| 51. |
- Saemi, Simindokht
(author)
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Development of the Pressure-Time Method for Flow Rate Measurement in Hydropower Plants by Numerical Simulation
- 2019
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Doctoral thesis (other academic/artistic)abstract
- Hydropower is a clean and sustainable energy resource developed since the late 19th century. To specify the hydraulic performance characteristics of hydraulic turbines, the volumetric flow rate as one of the few basic quantities should be determined. Discharge represents the most difficult quantity to measure. A good measurement accuracy and estimation is difficult to estimate compared to the power and head, especially in low head machines. Despite the developments in discharge measurement techniques, this part of the hydraulic machine performance tests is often a major challenge. The pressure-time method is one of the discharge measurement techniques, which is studied in this PhD thesis. Most of the researches, to improve the accuracy of this method, are performed experimentally, whilst limited one-dimensional numerical simulations are done on this method. Therefore, detailed investigation of this method has been difficult. The studies conducted in this thesis are divided in two experimental and numerical parts. Because the flow physics in the pressure-time method is a combination of decelerating flow with variable rate and water hammer phenomenon, at the first experimental studies are performed considering unsteady constant rate decelerating and accelerating flows. The results helped to better understanding the studies in the second part concerned with numerical simulations. In the second part, the physical phenomenon behind the water hammer and constant rate decelerating and accelerating flows is studied. Then the physical characteristics of the flow in the pressure-time method is investigated in detail based on the time variation of the wall shear stress and the γ parameter. The γ parameter represents the difference between the turbulence structure in a transient accelerating or decelerating flow and the one in the quasi-steady condition. It is demonstrated that for the pressure-time method, part of the flow decrease excursion can be characterized as quasi-steady and the rest is unsteady. The dominance of inertia and turbulence dynamics is investigated to evaluate the wall shear stress in the part of the excursion with the unsteady assumption. It is found that the inertia has a dominant effect during the excursion. The evaluation of the effective forces in the flow rate calculation in a straight pipe showed that the wall shear stress is a good approximation of the losses calculation and the other terms can be neglected. To extend the applicability of this method outside the limitations of the IEC41 standard, variable pipe cross-section and different friction loss calculation are also studied. A new method for the loss calculation in the penstocks with variable cross section is proposed. The errors induced by the proposed method are in an acceptable range provided that the contraction angle is less than ϴ=10°. The evaluation of the important forces showed that the variation of the momentum flux is the most significant term in the flow rate estimation in a pipe with a contraction. Then, the wall shear stress is the second most significant.
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| 52. |
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| 53. |
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| 54. |
- Sayahi, Faez
(author)
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Plastic Shrinkage Cracking In Concrete : Mitigation and Modelling
- 2019
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Doctoral thesis (other academic/artistic)abstract
- Early-age (up to 24 hours after casting) cracking may become problematic in any concrete structure. It can have a negative influence on the aesthetics of the concrete structure, as well as decreasing the durability and serviceability by facilitating the ingress of harmful materials into the concrete bulk. Moreover, these cracks may expand gradually during the member’s service-life due to long-term shrinkage and/or loading. Early-age cracking is caused by two driving forces: 1) plastic shrinkage cracking which is a physical phenomenon and occurs due to rapid and excessive loss of moisture, mainly in form of evaporation, 2) chemical reactions between cement and water which causes autogenous shrinkage. In this PhD project only the former is investigated.Rapid evaporation from the surface of fresh concrete causes negative pressure, known as capillary pressure, in the pore system. This pressure pulls the solid particles together and decreases the inter-particle distances, causing the whole concrete element to shrink. If this contraction is hindered in any way, the induced tensile stresses may exceed the low tensile strength of the concrete and cracking starts. The phenomenon, occurring shortly after casting while the concrete is still in the plastic stage, is mainly observed in elements with high surface to volume ratio such as slabs and pavements.Many parameters may affect the probability of plastic shrinkage cracking. Among others, effect of water/cement ratio (w/c), fines, admixtures, geometry of the element, ambient conditions (i.e. temperature, relative humidity, wind velocity and solar radiation), etc. has been investigated previously. In the presented research, in addition to studying the influence of various parameters, i.e. w/c, cement type, coarse aggregate content, superplasticizer dosage, admixtures, and steel fibres, effort is made to reach a better and more comprehensive understanding about the cracking governing mechanism. Evaporation, capillary pressure evolution and hydration rate are particularly investigated in order to identify their relationship.This project started with extensive literature study which is summarized in Paper I. Then, the main objective was set upon which series of experiments were defined. The utilized methods, material, investigated parameters, and results are presented in Papers II-IV. A model was, then, proposed in Paper V, to estimate the cracking severity of the plastic concrete.It has been observed that evaporation is the driving force behind the plastic shrinkage crackingin concrete. However, a correlation between evaporation, rate of capillary pressure development and the duration of dormant period governs the severity of the phenomenon. Among other things, the results show that rapid capillary pressure development in the pore network accompanied by slower hydration significantly increases the cracking risk.
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| 55. |
- Schillings, Audrey
(author)
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How does O+ outflow vary with solar wind conditions?
- 2019
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Doctoral thesis (other academic/artistic)abstract
- The entire solar system including Earth is enveloped in a region of space where the Sun’s magnetic field dominates, this region is called the heliosphere. Due to this position in the heliosphere, a strong coupling exists between the Sun and our planet. The Sun continuously ejects particles, the solar wind, which is composed mainly of protons, electrons as well as some helium and heavier elements. These high energetic particles then hit the Earth and are partly deflected by the Earth’s magnetosphere (the region around Earth governed by the geomagnetic field). Depending on the strength of the solar wind hitting our planet, the magnetosphere is disturbed and perturbations can be seen down to the lower atmosphere.The upper atmosphere is affected by short wave-length solar radiation that ionise the neutral atoms, this region is referred to as the ionosphere. In the ionosphere, some of the heavier ion populations, such as O+, are heated and accelerated through several processes and flow upward. In the polar regions (polar cap, cusp and plasma mantle) these mechanisms are particularly efficient and when the ions have enough energy to escape the Earth’s gravity, they move outward along open magnetic field lines. These outflowing ions may be lost into interplanetary space.Another aspect that influences O+ ions are disturbed magnetospheric conditions. They correlate with solar active periods, such as coronal holes or the development of solar active regions. From these regions, strong ejections emerge, called coronal mass ejections (CMEs). When these CMEs interact with Earth, they produce a compression of the magnetosphere as well as reconnection between the terrestrial magnetic field lines and the interplanetary magnetic field (IMF) lines, which very often leads to geomagnetic storms. The energy in the solar wind as well as the coupling to the magnetosphere increase during geomagnetic storms and therefore the energy input to the ionosphere. This in turn increases the O+ outflow. In addition, solar wind parameter variations such as the dynamic pressure or the IMF also influence the outflowing ions.Our observations are made with the Cluster mission, a constellation of 4 satellites flying around Earth in the key magnetospheric regions where we usually observe ion outflow. In this thesis, we estimated O+ outflow for different solar wind parameters (IMF, solar wind dynamic pressure) and extreme ultraviolet radiations (EUV) as well as for extreme geomagnetic storms. We found that O+ outflow increases exponentially with enhanced geomagnetic activity (Kp index) and about 2 orders of magnitude during extreme geomagnetic storms compared to quiet conditions. Furthermore, our investigations on solar wind parameters showed that O+ outflow increases for high dynamic pressure and southward IMF, as well as with EUV radiations. Finally, the fate of O+ ions from the plasma mantle were studied based on Cluster observations and simulations. These results confirm that ions observed in the plasma mantle have sufficient energy to be lost in the solar wind.
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| 56. |
- Schwanz, Daphne
(author)
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On Transfer Functions for Power Quality Studies in Wind Power and Solar PV Plants
- 2019
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Doctoral thesis (other academic/artistic)abstract
- As part of the decarbonisation of the energy system, wind and solar power are expected to play an important role. However, together with their growth, new challenges appear in the electric power system; this requires further research, development, and often studies before connection is possible. Two of those challenges, both impacts on the quality of power, are the subject of this dissertation: harmonic distortion and voltage unbalance.The growing use of solar power for electricity generation, especially in distribution systems, will result in increased voltage unbalance due to single-phase photovoltaic inverters (PVIs). Regulation on power quality and potential impact on other equipment, place a limit to the number of PVIs that can be connected to the system, the so-called “hosting capacity”. To include different uncertainties in the planning stage, a stochastic method based on the transfer-impedance matrix is proposed for quantifying this hosting capacity, with respect to voltage unbalance. The method has been illustrated by applying it to two typical Swedish low-voltage networks.Wind power plants (WPPs) consist of a collection grid and a number of wind turbines. These are known to be a harmonic source as power electronic devices are used to connect them to the power system. Earlier studies have shown that the actual emission at harmonic frequencies is low, but that the main issue is related to the spread of harmonics through the collection grid, especially the role of resonances. Regulation setting emission limits and the potential adverse impact of harmonics on equipment make that studies are needed to predict harmonic voltages and currents in and around a WPP. These studies are based on measurements performed on individual turbines under certain operating conditions. The main issue related to this determination is distinguishing the emission originating within the WPP (primary emission) from the emission originating elsewhere (secondary emission). A critical review has been performed on methods used for harmonic emission determination (i.e. distinguishing between primary and secondary emission) in WPPs. It was concluded that this determination cannot be solved without making assumptions. Transfers functions are independent of the emission from the individual turbines and can be obtained with less assumptions. These transfer functions have been used to estimate the spread of harmonics through a WPP and towards the public grid. Transfer functions were shown to be a suitable tool to quantify amplifications due to resonances and identify which harmonic orders can be an issue.Furthermore, information on the different transfer functions allows the selection of proper mitigation methods. This application of transfer functions has been illustrated for a specific advanced mitigation method: the use of inverter control techniques to emulate a “virtual resistor”. In this way it is possible to damp resonances without increasing fundamental-frequency losses.The ultimate aim of harmonic studies is to avoid interference between the grid and equipment connected to it, in this case between the power-electronics in the wind turbine and other equipment. However, these studies rarely address actual cases of interference, instead of this, measured or calculated harmonic voltages and/or currents are compared with limits set in regulations. These and regulations differ strongly between countries and even between individual network operators. A comparative study of regulatory methods has been performed presenting their advantages and disadvantages from the viewpoint of the network operator and from the viewpoint of the owner or operator of the WPP.
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| 57. |
- Skawina, Bartlomiej, 1985-
(author)
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Load-Haul-Dump operations in underground mines
- 2019
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Doctoral thesis (other academic/artistic)abstract
- The research presented in this thesis addresses several aspects of loading operations inunderground mines, in particular tools and equipment selection. It also addresses the flexibilityof the fleet when subject to substantial disturbances, such as ore pass loss, and proposes integration of the scheduling system with discrete event simulation. The thesis begins with a study of discrete event simulation (DES) tools for loading operations in an underground mining system. The results show the benefits of using simulation but also the drawbacks.The thesis presents an analysis of energy consumption and exhaust gas emissions from diesel and electric LHDs. The results show the potential energy savings with the use of electric LHDs. Next, it focuses on the LHD operations affected by long-term ore pass loss (unavailability). It shows the effects on the production system (the ventilation requirements, production and waiting times when too many LHDs operate in the area affected by an ore pass loss) and highlights the need for a flexible solution and a mitigation strategy. Finally, the thesis studies the integration of ABB’s Ability Operations Management System (OMS) with the SimMine simulation model and how this affects LHD operations. The results show the benefits of using the joined platformas a testbed and decision support system.
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| 58. |
- Soleimanmeigouni, Iman, 1988-
(author)
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Predictive Models for Railway Track Geometry Degradation
- 2019
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Doctoral thesis (other academic/artistic)abstract
- Railways are a vital and effective means of mass transportation and play a vital role in modern transportation and social development. The benefits of the railway compared to other transportation modes are a high capacity, high efficiency and low pollution, and owing to these advantages, railways are nowadays experiencing a higher demand for the transportation of passengers and goods. This is in turn imposing higher demands on the railway capacity and service quality. As a result, infrastructure managers are being driven to develop new strategies and plans to fulfil new requirements, which include a higher level of resilience against failure, a more robust and available infrastructure, and cost reduction. This can be achieved by making efficient and effective maintenance decisions by applying RAMS (reliability, availability, maintainability, and safety) analysis and LCC (life cycle cost) assessment.A major part of the railway maintenance burden is related to track geometry maintenance. Due to the forces induced on the track by traffic, the railway degrades over time, causing deviations from the designed vertical and horizontal alignment. When the track geometry degrades to an unacceptable level, this can cause catastrophic consequences, such as derailment. Maintenance actions are used to control the degradation of the track and restore the geometry condition of the track sections to an acceptable state.With the current advancements in the field of technologies for railway track geometry measurement, a large amount of event data and condition monitoring data is available. Such technologies, along with advances in predictive analytics, are providing the possibility of predicting the track geometry condition in support of a predictive maintenance strategy. The aim of the research conducted for this thesis has been to develop methodologies and tools for the prediction of railway track geometry degradation, in order to facilitate and enhance the capability of making effective decisions for inspection and maintenance planning. To achieve the purpose of this research, literature studies, case studies and simulations have been conducted.Firstly, a literature review was performed to identify the existing knowledge gaps and challenges for track geometry degradation modelling and maintenance planning. Secondly, a case study was conducted to analyse the effect of tamping on the track geometry condition. By considering the track geometry condition before tamping as the predictor, a probabilistic approach was utilised to model the recovery after tamping interventions. Thirdly, a two-level piecewise linear framework was developed to model the track geometry evolution over a spatial and temporal space. This model was implemented in a comprehensive case study. Fourthly, a data-driven analytical model was developed to predict the occurrence of track geometry defects. This model enables infrastructure managers to predict the occurrence of severe isolated geometry defects. Finally, an integrated model was created to investigate the effect of different inspection intervals on the track geometry condition.
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| 59. |
- Strandkvist, Viktor
(author)
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Hand grip strength and postural control among individuals with and without Chronic Obstructive Pulmonary Disease : Epidemiological and motion laboratory studies
- 2019
-
Doctoral thesis (other academic/artistic)abstract
- BackgroundChronic obstructive pulmonary disease (COPD) is estimated to affect approximately 8%–10% of all adults. The disease is considered to be a heterogenic syndrome with systemic effects, in which comorbidities including cardiovascular diseases, muscle dysfunction and fatigue are common. The majority of all individuals with COPD have mild to moderate disease (Global Initiative for Chronic Obstructive Lung Disease (GOLD) 1-2), and only a third are identified by health care, and this under-diagnosis contributes to a lack of knowledge of COPD in the population. COPD is related to skeletal muscle dysfunction, where muscle weakness, frequently assessed with hand grip strength (HGS), is one of the components. Evidence of postural control limitations among individuals with severe or very severe COPD has been emerging during the last decade. These limitations are partly related to muscle weakness, however, research of the other underlying systems of postural control is scarce, and an increased knowledge is of importance for the prevention of falls.AimThe main objective of the thesis was to increase the population-based knowledge about muscle strength, assessed as hand grip strength, and also postural control among individuals with and without COPD. Another objective was to investigate the value of hand grip strength measurements in relation to lower limb muscle strength and postural control among community-dwelling older adults.MethodsPaper I and II are based on the population-based Obstructive Lung disease in Northern Sweden (OLIN) COPD study. All individuals with obstructive lung function impairment (n=993) and an age- and sex-matched group without (n=993) (COPD and non-COPD) were identified after re-examination of population based cohorts during 2002-04. They (n=1,986) have been invited to recurrent examinations including structured interviews and spirometry. Paper I (n=1011) and II (n=831) include data from examinations during 2009-2010 and 2014, respectively, where measurements of HGS (Paper I & II) as well as assessment of fatigue and physical activity (Paper II) were included. Paper III and IV are based on the population-based Balancing Human and Robot (BAHRT) study. Randomly selected individuals over 70 years of age were invited to participate in a study of postural control aspects, in their homes and at the 2 Human Health and Performance Lab- Movement Science, at Luleå University of Technology. The examinations included a structured interview as well as measurements of postural control and sensory and motor systems including HGS, lower limb strength, proprioception, pressure sensitivity, visual acuity, vestibular function and reaction time. Participants in Paper III consisted of communitydwelling individuals over 70 years of age (n=45). Paper IV consisted of individuals with COPD recruited from both the OLIN COPD study as well as the BAHRT-study (n=22), as well as a control group without airway obstruction from the BAHRT-study (n=34).ResultsIn Paper I, there was no differences in mean HGS between the groups with and without COPD. However, HGS was related to disease severity; HGS was lower among those with severe to very severe COPD (GOLD 3-4) compared to nonCOPD, and HGS was associated with forced expiratory volume during 1 second % of predicted value (FEV1 % predicted) in regression models. Five years later, in Paper II, mean HGS was lower in COPD compared to non-COPD. Further, individuals with COPD and clinically relevant fatigue had lower HGS than those without clinically relevant fatigue, significantly so among men and close to among women. There was no clear association between HGS and level of physical activity. In Paper III, there was a strong and valid model showing association between HGS and lower limb strength, where all tested muscle groups were significantly correlated to hand grip strength. The regression model with HGS and postural control was significant, however, the model was weak. In Paper IV, individuals with COPD had worse postural control than those without COPD, significantly so regarding mediolateral amplitude in quiet stance with eyes open on soft/unstable surface, as well as anteroposterior limits of stability. In the COPD group, mediolateral amplitude was related to eye sight and the burden of tobacco smoking assessed as pack-years. Further, muscle strength associated with anteroposterior amplitude in the limits of stability test.ConclusionsIn the population-based OLIN COPD study in 2009-2010, mean HGS was significantly lower among individuals with severe to very severe COPD (GOLD 3-4) than those without COPD. Five years later, mean HGS was lower among all individuals with COPD (GOLD 1-4) compared to those without COPD, which may indicate altered aging process in COPD. HGS was associated to fatigue among individuals with COPD, while there were no associations between HGS and physical activity. HGS is a valid tool for the estimation of lower limb muscle strength among community-dwelling older adults, however, HGS is not 3 appropriate to use as an estimation of postural control. Postural control was impaired among individuals with COPD and different postural control assessments had different demands on the sensory and motor systems. The burden of tobacco smoking and visual acuity might be important for quiet stance trials with visual input, while muscle strength might be important for the more dynamic limits of stability test. Further research regarding the longitudinal aspects of muscle weakness and postural control among individuals with COPD is needed to gain knowledge for appropriate preventive or rehabilitative interventions.
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| 60. |
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