12 research outputs found
Solar updraft tower - structural optimisation under dynamic wind action
No full textAs fossil fuel reserves are rapidly being depleted, sustainable alternatives have to be found to fulfil the world's energy demand. Numerous concepts have been proposed to generate electricity by harnessing renewable energy sources such as solar or wind power. One of these concepts is the the so-called solar updraft tower (SUT). The SUT consists of three elements: a solar air collector, wind turbines and a chimney. The taller the chimney, the larger the stack effect and thus the more energy which can be generated by the turbines. The proposed concepts for this chimney schematise it as a reinforced concrete cylindrical shell, with the bottom half shaped like a hyperboloid and the top half as a flared cylinder, outfitted with ten stiffening rings evenly distributed over the height. Chimneys as tall as 1500m have been proposed, and, previous research shows that these tall structures have very low eigenfrequencies which come very close to the peak of the wind power spectrum. This makes them extremely vulnerable to resonance induced by storm actions. Two types of resonance can be distinguished in these structures; along-wind resonance, and across-wind resonance. Along-wind resonance is caused by turbulence in along-wind gusts. The second type, across-wind resonance, is caused by the alternating shedding of vortices. This leads to pulsating excitation forces in the across-wind direction, and, if the frequency of the vortex shedding is the same as one of the eigenfrequencies of the chimney, resonance will occur. In this thesis, a finite element model is created based on a pre-existing design. This so-called base model is then analysed to determine which key problem areas could benefit from improvement. The analyses show that especially the first two eigenfrequencies are critical for along-wind resonance as well as across-wind resonance. These eigenfrequencies are seen as two individual problem areas as improvements to one eigenfrequency not necessarily guarantee improvements to the second eigenfrequency. Furthermore, tension on the windward side leads to cracks in the stiffening rings which negatively influence the eigenfrequencies and thus the dynamic response. The last area which could benefit from optimisation is the cost of the chimney; an optimal solution does not use more material than necessary. A design tool called SUMAT (Solar Updraft Modal Analysis Tool) is created which enables the user to analyse multiple chimney configurations at once, subsequently being able to compare their results. Various sensitivity analyses are carried out to determine the influence of geometric and material parameters on the four key problem areas of the chimney. A multi-objective optimisation process is followed to optimise each of the key problem areas, ie. objective functions, by hand. The first step in optimising the structure is to subdivide the parameters which were researched into four categories, depending on their usefulness. The second step consists of gradually introducing these parameter changes into the base model. The optimisation process revealed that the objective functions can be maximised as follows: increasing the moment of inertia of the rings by changing their aspect ratio ensures that the chimney is fully loaded in compression. An increase in the throat height further improves the reduction of tension on the windward side and the first eigenfrequency. A reduction in wall thickness at the top of the chimney improves the first eigenfrequency while also reducing material use. Lastly, it appears that the stiffening rings at the bottom serve little to no purpose. Removing them leads to a reduction in material use while some of the material gained can be used to increase the dimensions of the top rings, consequently improving the second eigenfrequency and reducing tension. More thorough analyses revealed that the optimisation process has indeed led to an overall improved structure when compared to the original base model. While along-wind resonance does not pose as great a threat as was initially assumed, due to the influence of aerodynamic admittance, the results do show that the improved eigenfrequencies led to a smaller increase in deflection as a result of dynamic wind action. Vortex shedding also no longer poses a threat as the improved second eigenfrequency resulted in critical wind speeds which are much larger than could ever occur at the chosen reference location. Future optimisations should therefore focus more heavily on the second eigenfrequency than on the first, assuming that the accompanying mode shapes stay the same.Building Engineering - Structural DesignStructural EngineeringCivil Engineering and Geoscience
Nonlinear analysis of the solar updraft tower under static wind action
No full textThe near future is challenging: depletion of non-renewable energy depletion, electricity shortage and sustainability give the need of developing renewable energy technology. The solar updraft tower power generation technology using solar and wind generates a reasonable amount of electricity with zero carbon footprint during operation. The working principle is rather simple: the heated air underneath a large transparent roof (air collector) is sucked into a central vertical chimney; the driven wind generates electricity by wind turbines. The solar updraft towers are mainly suitable for high power output, which also requires a large scale structure. The solar updraft tower in this research project is a 1000m reinforced concrete tall tower stiffened by ten stiffening rings every 100m. The wind load is the most hazardous action for ultra-high ultra-thin SUT shells. The constitutive behavior of the reinforced concrete structure will give nonlinear responses under a certain wind loading level, which requires a careful study. Additionally, the SUT structural design under large wind load treated in a linear elastic stage could be costly. The dimensions of the reinforcement calculated by linear-elastic analysis can be improved by nonlinear analysis. It gives a real structural behavior including the stress redistribution caused by concrete cracking, crushing and reinforcement yielding. Nonlinear analysis can achieve a more economic design while maintaining sufficient safety. This main thesis objective is to perform a physical nonlinear analysis of the SUT under static wind action to acquire realistic structural behavior and conduct a further optimization. A finite element model is built and a linear analysis is done for the model verification. Then a detailed linear model is built to determine the necessity of detailing the stiffening rings. The analysis shows that by detailed modeling the stiffening rings provide a higher stiffness compared to modeling the simplified way, hence improving the overall structural behavior of the Solar Updraft Tower. A discussion about the feasibility of the chosen nonlinear material models has been done. The total strain crack rotating model is chosen whose crack is rotating continuously with the direction of principal stress. The simple working principle gives the model a robust and stable behavior. For concrete tensile behavior, a linear ultimate strain based stress-strain relation is used including the tension stiffening effect. A nonlinear stress-strain behavior is adopted for concrete in compression, considering effects of lateral crack and lateral confinement. For the reinforcement steel, elastoplastic material modeling is used with strain hardening. Few models according to the original design of Krätzig & Partner have been built. The shell walls have a one layer reinforcement grid located at the mid-surface of the shell walls with minimum reinforcement ratio both in the meridional and the circumferential directions. The difference between these models is the way of modeling the stiffening rings: one is modeled by 3D beam elements with 1% reinforcement ratio modeled in the longitudinal direction, and the other is modeled by curved shell elements with 1% reinforcement ratio modeled both in the longitudinal direction and the transverse direction. Both models experienced a large ovalization failure mechanism at the upper cone at the serviceability limit state. This is caused by 1) the low reinforcement ratio which provides a low stiffness to the stiffening rings; 2) The eccentricity of the stiffening rings which generates a large bending moment, thus weakening the ring-wall connections. The model with beam elements showed weaker structural response than the model with curved shell elements because the beam elements do not take the transverse stiffness into consideration. The model using curved shell elements is adopted for further nonlinear analysis. Models with increasing reinforcement ratio in two directions up to 3% on the stiffening rings are built to improve the structural behavior. The stiffness of the Solar Updraft Tower has improved, which gives less ovalization deformation. However, all models experienced the ovalization failure mechanism at a certain loading stage. Unfortunately, the concrete crushing and reinforcement yielding still occur, even with the maximum reinforcement ratio on the stiffening rings. It is because the large bending crack at the ring-wall connection caused by the eccentricity of the stiffening rings decreases the contribution of the stiffening ring to the overall structural behavior. Several models are built using the same reinforcement ratio in two directions on the stiffening rings with increasing tension stiffening effect. The nonlinear result shows that the influence of tension stiffening rings is high at a low loading level and is decreasing at a higher loading level. One possibility of removing the bending failure at the ring-wall connection is to move the mass center of the cross-section of the stiffening rings to the mid-surface of the shell walls. Two re-centered models with increasing reinforcement ratio are built. With 1% reinforcement ratio, the model with re-centered stiffening ring gives a similar structural response as the original design model with a reinforcement ratio of 2%. The re-centered model with 2% reinforcement ratio gives an almost linear behavior of the structure without ovalization failure mechanism at the upper cone. No cracking is found both at the ring-wall connection and the concrete shell wall, no concrete crushing occurs and also no reinforcement is yielding. A maximum deflection around 1m meets the requirement of the maximum allowable displacement in the serviceability limit state. However, a large crack width of 1.1mm is found in the stiffening rings which should be further improved by modeling the structure using two layer reinforcement or by prestressing. Last but not least, the effect of moving the stiffening rings inside the airflow on the power output should be further investigated. But re-centering geometry gives a triple time of improvement on the structural behavior, and eventually saved a high amount of material use.Civil Engineering and GeosciencesStructural Engineerin
Out of Sight, Out of Mind: Detecting Orphaned Web Pages at Internet-Scale
No full textSecurity misconfigurations and neglected updates commonly lead to systems being vulnerable. Especially in the context of websites, we often find pages that were forgotten, that is, they were left online after they served their purpose and never updated thereafter. In this paper, we introduce new methodology to detect such forgotten or orphaned web pages. We combine historic data from the Internet Archive with active measurements to identify pages no longer reachable via a path from the index page, yet stay accessible through their specific URL. We show the efficacy of our approach and the real-world relevance of orphaned web-pages by applying it to a sample of 100,000 domains from the Tranco Top 1M. Leveraging our methodology, we find 1,953 pages on 907 unique domains that are orphaned, some of which are 20 years old. Analyzing their security posture, we find that these pages are significantly ((p < 0.01) using (χ2)) more likely to be vulnerable to cross-site scripting (XSS) and SQL injection (SQLi) vulnerabilities than maintained pages. In fact, orphaned pages are almost ten times as likely to suffer from XSS (19.3%) than maintained pages from a random Internet crawl (2.0%), and maintained pages of websites with some orphans are almost three times as vulnerable (5.9%). Concerning SQLi, maintained pages on websites with some orphans are almost as vulnerable (9.5%) as orphans (10.8%), and both are significantly more likely to be vulnerable than other maintained pages (2.7%). Overall, we see a clear hierarchy: Orphaned pages are the most vulnerable, followed by maintained pages on websites with orphans, with fully maintained sites being least vulnerable. We share an open source implementation of our methodology to enable the reproduction and application of our results in practice.Information and Communication Technolog
Low Citrate Synthase Activity Is Associated with Glucose Intolerance and Lipotoxicity
No full textThis work was supported by Saudi Ministry of Higher Education Grant (to Y.A.), NHS Grampian Endowment Grant (no. 12/21) (to A.R. and S.R.G.), Kuwait Ministry of Health grant (to M.A.T.), and European Social Fund under the Global Grant Measure (VP1-3.1-ŠMM-07-K-02-057) (to A.L. and A.R.). The authors thank Shona Fleming of the School of Biological Sciences, University of Aberdeen, for the technical assistance during their study. The data used to support the findings of this study are available from the corresponding author upon request. A corrigendum for this article has been published. Volume 2019 | Article ID 9153809 | https://doi.org/10.1155/2019/9153809Peer reviewe
Cash for the Register? Capturing Rationales of Early COVID-19 Domain Registrations at Internet-scale
No full textThe COVID-19 pandemic introduced novel incentives for adversaries to exploit the state of turmoil. As we have witnessed with the increase in for instance phishing attacks and domain name registrations piggybacking the COVID-19 brand name. In this paper, we perform an analysis at Internet-scale of COVID-19 domain name registrations during the early stages of the virus’ spread, and investigate the rationales behind them. We leverage the DomainTools COVID-19 Threat List and additional measurements to analyze over 150,000 domains registered between January 1st 2020 and May 1st 2020. We identify two key rationales for covid-related domain registrations. Online marketing, by either redirecting traffic or hosting a commercial service on the domain, and domain parking, by registering domains containing popular COVID-19 keywords, presumably anticipating a profit when reselling the domain later on. We also highlight three public policy take-aways that can counteract this domain registration behavior.Electrical Engineering, Mathematics and Computer ScienceOrganisation & Governanc
Modelling of adsorbate-size dependent explicit isotherms using a segregated approach to account for surface heterogeneities
No full textIdeal Adsorbed Solution Theory (IAST) is a common method for modelling mixture adsorption isotherms based on pure component isotherms. When the adsorbent has distinct adsorption sites, the segregated version of IAST (SIAST) provides improved adsorbed loadings compared to IAST. We have adopted the concept of SIAST and applied it to an explicit isotherm model which takes into account the different sizes of the adsorbates: the so called Segregated Explicit Isotherm (SEI). The purpose of SEI is to have an explicit adsorption model that can consider both size-effects of the co-adsorbed molecules and surface heterogeneities. In sharp contrast to IAST and SIAST, no iterative scheme is required in case of SEI, which leads to much faster simulations. A comparative study has been performed to analyse the adsorption isotherms calculated using these three methods. The adsorbed loadings predicted by SEI and SIAST are in excellent agreement with the Grand-Canonical Monte Carlo (GCMC) simulation data. The loadings estimated by IAST show considerable deviations from the GCMC data at high pressures. Breakthrough curve modelling is used to compare the effects of these three models at dynamic conditions. The explicit model (SEI) leads to the fastest simulation run time, followed by SIAST.</p
Income distribution and the effect of the financial crisis on the Italian and Spanish labour markets
No full textThis paper aims at estimating the costs of the current crisis in terms of income distribution and poverty taking into account by means of microsimulation techniques - the change in employment status in Spain and Italy. We construct a micro simulation analysis on the impact of the crisis on unemployment, household income, and inequality using the European Statistics on Income and Living Conditions Surveys, and Labour Force Surveys data for Italy and Spain with reference to different types of households. We consider the effect of joblessness on household income and well-being and the impact of different systems of unemployment benefit on unemployment sustainability. Our focus is not only on the pecuniary dimension of well-being, but also in terms of the costs of limited access to medical and dental treatment and analyses
The Italian Labour Market and the Crisis
No full textThe aim of this paper is to analyse the effects of the crisis on the Italian labour market. The Italian labour market is characterized by deep gender differences and regional variability. The data show that the crisis lead to an increase in the gap of female employment rates and women?s inactivity rates with respect to Europe. The North of Italy experienced a higher increase in unemployment than the South, where many people withdrew from the labour market because of poor employment prospects. Moreover, in Italy, the increase in unemployment has been mitigated by the increase in the number of workers having access to the wage supplementation fund who are not computed within the unemployed. However, the heterogeneity in the system of unemployment benefits increased inequalities amongst the unemployed. Using a micro simulation techniques, we estimate the effect of the crisis on income distribution and poverty and find that at the national level, the population showed a reduction in equivalised household income by about 1 percent. The limited impact on household?s equivalent income can be connected to the relatively high share of unemployed who are young with relatively low income and sustained by other members of the householdlabour market, poverty, economic crisis
