. It is more complicated and figuratively speaking it can be written in c. 20 equations of twenty unknowns. Other problems arise in unknown coefficients to individual parameters and their effects. From another point of view it is the issue of changing air during the day, passing fronts, annual cycles and all of these in relation to internal environment of the building, materials used in the construction, land properties of rocks, and last but not least, relation to the terrain around the building. Then there are the effects of the building's altitudes etc. In total they are cycles with sine character of dampening and drying. The thesis discusses how to test the functionality of AIR insulations designed for the floor ventilation in historic buildings and follows on from the previously published articles. A flow analysis is being performed on an object of the city of Žirovnice which has been registered in the list of cultural monuments and was built as a brewery in the years 1589-1592 on the site of an older medieval building. This article brings new air-flow element. The aim of the article is to analyse in detail the air flow in a specific floor void with the aid of modern CFD programs and experimental measurements using the ALMEMO

This thesis examines the impact of spillway surface roughness on discharge capacity determination in hydraulic models. The study combines physical hydraulic modeling in a laboratory with Computational Fluid Dynamics (CFD) modeling using Fluent. The aim is to evaluate the effect of material roughness on spillway discharge determination for prototype spillways and outlets. The project includes a literature review, data collection, 3D modeling, model setup, numerical modeling, result analysis, comparison with other reports, and report writing. The results concluded that varying roughness heights as well as changing turbulence model and mesh settings did not significantly impact the final discharge (kg/s) at steady state. P-values less than 1e-7 for the average discharge at flowtime of [150 − tfinal[s]] suggests strong confidence in the statistical insignificance of varying roughness height affecting the discharge.

The purpose of this study was to examine the accuracy of English-speakers in determining the word recognition score of native Taiwan Mandarin-speakers. Digitally recorded Mandarin word lists were presented to 10 native Mandarin-speakers from Taiwan (five male, five female), from whom oral and written responses were collected. Oral responses were scored by 30 native English-speakers, 15 of which had no experience with Mandarin and 15 with two to three years of college-level Mandarin courses or equivalent knowledge of Mandarin. The judges who had experience with Mandarin were able to score the WRS tests with 97% accuracy (with scores ranging from 10% below to 4% above the actual score of the test). The judges without experience with Mandarin scored the WRS tests with 88.8% accuracy (with scores ranging from 34% below to 26% above the actual score of the test). An analysis of variance found that there was a significant difference between a judge's knowledge of Mandarin and his or her ability to accurately score the oral responses. An inspection of the performance of the judges in respect to the five different Mandarin tones indicated that there are some tone combinations that are more difficult to score correctly than others. While it is apparent that tone combination may play a role in the ability to accurately score WRS words in Mandarin, the implications of this for a clinical setting are uncertain because words with these tone combinations were not heard often. Tone perception training for the judges or simply making clinicians aware of this difficulty in tone identification may be of benefit in overcoming this obstacle.

Castings of aluminum alloys are widely used in the automotive and aerospace industries since they play a significant role in improving the performance and fuel efficiency. In aluminum industries, sampling is the most common method to evaluate the inclusion levels which is a key indicator for the quality of the aluminum alloys. Since how the filling process and solidification process will influence the inclusion characteristics during the sampling procedure is of great importance, the objectives of this work is to create a the two-phase flow model to simulate the filling process and solidification process, as well as calculate the particles movement in the whole sampling procedure. Computational Fluid Dynamics (CFD) modeling was used and this work was performed in the software ANSYS FLUENT. A numerical two dimensional (2D) axisymmetric model was built to simulate the sampling procedure with the assumption that the filling could be done along the main axis automatically. First, the initial solidification during the filling was taken into account without particle injection. The realizable k − ε turbulence model was used to model the effects of the turbulence. Several simulations with different inlet filling rate, different initial filling temperature and different inlet diameter was calculated to see the influence on the solidification behavior. Then, the whole sampling system was modeled with particle injection. The Discrete Phase Model (DPM) was used to simulate the particle motion in the melt and the focus was on the influence of the initial solidification on the inclusion distributions. Finally, the optimal sampling position inside the aluminum sampler mold was calculated. / Gjutningar av aluminiumlegeringar används ofta inom bil-, och flygindustrin eftersom de spelar en viktig roll för att förbättra prestanda och bränsleeffektivitet. Inom aluminiumindustrin är provtagning den vanligaste metoden att utvärdera mängden inneslutningar i smältan, vilket är en nyckelindikator för kvaliteten på aluminiumlegeringarna. Eftersom både fyllnads- och stelningsprocessen kommer att påverka inneslutningskarakteristiken är provtagningsproceduren av stor betydelse. Syftet med detta arbete är att skapa en två-fasflödesmodell för att simulera fyllnings-, och stelningsprocessen, samt att beräkna partikelrörelserna under provtagningsförfarandet. Computational Fluid Dynamics (CFD) modellering användes och arbetet har utfördes med mjukvaran ANSYS FLUENT. En numerisk tvådimensionell (2D) axisymmetrisk modell byggdes för att simulera provtagningsproceduren med antagandet att påfyllningen kan göras automatiskt längs huvudaxeln. Till att börja med betraktades det första stelnandet under fyllningen utan partikelinjektion. En antagen k - ε turbulensmodell användes för att modellera effekten av turbulens. Flera simuleringar med olika inloppshastighet, påfyllningstemperatur och inloppsdiametrar beräknades för att se påverkan på stelningsbeteendet. Därefter modellerades hela provtagningsmodellen med partikelinjektion. En Diskret Fasmodell (DPM) användes för att simulera partikelrörelsen i smältan och fokus var inverkan av det initiala stelnandet på inneslutningsfördelningen. Slutligen beräknades den optimala provtagningspositionen inuti aluminiumprovformen.