1,720,982 research outputs found
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
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
Wind Induced Heat Losses from Solar Dish-Receiver Systems
No full textParabolic dish-receiver concentrating solar power (CSP) plants are a promising technology for the generation of renewable electricity. However, the high operating temperatures of the cavity receivers mean the performance of these CSP systems is very sensitive to heat losses, in particular by wind.
A comprehensive literature review revealed a lack of work undertaken on wind flow around parabolic dish CSP systems and its impact on the heat loss from them. Previous studies investigating the effect of convective heat loss treated the receiver as an isolated entity, decoupled from the dish/reflector structure. Hence, the effect of the dish on the airflow around the receiver had not previously been considered.
This gap in the literature indicated a need to understand the effect of the dish in order to develop realistic heat loss models for the design of parabolic dish CSP systems. Hence, the research focused on the interaction between the wind and the dish structure causing local effects of air motion at the cavity inlet and the resulting in convective heat loss.
To verify the assertion that the dish would affect the flow of air around the receiver, and hence the heat loss, wind tunnel testing was performed on a scale model of a parabolic dish reflector. This study showed a significant disturbance to the flow field both qualitatively and quantitatively near where the receiver would be located. On this basis, a computational fluid dynamics (CFD) model of the airflow around a scaled dish and receiver was developed. These simulations showed good agreement with the quantitative measurements and qualitative visualization undertaken in the wind tunnel, thus validating the computational approach.
Having validated the simulation scheme, a detailed CFD study was undertaken to determine the heat loss from a 20m2 parabolic dish and receiver system, developed by the Australian National University for a range of dish orientations, wind speeds and incidence angles.
The CFD simulations confirmed that the dish’s presence had a significant impact on the convective heat loss experienced by the system. With the flow around the dish structure being considered, the heat loss experienced by the receiver was markedly different, and in some cases lower (up to 40%), than when it was assumed the receiver acted in isolation. Furthermore, it was found that for dish tilt angles and wind incidence angles between +/-30° and 0°, the heat loss significantly increased. This can be attributed to the receiver moving into the free stream and being subject to stronger forced flow than experienced in the dish’s wake.
In summary, the results delivered quantitative data as to the effect of the dish’s orientation, wind speed and wind incidence angle on heat loss. Using this information, a series of correlations were established to allow designers of parabolic dish CSP systems to incorporate the impact of the dish on the convective heat loss from the receiver. More broadly, the work demonstrated the importance of considering the influence of the dish when determining the heat loss from a parabolic dish receiver to avoid designing overly conservative, and hence costly, CSP systems
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
Development and Analysis of a Solar Humidification Dehumidification Desalination System
No full textIn this thesis, an investigation was performed in order to understand the performance of a solar humidification dehumidification (HDH) desalination system. Initially, a mathematical model of the system, including solar water heater, condenser, economizer and long duct humidifier was developed. Using a sensitivity analysis, it was found that improving the intensities of heat and mass transfer in the humidifier would significantly enhance the yield of the system. This led to the development of a novel cascading humidifier, in which air was directed through a series of falling water sheets.
An experiment was performed to first identify and characterise flow regimes in the crossflow interactions, and from this, to develop correlations to describe the heat and mass transfer for such interactions. Four flow regimes were identified and mapped based on the Reynolds number of the air and the Weber number of the water.
Subsequently, Buckingham’s π theorem and a least squares analysis was employed to develop a series of empirical relations for Nusselt and Sherwood numbers. This led to the proposal of three new dimensionless numbers named the Prandtl Number of Evaporation, the Schmidt Number of Evaporation and the Lewis Number of Evaporation. These describe the transfer phenomena in low temperature evaporation processes with crossflow.
Finally, the new correlations for Nusselt and Sherwood numbers were used to develop a model of a cascading humidifier, incorporated in a solar HDH system. It was found that a cascading humidifier enhances the yield of the HDH system by approximately 15%, while reducing the evaporation area to approximately a quarter of that required in a long channel humidifier
Natural Convection in Single Slope Solar Stills
No full textSolar thermal distillation systems remain one of the most viable solutions to mitigate water crises in arid locations. Previous research has shown that single slope solar stills remain the simplest and lowest-cost means of desalination in off-grid rural or remote environments. Improved potable water production from these structures is primarily dependent on the natural convection inside the still, and this has been the subject of numerous studies. Natural convection inside single slope solar still enclosures is affected by the geometrical parameters of the cavity. However, the numerous existing correlations that have been developed for solar stills, including the most used equation of Dunkle [1], do not consider the fact that the cover angle and the aspect ratio (the absorber length over the mean height of the still) may play a role in the volume of water produced by these systems. Moreover, a comprehensive review of the literature revealed that when cover angle and cavity aspect ratio of single solar stills was considered, the results were often in conflict between studies.
As a pilot study, this research first investigated the validity of the previously developed correlations (from literature) by constructing, instrumenting and operating a single slope solar still. The data from these experiments was compared with results from thermal models using the literature correlations. These results clearly indicated that the previous correlations overpredicted the magnitude of natural convection and the yield by 30% to 130%. From this pilot, it was clear that a much more detailed parametric study was required to determine and understand the relationship between natural convection and the internal geometry of the still.
To address this issue, computational fluid dynamics simulations (CFD) were undertaken for a single slope solar still with cover angles between 0⁰ and 60⁰ and aspect ratios ranging from 8 to 1.2, with the results validated experimentally using particle image velocimetry. It was found that both aspect ratio and cover angle significantly altered the heat transfer coefficient, with changes arising from mono- and multicellular flow patterns. These changes in the flow directly impacted the convective heat transfer coefficient, consequently, an improved correlation for Nusselt number was developed. The correlation was then verified against existing experimental data for different angles and aspect ratios, a good agreement was found in predicting distilled water production.
Having found that multicellular flow increased the natural convection in a single solar still geometry, it was decided to investigate the use of baffles as a means of altering the flow inside these devices and encouraging multicellular flow. To address this issue, the study examined the effect of vertically mounted passive baffles on the natural convection inside a single slope solar still geometry, again using CFD. These simulations explored the effect of baffle length and position on natural convection inside solar still with cover angles from 10˚- 60˚ and were validated experimentally using particle image velocimetry. The results showed that baffles did have a marked impact on the natural convection flow field and could increase the natural convection heat transfer coefficient. However, it was also found that in some cases the baffle obstructed the flow hence decreasing heat exchange between the boundary layers. The work led to the development of a relationship to describe the effect of baffles on natural convection, that will aid designers of single slope solar stills in the future.
Finally, a daily performance comparison between conventional and baffle single slope solar still was performed using both developed correlations. The results showed that both aspect ratio and cover angle have a direct effect on the water production and efficiency of both solar stills. Furthermore, the solar still equipped with a baffle delivered a higher yield of fresh water than that of the conventional single slope solar still geometry across different aspect ratios and cover angles. By investigating natural convection inside single slope solar stills in a generalisable way, this work has, for the first time, demonstrated the relationship between the aspect ratio, cover angle, convective heat transfer coefficient and still yield. Moreover, the study has shown that the inclusion of a passive baffle can enhance solar still performance. Combining these elements, this research constitutes a major advance in the knowledge of single slope solar still design
Unlocking the Aquatic Ballet of Sea Turtles: Harnessing Sea Turtle Biomechanics for the Evolution of Marine Technologies
No full textSea turtles are marvels of marine navigation, employing sophisticated biomechanical strategies that enable them to traverse thousands of kilometres across oceans. Despite their importance in oceanic ecosystems, all sea turtle species face the risk of extinction, largely due to human activities. This doctoral thesis provides an in-depth analysis of the biomechanics and hydrodynamics involved in sea turtle movement. It emphasises creating non-intrusive techniques for examining and mimicking their swimming behaviours. The study aims to enhance biological and biomechanical understanding and inspire technological innovations drawing from nature's design.
This research first introduces a novel, non-invasive procedure for studying the biomechanics of wild sea turtles by utilising underwater drones to film them in their natural habitat, the Great Barrier Reef. Through this approach, distinctive swimming patterns were observed, deviating from those recorded in captive juveniles. Our findings show that the flipper goes through a closed-loop trajectory with extended sweeping of the flipper tip towards the centre of the carapace to create a clapping motion. We have named this the "sweep stroke", and in contrast to previously described four-stage models, it creates a five-stage cycle swimming locomotion model.
Delving into the migratory prowess of sea turtles, the thesis then examines the biomechanical and hydrodynamic aspects of long-distance travel. Sea turtles achieve this remarkable feat despite a diet consisting primarily of low-energy foods. A model based on the green sea turtle (Chelonia mydas) and a custom testing rig was developed to investigate only the upstroke phase of their swimming. It was found that sea turtles likely utilise a passive upstroke, significantly reducing their drag coefficient and allowing them to maintain swim speed without generating thrust, thereby conserving energy.
The thesis further investigates the green sea turtle's incredible ability to swim up to 50 km per day on a diet of seagrass or microalgae. By factoring in the newly described five-stage swimming cycle, a soft-robotic sea turtle named Cornelia, capable of mimicking the real animal's form and function, was developed to provide biomechanical insights without invasive experimentation. The study reveals that the green sea turtle may only produce propulsion for about 30% of the limb beat cycle, with the rest of the time spent in a low-drag glide, minimising speed loss due to their large mass and low drag coefficient. These insights can potentially revolutionise oceanic exploration through a new generation of robotic systems that harness sea turtle-inspired propulsion strategies.
Furthermore, this work utilised Cornelia, to investigate the flow manipulation during the sea turtle's propulsive phase. By analysing the relationship between swim speed, flipper angle of attack, power consumption, and the production of thrust and lift, this research hypothesises how flow features contribute to the sea turtle's propulsive efforts and cost of transport. The findings indicate that sea turtles achieve exceptionally low cost of transport values, affirming the efficiency of their swimming technique and providing valuable data that could inform the design of high-efficiency underwater drones for extended missions.
Lastly, the thesis explores the development of prosthetic flippers for sea turtles that have lost a limb. Robotic testing demonstrated that a prosthetic could effectively mimic the sea turtle's downstroke and upstroke, allowing for regained manoeuvrability. Swim tests with the prosthetic attached to the robotic model yielded promising results, nearly matching the average swim speeds of wild sea turtles. This work aspires to lay the groundwork for open source prosthetic designs that could empower veterinary professionals worldwide to assist injured turtles. The broader ambition is to inspire further animal-based robotic designs, advancing technologies geared towards ecological conservation and rehabilitation.
In conclusion, this thesis presents a multifaceted investigation into the locomotion of sea turtles, yielding significant original insights that bridge biology, robotics, conservation, and bioinspired engineering. The findings have profound implications for understanding the biomechanical efficiency of these endangered species and offer a pathway toward developing sustainable technologies that could benefit both wildlife conservation and human engineering pursuits
Effects of Parapets on the Performance of Unglazed Solar Collectors
No full textThe useful energy output of a solar thermal collector is influenced by various factors, such as wind velocity. Elevated wind velocity results in substantial heat loss caused by wind, thereby affecting the performance of the collector. Solar thermal collectors that lack insulation or a glass covering (unglazed solar collectors) are especially susceptible. Considering the placement of these collectors on rooftops and their exposure to wind, it is imperative to examine methods for minimising this impact. Studies have demonstrated that perimetric parapets can modify wind loads on roofs and the structural support system of solar panels and collectors. Insufficient attention has been devoted to examining the influence of parapets on collector heat loss, as well as the optimal placement of collectors on roofs with respect to parapets and their effect on heat loss. Consequently, the focus of this study was directed towards examining the impact of parapets on the localised velocity surrounding roof-mounted unglazed solar collectors, as well as the subsequent heat loss.
The quantification of this effect was accomplished using Computational Fluid Dynamics (CFD) simulations validated through wind tunnel experiments. The aerodynamic properties of the roof were investigated across three distinct scenarios, namely those with high parapets, low parapets, and no parapets. The results of the study indicated that when perimetric parapets were not present, the vortex formed on the rooftop was located in closer proximity to both the leading edge and the surface of the roof. In contrast, the inclusion of parapets resulted in the elevation of the vortex above the surface of the roof. In each instance, the velocity at the central region of the roof exhibited higher velocities, while the velocity zones adjacent to the leading and trailing edges of the roof and parapets displayed lower velocities.
Measurements were taken at different roof mounting locations, representing 25%, 50%, and 75% of the roof length, for collector tilt angles of 5°, 20°, 40°, and 60° at different parapet heights and wind incidence angles (0°, 45°, 90°, 135°, 180°). The results of the simulations indicated that there was a direct correlation between the increase in tilt angle of unglazed solar thermal collectors and the corresponding increase in heat loss, irrespective of the height of the parapet. Additionally, the study demonstrated that lower parapets led to higher levels of collector loss compared to parapets with greater perimetric height. The research revealed that increased collector tilt angles resulted in a decrease in heat loss for the following row of collectors, which were positioned further from the incoming flow in rooftop solar arrays. In all instances, it was observed that the leading row of collectors positioned at the edge of the roof experienced a greater heat loss compared to all subsequent rows of collectors.
In summary, the results of this study demonstrated the aerodynamic impacts of parapets on the heat loss of roof-mounted standalone and array unglazed solar thermal collectors. Based on the acquired insight, a set of correlations was formulated to enable designers and architects to consider the influence of parapets on the convective heat loss of unglazed solar thermal collectors. In a more comprehensive context, the study showcased the importance of mitigating the impact of velocity on unglazed solar collectors, thereby enabling its use in various solar thermal applications. This is particularly noteworthy given that unglazed solar collectors are cost-effectiveness
Lightweight Composite Structure for Solar Central Receiver Heliostats
No full textOf all Concentrating Solar Power (CSP) technologies available today, central tower CSP systems are moving to the forefront as they have the capability to become the technology of choice for the generation of renewable electricity. The potential of central tower systems to achieve high temperatures offers a path to higher efficiencies, thereby providing an inherent advantage versus the other CSP systems. Achieving these high temperatures requires a large number of heliostats, and therefore the heliostats are considered the most crucial cost element of central tower CSP systems amounting up to 50% of a plant’s total cost. To address this issue and in order for the cost of energy from central tower plants to be competitive with that of other energy systems, there is a need for innovative heliostat designs that can reduce the heliostats’ cost without affecting its performance. One way of reducing this cost is by utilizing lightweight honeycomb sandwich composites in the heliostat structure, reducing the size of the drive units and their energy consumption. However, one of the challenges faced in implementing such systems is ensuring that they are able to cope with the aerodynamic forces imposed upon them during operation.
Despite the progress in heliostat development, a comprehensive review of literature revealed a lack of work undertaken on investigating the suitability of honeycomb sandwich composites for use as a heliostat mirror structure. This gap indicated a need to deliver a better understanding on the interaction between the wind and honeycomb sandwich composites employed as a heliostat mirror support structure by investigating their aero-structural robustness and behaviour characteristics.
The research first studied the flow behaviour and aerodynamic loads on a stand-alone heliostat using computational fluid dynamics (CFD), with particular emphasis on the effect of wind direction and its impact on the aerodynamic loading of a heliostat. This aspect of loading had not previously been explored in any detail. The model was validated by comparing the computation predictions of the heliostat’s aerodynamic coefficients with both experimental measurements and numerical results from previously published work. The study showed that, for a 0° wind incidence angle, the drag and base overturning moment coefficients decrease as the tilt angle alters from vertical to horizontal. The lift and hinge moment coefficients, on the other hand, showed an asymmetric behaviour about the 0° tilt angle with maximum values occurring at tilt angles of 30° and -30°. Increasing wind incidence angle affected the wind loading coefficients (drag, lift, base overturning moment and hinge moment coefficients) by decreasing their magnitudes at different rates. A subsequent non-linear regression analysis delivered a correlation for each of the coefficients based on the heliostat’s tilt and wind incidence angle was developed. These formulations provide a useful analytical tool for heliostat designers to determine the wind loads on heliostats and to assess structural forces and moments on the frame of the heliostat and its reflective surface. In summary, it was shown that wind incidence had a significant impact on the aerodynamic loads encountered by a heliostat and, therefore, needs to be accounted for when examining the structural integrity of heliostats.
Secondly, the study investigated the aero-structural behaviour characteristics of a proposed honeycomb sandwich composite-based heliostat structure by performing numerical fluid-structure interaction (FSI) simulations for several loading conditions at various tilt and wind incidence angles. The structural response of the heliostat’s honeycomb sandwich panel showed markedly different behaviour characteristics at various operational conditions. From the results, it was shown that the effect of heliostat’s tilt orientation on the sandwich panel’s maximum deflection and stresses becomes more pronounced as wind velocity increases above 10 m/s. This effect becomes more vital and the difference in the maximum displacement and stress values at different tilt angles escalates to a maximum at wind velocity of 20 m/s. Moreover, the wind velocity effect on the heliostat panel for the case of 0° tilt angle was negligible. This is because of the flow uniformity (the projected area of the reflector directly facing the wind is at its minimum) that leads to a significant decrease in the wind loading effect on the panel at this tilt orientation for all wind velocities (5-20 m/s). The study showed that increasing wind incidence angle affected the recorded maximum displacement and stress results by reducing their magnitudes at different rates. This is due to the fact that the heliostat’s projected area directly facing the wind decreases with the increase in wind incidence angle. This consequently reduces the effect of the blockage, causing a decrease in the wind loading effect on the heliostat. As the wind incidence angle gradually increases from 45° to 90° (the projected area of the reflector continues to decrease) for all tilt angles, the wind incidence angle influence on the maximum displacement and stress values gradually increased and the values notably decreased thus reaching its minimum at β = 90°. This implies that the heliostat panel at 90° wind incidence angle, regardless of any tilt angle, is not significantly influenced by wind loadings at wind velocities of 20 m/s and below. The study also showed that when wind strikes the heliostat structure at 0° and 45° incidence angles, the shielding effect caused by the supporting components and torque tube was clearly noticeable. When the incoming wind acted on the reflector’s back surface, the maximum displacement and stress values were slightly lower compared to the ones recorded when the flow acted on the heliostat’s mirror surface. In all of the operational conditions studied, it was concluded that the worst case was found to be at a tilt angle of 30° under the effect of wind flow at 0° to the heliostat surface with a velocity of 20 m/s. Despite this observation, it was found that the heliostat managed to maintain its structural integrity according to relevant optical and material failure standards.
Taking the worst case operational condition as a basis, and given that the mechanical properties of honeycomb core-based sandwich composites are highly dependent upon the honeycomb’s geometric configuration (e.g., cell wall angle (φ), cell wall length (a), cell wall thickness (t)) and the core thickness (D), a comprehensive parametric study was performed to investigate the effect that each of these parameters has on the aero-structural behaviour characteristics of the honeycomb sandwich composite-based heliostat. The study was carried out for three different core thicknesses (D) with various honeycomb configurations. From this the study revealed that varying the honeycomb’s cellular geometry significantly affected both the strength and stiffness properties of the sandwich composite-based heliostat structure, illustrating that it is attainable to control suitably the strength of the heliostat’s honeycomb sandwich panel to achieve superior mechanical properties by varying the cell’s configuration.
These variations in the heliostat’s structural response highlighted the necessity for a generalized model that can capture the influence of each of the honeycomb core’s geometrical parameters on the heliostat structure’s performance (i.e. optical, material failure and weight reduction). Having a predictive model that estimates the heliostat’s structural performance, under the worst case operational condition and based on the desired site’s maximum recorded wind speed, eradicates the need of going through the hurdles of establishing an FSI model for each of the honeycomb core’s geometrical parameters. This, in turn, runs down the implementation time and keeps off unnecessary computations. In this sense, and given that this approach is one of the prominent tools for modelling complex non-linear relationships, particularly in situations where the development of phenomenological or conventional regression models becomes impractical or cumbersome, artificial neural network (ANN) technique was utilized to establish a novel predictive model that predicts the structural performance of the honeycomb sandwich composite-based heliostat based on its honeycomb core’s physical parameters. The results showed that the established ANN model was capable of accurately predicting the structural performance of the honeycomb sandwich composite-based heliostat.
Finally, a rigorous investigation was carried out on the utilization of particle swarm optimization (PSO) algorithm to establish a novel prediction-optimization model that predicts and optimizes the structural performance of honeycomb sandwich composite-based heliostats. The model couples the ANN predictive model with the PSO algorithm for determining the optimum honeycomb core configuration leading to minimum self-weight of the heliostat’s sandwich composite panel while satisfying the structural performance requirements (i.e. optical and material failure). It was shown that the proposed integrated ANN-PSO model, which was encompassed as a user-friendly graphical user interface (GUI), delivers a useful, flexible and time-efficient tool for heliostat designers to predict and optimize the structural performance of honeycomb sandwich composite-based heliostats as per desired requirements.
In summary, the work presented is a significant milestone in the quest to develop cheaper lightweight heliostats that are strong and capable of withstanding wind loads and other environmental conditions, and a major step on the way to move central tower CSP systems to the forefront to become the technology of choice for energy production
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