1,720,964 research outputs found
Aerodynamics of two-dimensional sail wings
Get PDFThe purpose of this work is to obtain the exact solution for the profile and characteristics of a two-dimensional sail, immersed in an inviscid incompressible fluid. An experimental investigation was also performed and results are compared with theoretical predictions.
The sail was assumed to be an infinitely flexible, non-porous membrane of zero thickness, fixed at the leading and trailing edges, and stretched under a constant tension T in the sail surface. An aerofoil model was considered where the airflow remains smoothly attached over the entire profile.
All known previous studies have also used the inviscid fluid approximation, but made the further assumptions of small angle of attack and negligible profile slopes. These assumptions enable the use of thin aerofoil theory predictions for pressure distribution and linearisation of the sail equation.
In contrast this investigation obtains the exact solution. An iterative numerical method is devised whereby an initial estimate is made for the profile using thin aerofoil theory. The pressure distribution is then determined using Theodorsen's method and the profile recalculated using the full sail equation, so that tension and pressure forces are balanced. The cycle of redetermining the pressure distribution and profile is repeated until a convergent solution is obtained.
Results are shown for the profile, lift coefficient and centre of pressure, for various angles of attack and states of tension. For sails with non-negligible camber, values of lift coefficient and centre of pressure are found to differ significantly from those predicted by the linear approximations. Previous researchers have established the existence of a critical tension state where the tension force is unable to contain the pressure forces acting on the sail, and predicted that the value of this state (KTc) was independent of angle of attack. However this study indicates that KTc increases with increasing angle of attack. Centre of pressure calculations, for various angles of attack and states of tension, indicate that two-dimensional sails may possess either static longitudinal stability or instability, depending on the tension state. Experimental results for the profile, KTc, lift coefficient and centre of pressure are compared with theory, and areas of agreement and disagreement discussed. Experimental values for the drag coefficient and the lift to drag ratio were obtained and are discussed in detail
Coronal magnetic energy release by current sheet reconnection
Get PDFIn this thesis we investigate the rapid release of energy in the solar corona, with a particular view to understanding the solar flare in which magnetic reconnection is thought to play a key role. A review of existing reconnection solutions is given in Chapters 2 and 3, with new analytic and numeric results are presented in subsequent chapters.
Although much of the work in this thesis is computational, numerical investigations are always motivated theoretically. In Chapters 4 and 5 several aspects of two dimensional reconnection are investigated using a periodic time-dependent incompressible code. One of the main points is to check the veracity of the analytic solution of Craig and Henton (1995) by running the code from general initial conditions. Other aspects of 2-D merging covered include the tearing mode instability, osculation and the effects of finite compressibility.
We employ a 3-D time-dependent code, in Chapter 4, to check that the analytically predicted spine and fan forms develop from general initial conditions. Scalings with resistivity of the associated current structures are also investigated. Most of the analytic work so far has revolved around single null magnetic configurations. Chapter 6 focuses on reconnection solutions in the presence of multiple nulls. Finally, we look at an application of the analytic theory in the context of particle acceleration. In Chapter 7 we trace proton orbits using a physically plausible analytic current sheet solution
Current sheet formation in uniformly twisted magnetic flux tubes
Get PDFA viable mechanism for coronal heating is conversion of magnetic field energy into heat through magnetic reconnection in current sheets. Two processes capable of generating current sheets in the solar corona are studied here. The corkscrew kink instability of an axisymmetric magnetic flux tube, as a result of uniform twisting, is addressed first. Linear growth rates of the instability due to a helical perturbation are calculated for a variety of tubes. It is shown that plasma pressure and tube radius reduction are stabilising.
The helically symmetric geometry allows us to use an effectively two-dimensional formulation. A magneto-frictional code is developed to simulate relaxation to a second helical equilibrium of the tubes. Lower energy equilibria resulting from the kink are studied. Only one tube, whose twist decays with radius, is shown to result in a true field singularity or a current sheet. The Gold-Hoyle tube relaxes to a smooth kinked equilibrium. Tubes having reversal of a field component result in current concentrations as they kink and relax to a neighboring equilibrium. Plasma pressure reduces the strength of the kink in all cases.
The second process resulting in current sheet formation is the interaction of multiple flux tubes. The same frictional code is used to show that repelling tubes move away from each other as the superposed field relaxes to equilibrium. Twisting them together results in current concentrations. Attracting tubes with no neutral point also give rise to current concentrations when they are twisted together and relax to equilibrium. Current sheets are formed when attracting flux tubes, whose initial combined field have a neutral point, coalesce in the absence of plasma pressure. Introducing plasma pressure prevents current sheet formation when the tubes are not twisted together. In case the tubes are twisted together we again see current sheets, the lack of symmetry permitting plasma to flow out from between approaching flux surfaces, allowing them to come into contact. Interaction of unequal attracting tubes results in curved current sheets
Electromagnetic stirring using a travelling magnetic field
Get PDFElectromagnetic stirring of liquid metal is widely used during continuous casting in iron and steel industries to improve the quality of the products. In this thesis we aim to develop a theory for traveling linear stirrers in closed channels.
We consider a channel filled with an incompressible, electrically conducting viscous fluid. A two dimensional multi Fourier-component magnetic field is moved along the channel. Induced currents in the fluid interact with the field to give a Lorentz force which drives fluid motion. When the magnetic Reynolds number is large the imposed magnetic field is changed significantly.
In this study we consider uniformly-moving and accelerating fields.
For the initial fluid motion due to a two Fourier-component source moving at a constant velocity, with Rm small, an analytical solution is derived. The LaxWendroff numerical scheme is used to calculate the flow until a steady state is reached. For effective stirring the width of the channel has to be less than the field wavelength.
The problem of a solid conductor in a steadily-moving field at finite Rm is investigated next. It is shown that as the magnetic Reynolds number increases the flux inclines in the direction of the motion. In the study of the fluid flow at finite Rm it is shown that the narrower the channel the better and stronger will be the magnetic field's penetration. Most effective stirring occurs when magnetic Reynolds number is around 100.
When the magnetic field is given a time-dependent velocity at large Rm we show that more eddies are created immediately after a velocity reversal. In the process, the layers of magnetic fieldlines on the boundaries incline in the opposite direction of the applied field. Simple harmonic motion of the field is also effective, and becomes more so as the frequency increases (at fixed wavelength) but at the expense of overall fluid velocity
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Reduction of water and bed levels in the lower Waikato River
No full textThe major aims of this thesis are to determine the historical qualitative and quantitative reduction in water and bed levels of the lower Waikato River, to identify probable factors influencing the reduction, and to predict the future water and bed levels at Huntly in relation to the Huntly Power Station management
Sediments in the Waikato River are composed of gravelly pumiceous sands. In terms of the mean and median grain sizes, generally sands dominate the lower river b~ from the river mouth to the cross section at about 100 km upstream of the mouth, while gravels dominate the bed upstream onward. A hiatus in the longitudinal mean and median grain size distributions from sand to gravel is apparent around Horotiu. In the Waipa River, the texture of bed materials was found to be similar but probably finer to those in the lower parts of the Waikato River.
On the basis that ∂n / ∂Q= ∂W/ ∂Q =∂S / ∂Q=0, where n, W, S and Q are, respectively, Manning's coefficient, width, energy slope and discharge, water level of each gauging can be adjusted to one for an index flow of 350 m³/s ( WLQ₌₃₅₀ ). Further assuming that ∂n / ∂T = ∂W / ∂T = ∂S / ∂T=0 where T is time, variations in the time series of WLQ₌₃₅₀ represent the mean bed level changes over time. These assumptions seem to be acceptable for those gauging data within a certain range of discharges in the lower Waikato River.
A quantitative analysis of the available river survey data, water level profile measurements, and gauging records has indicated that, in general, there was a continuous trend of reduction in water and bed levels on the lower Waikato River since the 1960s. The reasons for these reductions likely include sand mining operation along the river, consequent disturbance of the river bed surface, long term sand extraction around Mercer and further downstream, upstream effects of the significant bed level lowering at Mercer, and downstream effects of the 1947 Karapiro Dam closure.
Analysis of river bulk volume changes suggested that the amount of bedload transported into the river downstream of Ngaruawahia was about 160 000±24 000 m3/yr between 1964-1989. Nearly two-thirds of this was contributed by the bed materials stored in the upstream Waikato River course by an analysis of data from 1974-1989, and the remainder by the Waipa River and the catchment yield from the Hamilton basin.
Two disparate time scales for water and bedload movements in practice result in singularperturbation characteristics of the system. With a quasi-steady flow approach, linearization of the water and bedload movement system produces a hyperbolic equation or even a parabolic equation (uniform flow). The parabolic equation is a good approximation of the hyperbolic equation under the condition of large values of time or a large distance from the original disturbance. Variations along the Waikato River of an average mean bed level within a certain length of channel are expected to be small. Therefore the linear models can be applied. In the domain between 48.25-94.45 km upstream of the Waikato River mouth, the parabolic model has been used numerically to predict the future bed levels at Huntly in December 2040 for different scenarios. Potential effects on operation of the present cooling water system have been assessed for given discharges at that time.
The ratio of river width to water depth for the formation of alternate bars at the Huntly Railway Bridge has been found to be exceeding 100 by an analysis of the gauging data at this site and at the Ngaruawahia Cableway. This critical width-depth ratio, 100, is much bigger than those suggested in the literature. The corresponding conditions of discharge and mean water depth are, respectively, less than about 350 m³/s and 2.30 m. However further research is required to confirm these conclusions
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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