| 318051. |
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| 318052. |
- Reidy Liermann, Catherine, 1972-
(författare)
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Ecohydrologic impacts of dams : A global assessment
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This dissertation aims to improve our understanding of how dams and reservoirs impact freshwater systems worldwide. The following questions were addressed specifically: 1) what are the spatial patterns and magnitudes of flow regulation and channel fragmentation by dams globally; 2) how are dam impacts distributed biogeographically, and which are the biogeographic regions and taxa most threatened by dam impacts; and 3) how can climate change and dams be expected to interact in basins, and what management actions would mitigate adverse interactions? Results show that the majority of the world’s large river systems are fragmented and have their flow altered by dams. Exceptions to this tend to lie in regions inhospitable to hydropower development, such as northern tundra, or in the least economically active regions. The biogeographic distribution of dam impact is widespread, both at terrestrial and freshwater scales, representing significant threat to global biodiversity. Relatively species-poor tundra is the world’s only terrestrial ecoregion which remains predominantly unaffected by dams. Nearly half of the world’s freshwater ecoregions are internally fragmented by dams, and ecoregional distinctions may be artifically imposed by dams in many cases. Freshwater ecoregions with the highest counts of total and endemic species remain relatively unobstructed, representing significant conservation potential. Diadromy is one of the few fish traits indicative of vulnerability to dams for which data are sufficient for global scale analysis. Lampreys (Lampetra spp.), Eels (Anguilla spp.) and Shad (Alosa spp.) are examples of genera particularly vulnerable to dams because their distributions coincide with the most heavily fragmented freshwater ecoregions, and a large proportion of the coincident species for each genera are diadromous. Due to changes in discharge and water stress, the area of large river basins in need of management interventions to protect ecosystems or people will be much greater for basins impacted by dams than for basins with free-flowing rivers. Proactive measures that restore the natural capacity of rivers to buffer climate-change impacts are more desirable than reactive actions since they may also lead to environmental benefits such as higher water quality and restored fish populations – benefits which may later be unattainable.
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| 318053. |
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| 318054. |
- Reierstam, Helena, 1972-
(författare)
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Assessment in Multilingual Schools : A comparative mixed method study of teachers’ assessment beliefs and practices among language learners - CLIL and migrant students
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents the results from two research projects on teachers’ assessment beliefs and practices in multilingual education. Study I involved teachers of biology, history or English as a foreign language (EFL) in Swedish upper secondary Content and Language Integrated Learning (CLIL) schools, grades 10-12, where English is used as the medium of instruction. In Study II teachers from schools with newly-arrived migrant students (NAS) grades 7 to 12 in the natural and social sciences participated. In both contexts, students are learning the language of instruction at the same time as they are expected to develop subject area knowledge why issues in relation to the role of language in assessment come to the fore. The aim is to contribute to the knowledge of an underexplored research area on subject matter assessment in multilingual schools and draw attention to the consequences varying language policies and pedagogies may have on fairness in access opportunities and validity in assessment outcomes.In this thesis teachers’ language beliefs and practices as expressed in interviews, questionnaires and assessment samples were compared and analyzed in relation to the cognitive and linguistic requirements of language functions in syllabi and the assessment tasks. Whereas Study I was mainly qualitative in nature, involving 12 teachers, a mixed method approach was adopted in Study II where 196 teachers participated in a survey and 13 in follow-up interviews. The responses in the survey were analyzed using descriptive and inferential statistics. Thematic content analysis was used for the interview data, the assessment samples and open-ended survey questions.The findings from the two studies indicate that although teachers state that language is not part of the assessment, they maintain that students need to use language to show proof of critical thinking and more advanced analytical skills. All teachers regardless of discipline shared the weight attached to covering course content as expressed in syllabi which points at a shared accountability culture. One of the main concerns expressed by the teachers is how to construct assignments where language does not represent a hindrance to show content knowledge. Teachers in both studies claimed to focus mainly on subject concepts, not general academic language, and the assessment beliefs and practices seemed to be closely related to the character of the subject. The non-parametric tests of association revealed that teachers with a dual language and subject content certification displayed significantly higher results in relation to all activities involving a visible language pedagogy, e.g. looking at useful sentence structures and providing model texts. Oral follow-up was used by all teachers to remedy poor written results. Although the use of the students’ strongest language is advocated in guidelines for the instruction of NAS, most teachers in Study II referred to a monolingual Swedish language norm. In a society where the educational discourse has become characterized by diversity, inequality and segregation, these two studies underline the need for a shared language policy and pedagogy across subjects and school contexts. They also suggest that an organization and teacher profession with an explicit responsibility for academic language is needed to provide equal access to subject content and validity and comparability in assessment in multilingual schools.
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| 318055. |
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| 318056. |
- Reifarth, Simon, 1980-, et al.
(författare)
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Analysis of EGR/Air Mixing by 1-D Simulation, 3-D Simulation and Experiments
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The use of EGR for NOX reduction is today a standard technology for diesel engines. The mixing of air and EGR is an important issue, especially for high-pressure EGR systems. Anuneven distribution of EGR between the cylinders can lead tohigher overall engine emissions when some cylinders producemore soot, others more NOX than they would with a perfectlyeven distribution.It is therefore important to understand the processes thatcontrol the mixing between air and EGR. The mixing isinfluenced by both the geometry of the mixing area and thepulsating nature of the flow.The aim of this work is to point out the high importance of thepulses present in the EGR-flow. By simulation in 1-D and 3-Das well as by a fast measurement method, it is shown that theEGR is transported in the air flow in packets. This implies thatthe timing between intake valve opening and the positioning ofthe EGR packets has a high influence of the distribution ofEGR between the cylinders.The ability of 1-D and 3-D simulation to predict the behavior isevaluated. It is shown how standard 1-D simulations fail topredict the pulsation effects. Furthermore, it is shown how 1-Dmodels can be modified to give results reasonably close to the3-D simulation results.
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| 318057. |
- Reifarth, Simon, 1980-
(författare)
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Efficiency and Mixing Analysis of EGR-Systems for Diesel Engines
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The reduction of fuel consumption and the reduction of toxic emissions are the main goals of research and development in the area of internal combustion engines. The use of exhaust gas recirculation (EGR) to come further in that direction is today an established method for diesel engines. EGR reduces the emissions of nitrogen oxides with a low penalty in fuel consumption.The increasingly hard regulations on emissions put high pressure on the manufacturers to improve these systems. The present work aims at increasing the knowledge in the area of EGR. Two of the main challenges when applying EGR are addressed, efficiency and mixing.The efficiency of the EGR-system is analyzed, focusing on keeping the fuel penalty low for a given EGR-rate. Different layouts of the EGR system are studied and compared regarding their stationary and transient properties. Exergy analysis is used to show the potential for improvement in different system components. In the same time, exergy analysis as a tool is introduced and compared to energy analysis of a system. The usefulness of exergy analysis of the entire gas exchange is shown by the example of a heavy-duty diesel engine.The problem of EGR and air mixing is approached by a detailed study of the mixing process in a heavy-duty diesel engine. Different methods for the measurement of EGR distribution are presented and compared. Additionally, the possibility to predict the mixing effects by 1-D and 3-D simulation is assessed. It is shown that the mixing between air and EGR is highly dependent on the pulsating nature of the flow. The EGR is shown to be transported in packets in the air flow. This leads to the conclusion that mixing not only at the mixing point, but also mixing in flow direction needs to be optimized, as the distribution of EGR between the cylinders is dependent on the timing between the passage of the EGR packets and the valve opening time.
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| 318058. |
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| 318059. |
- Reifarth, Simon, 1980-, et al.
(författare)
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Exergy and Energy Analysis of HP and LP EGR-system
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The emission legislation throughout the world is getting moreand more stringent. Especially in the US and Europe, dieselengine manufacturers are facing big challenges in order tokeep their engine emissions within the limits. In addition totraditional legislation of harmful emissions, the emission ofCO2, i.e. the fuel consumption, is starting to be subject of newlegislation. Many measures that control the harmful emissionscounteract the reduction of fuel consumption. After treatmentsystems for example increase backpressure, thus lowering theengine efficiency.Exhaust gas recirculation (EGR) is an important tool foremission control in modern diesel engines. The EGR-loopinduces some pumping losses thus decreasing the overallengine efficiency. Many studies have been published that aimat minimizing these losses. The use of a low-pressure EGRloop is one of the most common ways to alternate the system.This study compares a low-pressure (LP) and a high-pressureHP) EGR-system with focus on single components.To analyze the gas exchange system, both energy and exergybalance methods can be used. In this study, both methods arecompared and specific advantages and disadvantages areshown. It is shown that the exergy analysis contains usefulinformation for engine development regarding the efficiencyof single components and their influence on the entire systems performance.
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| 318060. |
- Reifarth, Simon, 1980-, et al.
(författare)
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Measuring and simulating EGR-distribution on a HD-diesel engine
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Ingår i: SAE technical paper series. - 0148-7191.
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The distribution of EGR between the cylinders of an internalcombustion engine has been shown to have large impact onthe engine emissions. Especially at high EGR, the combustionreacts sensibly to variations in the EGR-rate. A cylinder thatreceives excessive EGR produces soot particles while acylinder with too little EGR has increased NOX-emission. It istherefore important to have knowledge about the mixing in anengine.This study compares two different EGR-mixing measurementmethods. The first is based on CO2 measurement withstandard probes, placed at 36 different locations in the engine.The second method uses a laser beam and a detector to gaininformation about the mixing with a high time-resolution.Additionally, 1-D simulations are used to gain informationabout the mixing process.To vary the mixing process on the engine, two differentair/EGR mixers are used and their mixing performance isevaluated.
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