445 research outputs found
Afstandbestuurde elektronische programma schakelaar voor kabel-T.V. systemen
No full textBij een kabelnet volgens het sterprincipe kan men kies-T.V. toepassen. De abonnee bedient thuis een in de centrale aanwezige schakelaar; deze schakelaar moet aan hoge overspraakeisen voldoen. De mogelijkheden van een ’space’-schakelaar worden hier onderzocht: op alle ingangen komt een programma terwijl 1 ingang met de uitgang wordt doorverbonden. Aangetoond zal worden dat een dergelijke schakelaar voor 10 programma’s thans op eenvoudige wijze te realiseren is, tegen redelijke kosten, terwijl het aantal programma’s desgewenst uit te breiden is.Electrical Engineering, Mathematics and Computer ScienceTransmissie van Informati
T.V. Bulpin, Storm over the Transvaal
No full textThomas Victor ("T.V.") Bulpin (1918-1999), the well-known and prolific South African author, wrote nearly thirty books and more than two thousand articles and other publications, mostly on South African hunting, travel and history. Among his well-known “classics” are books, such as Lost Trails on the Lowveld (first published in 1950); The Ivory Trail (1954); Islands in a Forgotten Sea (1959); The Hunter is Death (1962); Natal and the Zulu Country (1966); The Great Trek (1968); Illustrated Guide to Southern Africa (1980) and Tavern of the Seas: The Story of Cape Town, Robben Island and the Cape Peninsula (published posthumously, 2003). Since 2010, several of Bulpin’s well-known books have been republished by Protea Book House in Pretoria – the latest being Storm over the Transvaal (which was first published in 1955 by Howard Timmins in Cape Town)
Flow patterns around longitudinal training dams
No full textWith the intention to reduce the negative effects of ongoing bed erosion, as well as to improve several other river functions such as protection against floods, provision of safe and efficient navigation and ecology, a ‘pilot project longitudinal training dams’ was initiated. The training dams have recently been implemented in the Waal between Tiel and Sind Andries. In this project, river groynes have been completely removed and replaced by dams that lie parallel to the river bank. With help of the longitudinal training dams, a two-channel river system is created in which the river is divided into a main and side channel. The dams are placed in a continuous manner with openings in between that are relatively small compared to the dam length. At the beginning and end of the dam an inlet and outlet region is situated, as shown in Fig. 1. The combination of inlet and openings allows for water and sediment to be divided between the main and the side channel. Both inlet and openings are constructed with the help of a porous rock-layer. The crest heights can be altered by adding or removing stones. This is expected to influence the amount of water and sediment entering the side channel and can therefore be used as a regulation tool. A.J.F. Hoitink, T.V. de Ruijsscher, T.J. Geertsema, B. Makaske, J. Wallinga, J.H.J. Candel, J. Poelman (Eds.) NCR days 2017, Febr. 1-3, 2017. Book of abstracts, NCR publication 41-2017.Rivers, Ports, Waterways and Dredging EngineeringHydraulic Structures and Flood RiskEnvironmental Fluid Mechanic
Flow past a square prism: A numerical study
No full textThe ERCOFTAC test case for flow past a square-prism at a Reynolds number of 21,400 was studied numerically using the flow solver ReFRESCO. The main objectives were to evaluate the performance of a Reynolds Averaged Navier-Stokes (RANS) based turbulence models in Unsteady RANS (URANS) simulations, identify sensitivities to numerical parameters and find out if Detached-Eddy Simulation (DES) based turbulence models provide lower modelling errors than in URANS simulations for this practical test-case. In this respect, the effect of the iterative error and the discretization error using k-omega SST-2003 model [15] was studied and turbulence models such as the SST-DDES turbulence model [19] and the TNT-XLES turbulence model [20] were also applied. The results using the URANS results have highlighted the importance of small temporal and spatial resolution in the calculation domain in order to reduce the effect of the discretization errors. Moreover, large time-integration periods (larger than 60 shedding cycles) should be required to ensure that the statistical noise does not affect the solution Verification procedure which would otherwise hamper the estimated numerical uncertainties. As expected from the studies of Pereira et al. [7], the results with the DES based turbulence models have shown that numerical settings for URANS are insufficient for DES and large comparison errors have been found for many quantities such as the time-averaged pressure distribution at the surface and the integral quantities. The numerical studies with the k-omega SST model [15] have shown that if the numerical uncertainties identified in the study are controlled, Verification & Validation studies using this model could provide low modelling errors for this test-case for the investigated mean flow quantities. For the DES results, since the poor comparisons with experimental results with respect to URANS results using the k-omega SST (2003) turbulence model are caused by poor numerical settings, studies using finer grids, smaller time-steps and lower iterative criteria than the ones used in the present work are recommended.Mechanical, Maritime and Materials EngineeringMaritime Technolog
Criteria of Grain Refinement Induced by Ultrasonic Melt Treatment of Aluminum Alloys Containing Zr and Ti
No full textIt is well known that ultrasonic melt treatment (UST) promotes grain refinement in aluminum alloys. Cavitation-aided grain refinement has been studied for many years; however, it is still not being applied commercially. The current article summarizes the results of experimental work performed on various alloying systems at different stages of solidification. The influence of UST parameters and solidification conditions on the final grain structure is analyzed. It was found that small additions of zirconium and titanium can significantly increase the efficiency of UST, under the stipulation that grain refinement is performed in the temperature range of primary solidification of Al3Zr. The possible mechanisms for this effect are discussed.Materials Science and EngineeringMechanical, Maritime and Materials Engineerin
FireFly Dating: Bachelor End Project
No full textThe FireFly company was created on the idea of blind dating. Most dating apps need the user to have an extensive conversation through a chat box before settling for a date. The product of FireFly tries to remedy that by having the first conversation be face to face. Over ten weeks, we have built the FireFly Dating application. We started researching other dating applications and matching algorithms to decide the requirements for FireFly Dating. After this, we implemented these requirements from scratch. In the final product, users can register an account, like or dislike other users profiles to indicate preferences for the matching algorithm, enroll for a blind date, and get matched. The application also includes an automated emailing system and a full reporting and feedback system. Furthermore, the system allows for an Administrator to get an overview of the application’s data. The Administrator can insert, edit or archive Dating Establishments and Time Slots. As well as view user reports and ban users if necessary. The final application is a product that portrays the Product Owner’s vision in that dating should be easily accessible and offline. At the end of the project, together with the Product Owner, it can be concluded that the FireFly Dating application was successful
Unequal networks: Spatial segregation, relationships and inequality in the city
No full textDoes the neighbourhood in which people live matter for the resourcefulness of their personal network and thus for their opportunities in life? Do residents of a multi-ethnic ‘problem’ area maintain fewer relationships with fellow-residents compared to residents of a homogeneous problem-free neighbourhood? And do ‘diversity-seekers’ who choose to live in a mixed neighbourhood translate their liking for diversity into more mixed networks and more bridging ties? This book brings together key insights from urban studies and network studies in order to understand whether and how spatial segregation matters for personal networks and inequality. By approaching these questions through different urban sociological perspectives, the book engages with current debates on poverty concentration as well as ethnic diversity, gentrification and social capital. The study is based on detailed quantitative and qualitative data on the personal networks of people living in three differently composed neighbourhoods in Rotterdam, the second largest city in the Netherlands.Urban Renewal and HousingOTB Research Institut
Providing cable TV service in large metropolitan areas: a study of consumer demand, 1990
No full textCable television was introduced in America in 1948. Since its beginning, the cable industry has reaped substantial gains in sales and revenue. Its expected to grow even more in the future, due to the technological breakthrough in video two-way communication. Although the future of cable T.V. seems bright, the implementation of cable systems in large urban areas has been lagging. The reason given is that providing cable service in such areas will not be cost effective. The intent of this study is to facilitate cable operators and others in evaluating future potential markets for cable T.V., and to provide a foundation in which subsequent studies can be conducted. The approach in this study is to develop a consumer demand model to investigate the relationships between demand for cable T.V. and those factors which influence consumer demand, for the purpose of predicting demand for cable T.V. in large urban areas. The results of this study provide sufficient evidence that consumer demand for cable T.V. does exist in large urban centers, and contracts the notion that operating a cable T.V. system in these areas would not be cost effective. This thesis was originally written in 1982, and was successfully defended. However, due to unfavorable circumstances it was retyped, revised and resubmitted in the spring of 1990
Towards the Industrial Application of Spark Ablation for Nanostructured Functional Materials
No full textNanostructuring of functional materials is an essential part in the design of energy related devices – but the industrial tools we have to make these materials are lacking. This dissertation explores the green, flexible, and scalable spark discharge process for the fabrication of complex nanostructured materials, and the application of said materials in energy devices. A novel spark generator concept with a 60-fold increased mass production rate was developed, where spark energy and spark repetition rate have been decoupled from gas and material properties. The application of spark discharge materials in two types of energy storage and conversion devices was studied: amorphous-Si photovoltaic cells, MgH2-based hydrogen storage. The possibility of using spark discharge to functionalize nanoparticles with metal coatings was investigated using two spark generators in series. The new spark generator provides true scaling: it produces materials identical to that of the old designs, and the mass production rate – about a gram per day – scales linearly with the spark repetition rate. Arrays of ?100 nm high-purity silver nanospheres were deposited as scattering agents in solar cells, improving their external quantum efficiency by 30 %. A method for synthesizing MgH2 nanoparticles was developed, which show promising hydrogen storage properties. Nanocomposites of Mg with NbOx catalyst nanoparticles were synthesized using two sparks in series, increasing the H2 desorption rate of MgH2. Using a hollow electrode spark, 40 nm gold nanoparticles were coated with silver, and vice versa. The possibility to make useful quantities of high-quality nanomaterials – e.g. high-purity metals or light metal hydrides – from nearly any element makes spark discharge a powerful tool in materials design. The new spark generator provides sufficient quantities to make it economical to develop an industrial nanoparticle facility using multiple sparks in parallel.ChemEApplied Science
Cavitation-aided grain refinement in aluminium alloys
No full textThis thesis deals with grain refinement under the influence of ultrasonic-driven cavitation in aluminium casting processes. Three major goals of this research were: (1) to identify the mechanism of the cavitation-aided grain refinement at different stages of solidification; (2) to reveal the conditions of the stable grain refinement effect in different alloying systems; and (3) to apply the knowledge gained as a result of an experimental work on a small scale to direct chill (DC) casting process. The research is experimental. Experiments were performed both in water and aluminium melts with specially designed ultrasonic installation, including an 5 kW ultrasonic generator, a 5 kW magnetostrictive transducer at a resonance frequency of 17.5 kHz, sonotrode and a selection of melting and casting facilities. DC casting was used to study the influence of cavitation treatment on a pilot scale. The general description of experimental procedures are highlighted in Chapter 2. To increase the efficiency of ultrasonic melt treatment (UST) both solidification conditions and parameters of sonication should be optimized, i.e. cooling rate, treatment temperature, amplitude and frequency of vibrations, treatment and holding time. The influence of parameters of UST on the effects of sonication in liquids (acoustic streaming and cavitation), were studied both in water and model Al-Cu alloys. The fluid flow patters produced by ultrasonic treatment and mixing were discussed based on the experiments in water, while the impact of cooling conditions and amplitude of vibrations is studied on aluminium alloys. The main conclusions that can be drawn from the results are: (1) the amplitude of vibrations should be high enough to promote cavitation, according to our study in aluminium melts it should be 20 ?m or higher; (2) the efficiency of UST increases with prolonged treatment time; (3) the smaller the treated volume, the finer is the grain size; (4) the effect of UST is quite stable, two minutes between UST of the same volume and casting result in only marginal grain coarsening. Based on the results of experimental work the criteria of efficient UST were suggested in Chapter 3. The influence of cavitation melt treatment performed from temperatures above liquidus until almost complete solidification (75 % solid fraction) on structure formation were studied on pure aluminium and model Al-Cu alloys. In all cases studied UST led to formation of equiaxed grain structure with four to eight times smaller grain size. During such a treatment two mechanisms are involved, namely cavitation-induced heterogeneous nucleation and dendrite fragmentation. The possible mechanisms are discussed in Chapter 4 based on the results of experiments performed isothermally in the semi-solid state and in the temperature range from the temperatures above liquidus until complete solidification. The up-scaling of the UST to direct chill casting can be done more efficiently if the processing occurs outside the primary solidification range of aluminum, when the alloy is still fluid. Thus, in order to be able to apply UST for commercial casting techniques (DC-, investment casting), grain refinement should be achieved after the treatment in the liquid state (Chapter 5). One of the mechanisms explaining enhanced nucleation under cavitation is related to activation of nonmetallic inclusions. To investigate the influence of non-wettable particles on the efficiency of cavitation-aided grain refinement in aluminium alloys, we performed a set of simple experiments. In our experiments aluminium films were manually introduced in the liquid metal by mixing the oxide film into the melt during two minutes, in order to increase the amount of oxides. As a result moderate grain refinement has been achieved, which is an indirect evidence of the cavitation-induced heterogeneous nucleation through activation of oxides. In Chapter 5 it has been shown that UST applied during 10 s above liquidus does not change the grain size and morphology of pure aluminium. When applied in Al-Cu alloys, it results in moderate grain refinement (~20 %). Additions of transition metals (Zr, Ti etc.) can significantly improve the efficiency of cavitation treatment. Many commercial wrought aluminium alloys contain these elements because they also prevent recrystallization. Chapter 5 describes the results on the influence of Zr and Ti on structure formation during ultrasonic melt treatment (UST) in the liquid state. Ultrasonic processing is performed in the temperature range of the primary solidification of an intermetallic phase, i.e. normal casting temperatures of aluminum alloys. Based on the investigations of the influence of UST on formation of primary intermetallics in aluminium alloys with high amount of Zr and Ti and an X-ray and EDX analysis of formed phases, a mechanism was proposed for grain refinement in Al–Zr–Ti alloys produced with cavitation treatment in the temperature range of solidification of primary intermetallics. Ultrasonic melt treatment promotes fragmentation of Al3Zr particles, decreasing the size of potential solidification sites and increasing their number, while Ti substitutes Zr in Al3(Zr1-xTix) particles, which leads to the delay of nucleation to larger undercooling, when the smaller and more numerous intermetallic particles become active. Based on the results of Chapters 3 to 5 the criteria of efficient cavitation-assisted grain refinement of aluminium alloys were formulated and used for: (1) production of commercial aluminium alloys on a small scale in permanent volume (90 cm3), (2) UST in the launder, and (3) DC casting of Al-Cu alloys with small additions of either transition metals (Zr. Ti) or Al-3Ti-B. Effects of ultrasonic melt treatment on structure formation and macrosegregation of aluminium alloys were studied. Five billets 195 mm in diameter were cast at a pilot vertical DC caster. In separate experiments UST was applied in the sump and in the launder. Three different launder designs were used. It was shown that UST during DC casting can refine the structure, if applied either in the hot top or in the launder. Further work is required to optimize the process in terms of treatment time, volume and casting speed. A possible solution might be either optimization of current launder design or introduction of an intermediate chamber, where cavitation treatment can be realized in a larger volume with prolonged treatment time.Materials Science & EngineeringMechanical, Maritime and Materials Engineerin
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