1 - 22 of 22
rss atomLink to result list
Permanent link
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
  • Public defence: 2019-12-09 10:00 Ka-Sal B (Peter Weissglas), Kista
    Reinhold, Ingo
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Electronics and Embedded systems, Electronic and embedded systems. XaarJet ltd..
    Industrial Digital Fabrication Using Inkjet Technology2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The use of acoustic waves initiated by the deformation of a microchannel is one method for generating monodisperse, micrometer-sized droplets from small orifices and is employed in piezo-electric inkjet printheads. These printheads are used in both graphical printing and digital fabrication, where functionalities, such as optical, biological, electrical or mechanical, are being produced locally. The processes leading to detrimental artifacts such as satellite droplets or nozzle outages, however, are not fully understood and require profound experimentation. This thesis presents both novel techniques to study jetting for optimal droplet formation and reliability, as well as the post-processing techniques required for solution-based production of a conductive feature on low-cost polymeric substrates.

    A multi-exposure imaging system using laser light pulses shorter than 50 ns and a MEMS micro-mirror enabled the imaging of the droplet formation at ten instances on the droplet's  travel towards the substrate. The technique allows for the study of droplet formation, satellite droplet break-up and secondary tail formation allowing for better control and understanding of the process.

    Reliability measurement using a linescan camera was introduced to record every droplet ejected from the width of a printhead. The variations in droplet velocity and misalignment of the printhead required the use of a constant background illumination to reliably capture the droplets. The resulting low-contrast images were post-processed using statistical analysis of the graylevel distributions of both, the droplet and background pixels, and were subsequently used in a histogram matching algorithm to enable reliable identification of the threshold value required for unhindered detection of missing droplets based on the printed image. Using temporal oversampling the technique was shown to qualitatively describe droplet velocity variations introduced by the actuation of the printhead.  

    The conversion of inkjet-printed metallic nanoparticle inks to conductive structures was investigated with a focus on the applicability to industrial processes. Intense pulsed light (IPL) processing achieved comparable results to convective oven sintering in less than ten seconds. The dynamics of IPL sintering were found to be strongly dependent on the spectral composition of the light resonating in the processing chamber. By implementing a passive filtering concept, thermal runaway was prevented and the line conformation was optimized irrespective of the underlying substrate. Alternatively, pulse-shaping, to tailor the energy flux into the deposit and incorporate drying in the IPL process, was found to generate conductive copper features without pre-drying.

    The findings were applied to applications comprising small droplet generation for nanoimprint lithography, the fabrication of conductors for blind via connections to buried LED dies as well as the hybrid generation of hyperbolic ion-trap electrodes for  mass spectrometry applications. The addition of the non-contact and high accuracy of the inkjet process enabled suitable performance that lies beyond that of conventional processes.

  • Public defence: 2019-12-10 10:00 H1, Stockholm
    Zografos, Dimitrios
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Electric Power and Energy Systems.
    Power System Inertia Estimation and Frequency Response Assessment2019Doctoral thesis, monograph (Other academic)
    Abstract [en]

    Power plant emissions constitute a major source of environmental pollution. This renders the gradual replacement of such power plants by renewable energy sources imperative. Changes in the quota between conventional generation and renewable energy sources introduce challenges that the modern power systems have to encounter. For example, conventional power plants are replaced by wind turbines and photovoltaics, which do not contribute to system's inertia. As a result, power system inertia decreases and frequency stability becomes a concern. Frequency stability is affected by the amount of power system inertia, along with the response of controllable frequency reserves and the amount of power imbalance. Therefore, the estimation of power system inertia, as well as the frequency response assessment is necessitated, so that appropriate actions can be taken to ensure frequency stability.

    The first part of this thesis focuses on power system inertia estimation. Four disturbance based inertia estimation methods are proposed. The methods accommodate the frequency and/or voltage variations that arise after a disturbance and estimate both the total inertia constant and the total power imbalance of the system. This is achieved by considering suitable functions that can approximate the voltage and frequency dependency of the loads, as well as the response from the governors. The proposed methods are applied on frequency responses from simulations of a test system under several different scenarios. The performance of the methods under lack of certain data is investigated, in order to examine if they can be employed under realistic conditions. An extensive analysis is performed, which enables the selection of the most appropriate method, depending on the information that is available.

    The second part of the thesis deals with frequency response assessment. First, the use of simplified dynamic equivalent models is examined. The parameters of either governor or frequency response models of the system are identified and validated by employing historic events. Data are obtained from events from the power systems of Sweden and Texas. After the identification, the frequency response model of the system can be simulated to assess frequency stability. Secondly, the thesis examines methods that deal solely with the prediction of frequency nadir. The examined methods either use neural networks or linear regression. The accuracy of the methods, as well as the uncertainty that is introduced by system non-linearities, are assessed through simulations.

    By proposing methods for estimating power system inertia and frequency response, this thesis attempts to provide additional solutions to the challenges that modern power systems have to face. It offers supplementary tools to increase the system awareness, in order to take appropriate actions in case of frequency events.

  • Public defence: 2019-12-11 10:00 F3, Stockholm
    Shipsha, Anton
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures. KTH, Superseded Departments (pre-2005), Vehicle Engineering.
    Compressive Failure of NCF Composites2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The necessity to reduce environmental impact promotes transportation industry to reduce energy consumption of vehicles. One possible way to improve vehicles' structural efficiency is to utilize modern composite materials that offer excellent mechanical performance-to-weight ratio. Mass production of composite parts requires cost effective manufacturing methods. One potential rational method is to use dry textile preforms and liquid moulding methods, e.g vacuum infusion or resin transfer moulding. Among different types of textile preforms, non-crimp fabrics (NCFs) are most attractive for load bearing applications as they offer considerably higher in-plane mechanical properties compare to other textiles such as wovens or random mats. Composites manufactured with NCF fabrics are characterised by distinct fibre bundles separated by resin rich areas. These bundles are not perfectly straight but have a small yet significant waviness, both in-plane and out-of-plane. The waviness will influence the performance of NCF composites and especially the compressive properties. Design of structural parts made of NCF composites requires both a thorough understanding of the compressive failure process and effective failure prediction models. This is particularly relevant for the critical compressive loaded parts, such as bolted joints.

    The present work is concerned with the compressive failure of NCF composites and focuses on two major goals. First is to experimentally characterise the compressive failure process of various NCF composites and identify relevant damage modes and mechanisms. Secondly is to develop and propose suitable failure prediction models for reliable design of NCF composite parts with special emphasis on cost-effective methods relevant for industrial design processes.

    In the present work, a combination of experimental studies, modelling methods development and implementation of advanced state-of-the-art failure criteria have been performed. Optical methods were used to characterise the damage mechanisms in the material at different stress levels. This allowed both identification of the critical damage mechanisms and the whole damage progression sequence. Engineering models were developed to predict the compressive failure of NCF composites. In the models, the fibre bundles' waviness was dealt with in a cost-effective way. The models utilise a state-of-the-art failure criteria that predict both intra-laminar and inter-laminar damage. The proposed models demonstrated good accuracy in the predictions of both compressive and bearing failures. In addition, a cost-effective high-fidelity meso-scale modelling methodology was developed for bearing failure prediction of NCF composites. The methodology demonstrated a potential for cost-effective and highly detailed analysis of the bearing failure process and possible method for parameter studies of mechanical properties and their relation to the reinforcement architecture.

  • Public defence: 2019-12-11 13:30 Kollegiesalen, Stockholm
    Ebrahimpouri, Mahsa
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Electromagnetic Engineering.
    High Frequency Microwave and Antenna Devices based on Transformation Optics and Glide-Symmetric Metasurfaces2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The new generation of wireless communication networks intends to support data rate of Gbit/s. One solution to make it possible is to move upwards in frequency range to employ the unused spectrum in mm-wave frequencies. This brings new challenges in the design of hardware for the communication networks, namely high free space path loss and expensive manufacturing. In this thesis, transformation optics and glide symmetry are employed to address these new challenges for the design of high frequency microwave components and lens antennas.

    Transformation optics provides a systematic tool to manipulate electromagnetic waves in a desired way. In this thesis, this tool has been used to improve the radiation properties of conventional homogeneous three-dimensional lenses and compress the size of two-dimensional graded-index lenses.

    Glide symmetry is a subset of higher-order symmetries and is described by a translation followed by a reflection with respect to a defined plane. Periodic structures possessing glide symmetry exhibit interesting properties. In this thesis, four of these properties are explored and possible applications are discussed.

    First, it is demonstrated that the first mode in a glide-symmetric periodic structures is significantly less dispersive than the corresponding conventional non-glide structure. This property was employed to design fully metallic wideband metasurface-based antennas. The losses in this type of antennas are only ohmic which make them suitable for high frequency applications. Second, it is shown that anisotropic glide-symmetric periodic structures can provide higher levels of anisotropy compared to their conventional periodic counterparts. This property is employed to design compressed two-dimensional lenses. Third, it is demonstrated that glide symmetry can be used to match the impedance of two vastly different dielectric media in a parallel plate waveguide configuration by enhancing the magnetic properties. This property was used to match the profile of two-dimensional homogeneous lenses. Fourth it is shown that glide-symmetric holey metallic structures achieve a significantly wider stop-band compared to conventional non-glide periodic structures. This property is exploited to design cost-effective waveguiding structures and microwave components at mm-wave frequencies. Furthermore, using this property, a flange design that provides contact-less measurement at mm-wave frequencies is presented.

  • Public defence: 2019-12-12 10:00 F3, Stockholm
    Zhou, Tao
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy. KTH Royal Institute of Technology.
    Integrated Experimental and Computational Study of Precipitation in Martensitic Steels2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Precipitation is a phase transformation process in metallic materials that significantlyaffects properties. The precipitation process that includes nucleation, growth andcoarsening of small particles can be tuned by alloying, deformation, thermal treatment.This opens opportunities for optimizing the properties of metallic materials bytailoring precipitation. An example of high-performance metallic materials withcontribution from precipitation is tempered martensitic steels. By means of highlydispersed nanoscale precipitates within the hierarchic martensitic microstructure,these steels achieve an excellent combination of ultra-high strength and hightoughness. With the objective of accelerating the development of these high-performance steels, an integrated computational materials engineering (ICME)approach, combining advanced characterization, physically based/semi-empiricalmodelling, theory and databases, is used in this thesis to develop computationallinkages from heat treatment to precipitation to strength.Two multicomponent steels, a Cu precipitation-hardened maraging stainless steel anda carbide-strengthened low alloy Cr – Mo – V martensitic steel, are studied in this thesisusing quantitative characterization and modelling. The results suggest that theprecipitation simulations using Langer-Schwartz-Kampmann-Wagner (LSKW)modelling have good agreements with the experiments and show promise for futurepredictive modelling to be used for materials design. The semi-empirical models forindividual strengthening mechanisms and an integration of the strengtheningmechanisms used in this work may also represent the trends in the yield strength offresh and tempered martensite, but it is difficult to predict the early yielding of freshmartensite and the correlation of hardness and strength. This indicates the need tofurther develop the models. Overall, this thesis shows that the ICME approach can beused to study and predict precipitation and precipitation-strengthening inmulticomponent steels. The applied approach differs from traditional trial-and-errortesting and has the potential to save time, money and resources in steel development.

  • Public defence: 2019-12-13 10:00 Lecture Hall K1, Stockholm
    Liu, Min
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Surface and Corrosion Science.
    DFT calculations of initial localized corrosion of aluminum: Influence of aqueous ad-layer, chloride ions, and intermetallic particles2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Localized corrosion of aluminum (Al, here including Al alloys) involves a series of physico-chemical processes at the interface between the metal and the aqueous ad-layer or the aqueous solution. The mechanisms that govern localized corrosion are quite complex and have been the subject of many experimental studies. Efforts to improve our understanding through computational studies have so far been much more limited. The primary aim of this Doctoral Thesis was to apply Density Functional Theory (DFT), together with some Molecular Dynamics calculations (limited effort), to gain a deeper mechanistic understanding of some of the most influential factors for the initiation of localized corrosion of Al: chloride ions, intermetallic particles (IMPs) and the presence of an aqueous ad-layer on the solid phase.In the scientific literature three scenarios have been proposed for the interaction of chloride ions with an aluminum and/or passive aluminum surface: through adsorption onto the passive layer, through breakdown of the same layer or through migration of chloride ions into the layer. DFT-calculations have been able to explore these scenarios in more detail, and provide evidence that chloride ions induce partial de-passivation in several ways. On the bare Al surface, chloride ions may inhibit the re-passivation through competitive adsorption with oxygen molecules, as suggested by density of state calculations. Chloride ions are also found to migrate via oxygen vacancies into the inner part of the investigated aluminum oxide films (α- and γ-Al2O3), where a critical amount of accumulated chloride can promote meta-stable pitting propagation. γ-Al2O3 exhibits a more open structure than α-Al2O3, resulting in a lower energy barrier for chloride migration.Micro-galvanic effects induced by Volta potential differences between representative intermetallic particles (Mg2Si and Al2Cu) and the surrounding Al matrix were predicted by calculating the work function of the bare surfaces of these phases with DFT. These values vary with crystalline face orientation and with terminal atomic configuration in the outmost surface layer. Calculated Volta potential differences between IMPs and Al show a reasonable agreement with reported experimental data, and suggest the possibility of predicting the nobility of specific IMPs relative to Al. Moreover, both DFT and scanning Kelvin probe force microscopy show evidence of electrochemical nobility inversion of Mg2Si versus Al upon adsorption of pure water ad-layers. This implies that an originally- 2 -cathodic Mg2Si phase becomes anodic compared to Al upon water adsorption, and is attributed to surface relaxation according to DFT calculations. With subsequent introduction of chloride ions into the water ad-layer, the nobility inversion of both Mg2Si and Al2Cu retains. This is due to a strong oxidizing effect of water on Al, while the effect of chloride seems less pronounced.In all, these and other examples presented show that DFT-calculations can provide more detailed atomistic and molecular information on physico-chemical processes governing localized corrosion of Al than experiments alone can do.

  • Public defence: 2019-12-13 10:15 F3, Stockholm
    Gaborit, Mathieu
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. LAUM UMR CNRS 6613, Le Mans Université, Le Mans, France.
    Modelling strategies for thin imperfect interfaces and layers2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The global trend towards quieter environments has been one of the key topics of acoustics research for years. The recent tightening of the regulations on noise exposure as well as the many reports on the impact of noise on human health confirm this situation and stress ever more the need for innovative mitigation strategies. Numerous efforts from many teams allowed to refine existing solutions and explore new approaches towards a lower noise level ultimately leading to a number of promising concepts. Central to this field, the use of poroelastic media and the development of realistic meta-materials are paving the way to tackle the problem. In the meantime, a great part of the most widely adopted systems to mitigate noise, such as acoustics panels for instance, resort to thin resistive screens placed on the surface to protect the bulk and control the properties. Despite often being one of the thinnest components of the systems, they have a non-negligible impact on the overall response and are subject to a number of uncertainties.The approach chosen in this thesis differs from the global trend of designing new solutions and conversely relies on investigating the effect of uncertainties inherent to all these sound proofing systems. More precisely, the work performed focuses on modelling the impact of uncertain interfaces and uncertain parameters in the thin layers used as protective, tuning or aesthetic elements. These acoustic films, and to a certain extent the thin interface zones resulting from the assembly process, are notably challenging to characterise with precision. The main goal of this thesis is then to propose strategies to account for uncertainties on the parameters of the films and interfaces and predict their impact on the overall response of the systems.Three different scientific contributions are presented in this thesis. Together they discuss modelling aspects related to the films, propose possible simplifications and demonstrate the effect of parameter uncertainties. Finally they introduce numerical strategies to efficiently account for uncertainties in computations within the context of poroelastic and meta-poroelastic media.

    The full text will be freely available from 2019-12-13 14:00
  • Public defence: 2019-12-13 13:00 Kollegiesalen, Stockholm
    Chen, Chao
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology, Wood Chemistry and Pulp Technology.
    Development of Non-leaching Antibacterial Approaches on Cellulose-based Substrates and Their Mechanisms2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The layer-by-layer (LbL) technique is becoming a powerful tool that has been applied in many surface coatings and functionalizations in recent years. It has many advantages including a fast and mild process, the flexibility of choice of substrate, and the easiness to scale-up. Novel antibacterial materials can be achieved using this technique, by immobilizing selected antibacterial agents on surfaces of desired substrates. An ideal antibacterial agent, a cationic polyelectrolyte, can be LbL-deposited onto the surfaces in mono or multi layers, make the surfaces lethal to the bacteria due to their positive charge. This approach is able not only to effectively control the spreading of bacteria but also to minimize bacterial resistance as well as the environmental impact.

    Cellulose fibres modified by different cationic polyelectrolytes including PDADMAC, PAH, PVAm as either monolayer or multilayer assembled with PAA using LbL deposition have shown more than 99.99 % bacterial removal as well as the inhibition of bacterial growth. Among these modifications, two layers of PVAm assembled with one layer of PAA have shown the highest antibacterial efficiency due to the highest adsorbed amount and charge density. Secondly, PAA was replaced by a bio-based cellulose nano-fibril (CNF), as a middle layer between two layers of PVAm, which decreases the carbon-footprint and expands the possibility of using LbL technique in antibacterial applications, since the LbL technique can be used long as the alternate layers are oppositely charged. The fibres modified with this approach have shown similar and even better antibacterial properties than those of PAA.

    To develop the antibacterial approach using LbL on cellulose fibres, it is also essential to understand the antibacterial mechanism. It was found that the charge density and surface structures are two important factors affecting bacterial adhesion and the bactericidal effect. To study this, different charged cellulose model surfaces were made by coating oxidized, regenerated cellulose followed by PVAm/CNF/PVAm LbL deposition, and a better antibacterial effect was observed on the higher charged surface. By calculating the force between the bacteria and charged surface, it was suggested that a higher interaction due to the higher surface charge causes a large stress on the bacterial cell wall which leads to the disruption of the bacteria. To further improve the bactericidal effect, the flat surfaces were patterned with micro and nano structures using a femtosecond laser technique. The weakening of the bacterial cell wall caused by the charged surface makes the bacteria more vulnerable and easier to disrupt. This approach has been shown to be valid on both Gram-positive S. aureus, and Gram-negative E. coli. The effect was greater on E. coli with a weaker membrane structure and higher surface potential, which shows that the antibacterial mechanism is a physical disrupt of the bacterial cell.

    The full text will be freely available from 2019-12-13 23:54
  • Public defence: 2019-12-13 14:00 F3, Stockholm
    Hägg Mameng, Sukanya
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Surface and Corrosion Science.
    Localised corrosion and atmospheric corrosion of stainless steels2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This research is focused on defining limiting conditions for corrosion of stainless steels. The aim of the first part (Papers I-IV) was to understand the role of alloying level and environmental parameters on localised corrosion in aqueous conditions. Testing was done with a combination of short-term electrochemical and long-term immersion experiments. Results show that the chloride ion concentration and temperature are the main factors that affect the localised corrosion resistance. The presence of residual chlorine is also significant, since it leads to the ennoblement of the corrosion potential. If the corrosion potential exceeds the breakdown potential, the stainless steel will suffer from localised corrosion. Oxygen content and pH were identified as additional factors influencing the corrosivity of the environment.

    The aim of second part (Papers V-VIII) was to present information about the effect of alloying level, surface condition and environmental conditions on atmospheric corrosion resistance in Middle-East environments. Field exposure tests were performed and supplemented by laboratory aqueous corrosion tests. Results demonstrate that a higher alloying level (in both the bulk material and the surface), plus a smoother and cleaner surface gave an improvement in the corrosion performance and resistance to aesthetic degradation. Chloride and sulphate are the main surface contaminants found in the Middle East atmosphere. Stainless steels exposed in sheltered conditions showed a better atmospheric corrosion resistance than in open conditions. Three factors are considered to contribute to this difference: a very low rainfall, more condensation leading to corrosion in the open conditions and a higher level of beneficial sulphate in the deposits in sheltered conditions. The results showed a reasonable correlation between laboratory pitting corrosion tests and atmospheric field tests.

    The results from this thesis provide reference data to aid selection of appropriate stainless steel grades. Results can also be used to help understand the limits for use of stainless steels in different conditions in both aqueous environments and in the atmosphere.

  • Public defence: 2019-12-16 10:00 FB42, Stockholm
    Rzeszutek, Elzbieta
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Glycoscience.
    Cell wall biosynthesis in the pathogenic oomycete Saprolegnia parasitica2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The oomycete Saprolegnia parasitica is a fungus-like microorganism responsible for the fish disease saprolegniosis, which leads to important economic losses in aquaculture. Currently, there is no efficient method to control the infection and therefore methods for disease management are urgently needed. One of the promising approaches to tackle the pathogen is the inhibition of cell wall biosynthesis, specifically the enzymes involved in carbohydrate biosynthesis. The cell wall of S. parasitica consists mainly of cellulose, β-1,3 and β-1,6-glucans, whereas chitin is present in minute amounts only. The available genome sequence allowed the identification of six putative chitin (Chs) and cellulose (CesA) synthase genes. The main objective of this work was to characterize CHSs and CesAs from S. parasitica and test the effect of cell wall related inhibitors on pathogen growth. The tested inhibitors included nikkomycin Z, a competitive inhibitor of CHS as well as inhibitors of cellulose biosynthesis, namely flupoxam, CGA325'615 and compound I (CI). All drugs strongly reduced the growth of S. parasitica and inhibited the in vitro formation of chitin or cellulose, demonstrated by the use of a radiometric assay. The chemicals also affected the expression of some of the corresponding Chs and CesA genes.

    One of the CHSs, namely SpCHS5, was successfully expressed in yeast and purified to homogeneity as a full length protein. The recombinant enzyme was biochemically characterized and demonstrated to form chitin crystallites in vitro. Moreover, our data indicate that SpCHS5 most likely occurs as a homodimer which can further assemble into larger multi-subunit complexes. Point mutations of conserved amino acids allowed us to identify the essential residues for activity and processivity of the enzyme.

    In addition to the cell wall related inhibitors, a biosurfactant naturally produced by Pseudomonas species, massetolide A, was tested, showing strong inhibition of S. parasitica growth.

    Altogether, our data provide key information on the fundamental mechanisms of chitin and cellulose biosynthesis in oomycetes and the biochemical properties of the enzymes involved. They also demonstrate that the enzymes involved in cell wall biosynthesis represent promising targets for anti-oomycete drugs, even when the corresponding polysaccharides, such as chitin, occur in small amounts in the cell wall.

  • Public defence: 2019-12-16 10:00 F3, Stockholm
    Sommerfeldt, Nelson
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Solar PV in prosumer energy systems: A techno-economic analysis on sizing, integration, and risk2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In the transition towards a sustainable energy system, building mounted solar photovoltaics (PV) have unique benefits; they require no additional land and the energy is generated directly at load centers. Within residential buildings, multi-family homes (MFH) are particularly interesting because of the economies of scale and their greater potential for emissions reductions.

    This thesis identifies and describes value propositions for solar PV within Swedish multi-family houses via three branches of inquiry; system sizing optimization, quantification of investment risk, and the techno-economic potential of PV/thermal (PVT) collectors integrated with ground source heat pumps (GSHP). Underpinning these investigations is a comprehensive review of technical and economic models for solar PV, resulting in a catalogue of performance indicators and applied techniques.

    From the sizing analysis, no objective, techno-economically optimal PV system size is found without including the prosumer’s personal motives. Prioritizing return on investment results in small systems, whereas systems sized for net-zero energy can be profitable in some buildings. There is also a strong economic incentive to adopt communal electricity metering to increase self-consumption, system size, and economic return. Monte Carlo analysis is used to quantify investment uncertainty, finding that well-designed systems have an 81% chance of earning a 3% real return on investment, and even without subsidies there is a calculated 100% chance of having a positive return. PVT integrated GSHP can reduce the land needed for boreholes by up to 87% with a lower lifecycle cost than district heating, thereby broadening the heat pump market and reducing barriers to heating electrification.

    The quantitative results provide guidance for Swedish MFH owners while the methodology presents solar PV value in a more useful manner for prosumers to identify their personal motives in decision making. This approach is also useful for researchers, business leaders, and policy makers to understand the prosumer perspective and promote adoption of PV in the built environment.

  • Public defence: 2019-12-16 10:00 Kollegiesalen, Stockholm
    Aurell, Alexander
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Mathematical Statistics.
    Topics in the mean-field type approach to pedestrian crowd modeling and conventions2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis consists of five appended papers, primarily addressingtopics in pedestrian crowd modeling and the formation of conventions.The first paper generalizes a pedestrian crowd model for competingsubcrowds to include nonlocal interactions and an arbitrary (butfinite) number of subcrowds. Each pedestrian is granted a ’personalspace’ and is effected by the presence of other pedestrians within it.The interaction strength may depend on subcrowd affinity. The paperinvestigates the mean-field type game between subcrowds and derivesconditions for the reduction of the game to an optimization problem.The second paper suggest a model for pedestrians with a predeterminedtarget they have to reach. The fixed and non-negotiablefinal target leads us to formulate a model with backward stochasticdifferential equations of mean-field type. Equilibrium in the game betweenthe tagged pedestrians and a surrounding crowd is characterizedwith the stochastic maximum principle. The model is illustrated by anumber of numerical examples.The third paper introduces sticky reflected stochastic differentialequations with boundary diffusion as a means to include walls andobstacles in the mean-field approach to pedestrian crowd modeling.The proposed dynamics allow the pedestrians to move and interactwhile spending time on the boundary. The model only admits a weaksolution, leading to the formulation of a weak optimal control problem.The fourth paper treats two-player finite-horizon mean-field typegames between players whose state trajectories are given by backwardstochastic differential equations of mean-field type. The paper validatesthe stochastic maximum principle for such games. Numericalexperiments illustrate equilibrium behavior and the price of anarchy.The fifth paper treats the formation of conventions in a large populationof agents that repeatedly play a finite two-player game. Theplayers access a history of previously used action profiles and form beliefson how the opposing player will act. A dynamical model wheremore recent interactions are considered to be more important in thebelief-forming process is proposed. Convergence of the history to acollection of minimal CURB blocks and, for a certain class of games,to Nash equilibria is proven.

  • Public defence: 2019-12-17 10:00 Kollegiesalen, Stockholm
    Su, Chang
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Building heating solutions in China: A spatial system analysis2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Modern, clean, accessible and affordable building space heating is key tofuture sustainable development in China. However, it is impossible to recommendidentical building space heating solutions for all spaces in such alarge country as China. The decision making for choosing the most feasiblebuilding space heating solution is associated with a number of local characteristic spatial parameters, and stakeholders are still suffering from insufficient understanding of at which locations and under what conditions to choose a certain technology. Therefore, the present thesis aims at llingthis research gap by four steps: first, review current space heating situationin China; second, develop a systematic evaluation method for proper choice on building heating solution in different geolocations of China; third,demonstrate the efficacy of proposed method by case studies; fourth, analysethe Chinese energy sector administration infrastructure and its influence on building heating solutions.

    Step one is to understand the current status of building space heating in China, including what technologies currently prevail and where they are implemented, as well as their application scales. It is found that under existing energy structures, coal as the primary energy source is extensively consumed in space heating systems. Coal-based regional boilers and combined heat and power district heating is prevalent in North China. Distributed heating, such as reversible air-conditioners, is still dominating South China. During past decade, sustainable energy space heating is increasing rapidly under a series of national policy initiatives, and will continue to grow in the future.

    Following the current status review, a systematic method featured by spatial analysis is developed to compare the various heating options and find the best alternative. The method contains three system boundary levels, which reflect the characters of space heating technology, heat source, heat sink as well as the primary energy system. In each system level, local spatial parameters are analyzed. A set of key performance indicators is selected to quantitatively compare the relative advantages and disadvantages of implementing one building space heating solution over another from techno-economic-environmental as well geographical perspectives.

    Case studies are then carried out to demonstrate the application of the method. In case study one, two Chinese cities with different local spatial conditions are chosen. Ground source heat pumps and air source heat pumps are compared with status-quo space heating solutions, which are coal boilers and electric boilers. The results lie in three aspects. Technically, heat pumps are more efficient than boilers from a primary energy point of view. Economically, ground source heat pumps have to reach a satisfying seasonal coefficient of performance value of 3.7 for a competitive payback period against existing heating solutions. Environmentally, heat pumps have to reach a critical seasonal coefficient of performance value around 2.5 to guarantee their environmental advantages compared with directly burning coal for space heating as long as coal is the dominant source of energy to produce electricity. Such a threshold is fairly easy to reach considering the coefficient of performance of the heat pumps in the market.

    Case study two investigates seawater heat pumps potential in four coastal cities from north to south China. From techno-economic perspective, in North China seawater heat pumps can save primary energy use upto 18% in space heating, and can have a discounted payback period as short as 4 years compared with coal boilers. In southern Chinese cities on the other hand, seawater heat pumps can save primary energy use upto 14% in space heating but the discounted payback period is often more than 10 years compared with status-quo system. Environmentally, in North China seawater heat pumps have to reach a critical seasonal coefficient of performance value around 2.4 to guarantee their potential in carbon emissions saving when compared with fossil fuel boilers. In South China, seawater heat pumps generally emit less greenhouse gases than competing technologies. Geographically speaking, northern coastal cities are more feasible for seawater heat pumps applications compared with southern cities, as many buildings in northern coastal cities are within a proper distance to the seawater for efficient utilization of seawater for space heating and cooling.

    Energy administration structure and energy policies in China are anavilyzed in parallel with case studies, in order to understand how energy management in China is regulated and how effective such energy policies can be. It is shown that energy administrations in China have great influence on the implementation of energy technologies and many energy policies are quite effective in promoting renewable space heating technologies.

    In conclusion, stakeholders are suggested to adopt the system method proposed in this thesis, to promote the best building heating solution based on local spatial characteristics. By using the method in case studies, it is concluded that for heat pumps, a number of prerequisites have to be fullled for a more successful application in China. Future emphasis should be placed on heat pumps efficiency improvements, operation management and cost reduction. Meanwhile, increasing the share of zero-carbon electricity in the energy system should be a long-term goal so that the environmental benefits of heat pumps can be more prominent.

  • Public defence: 2019-12-17 10:00 Sal C, Electrum, Kista
    Ghoorchian, Kambiz
    KTH, School of Electrical Engineering and Computer Science (EECS), Computer Science, Software and Computer systems, SCS.
    Graph Algorithms for Large-Scale and Dynamic Natural Language Processing2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In Natural Language Processing, researchers design and develop algorithms to enable machines to understand and analyze human language. These algorithms benefit multiple downstream applications including sentiment analysis, automatic translation, automatic question answering, and text summarization. Topic modeling is one such algorithm that solves the problem of categorizing documents into multiple groups with the goal of maximizing the intra-group document similarity. However, the manifestation of short texts like tweets, snippets, comments, and forum posts as the dominant source of text in our daily interactions and communications, as well as being the main medium for news reporting and dissemination, increases the complexity of the problem due to scalability, sparsity, and dynamicity. Scalability refers to the volume of the messages being generated, sparsity is related to the length of the messages, and dynamicity is associated with the ratio of changes in the content and topical structure of the messages (e.g., the emergence of new phrases). We improve the scalability and accuracy of Natural Language Processing algorithms from three perspectives, by leveraging on innovative graph modeling and graph partitioning algorithms, incremental dimensionality reduction techniques, and rich language modeling methods. We begin by presenting a solution for multiple disambiguation on short messages, as opposed to traditional single disambiguation. The solution proposes a simple graph representation model to present topical structures in the form of dense partitions in that graph and applies disambiguation by extracting those topical structures using an innovative distributed graph partitioning algorithm. Next, we develop a scalable topic modeling algorithm using a novel dense graph representation and an efficient graph partitioning algorithm. Then, we analyze the effect of temporal dimension to understand the dynamicity in online social networks and present a solution for geo-localization of users in Twitter using a hierarchical model that combines partitioning of the underlying social network graph with temporal categorization of the tweets. The results show the effect of temporal dynamicity on users’ spatial behavior. This result leads to design and development of a dynamic topic modeling solution, involving an online graph partitioning algorithm and a significantly stronger language modeling approach based on the skip-gram technique. The algorithm shows strong improvement on scalability and accuracy compared to the state-of-the-art models. Finally, we describe a dynamic graph-based representation learning algorithm that modifies the partitioning algorithm to develop a generalization of our previous work. A strong representation learning algorithm is proposed that can be used for extracting high quality distributed and continuous representations out of any sequential data with local and hierarchical structural properties similar to natural language text.

  • Public defence: 2019-12-18 09:00 Kollegiesalen, Stockholm
    Filipović, Marko
    KTH, School of Electrical Engineering and Computer Science (EECS), Computer Science, Computational Science and Technology (CST). Bernstein Center Freiburg and Faculty of Biology, University of Freiburg, Germany.
    Characterisation of inputs and outputs of striatal medium spiny neurons in health and disease2019Doctoral thesis, monograph (Other academic)
    Abstract [en]

    Striatal medium spiny neurons (MSNs) play a crucial role in various motor and cognitive functions. They are separated into those belonging to the direct pathway (dMSNs) and the indirect pathway (iMSNs) of the basal ganglia, depending on whether they express D1 or D2 type dopamine receptors, respectively. In this thesis I investigated the input processing of both MSN types, the characteristics of dMSN outputs, and the effect that aberrant iMSN activity has on the subthalamic nucleus-globus pallidus externa (STN-GPe) network.In order to verify a previous result from a computational study claiming that dMSNs should receive either more or stronger total input than iMSNs, I performed an analysis of in vivo whole-cell MSN recordings in healthy and dopamine (DA) depleted (6OHDA) anesthetized mice. To test this prediction, I compared subthreshold membrane potential fluctuations and spike-triggered average membrane potentials of the two MSN types. I found that dMSNs in healthy mice exhibited considerably larger fluctuations over a wide frequency range, as well as significantly faster  depolarization towards the spiking threshold than iMSNs. However, these effects were not present in recordings from 6OHDA animals. Together, these findings strongly suggest that dMSNs do  receive stronger total input than iMSNs in healthy condition.I also examined how different concentrations of dopamine affect neural trial-by-trial (or response) variability in a biophysically detailed compartmental model of a direct-pathway MSN.  Some of the sources of trial-by-trial variability include synaptic noise, neural refractory period, and ongoing neural activity. The focus of this study was on the effects of two particular  properties of the synaptic input: correlations of synaptic input rates, and the balance between excitatory and inhibitory inputs (E-I balance). The model demonstrates that dopamine is in  general a significant diminisher of trial-by-trial variability, but that its efficacy depends on the properties of synaptic input. Moreover, input rate correlations and changes in the E-I balance by themselves also proved to have a marked impact on the response variability.Finally, I investigated the beta-band phase properties of the STN-GPe network, known for its exaggerated beta-band oscillations during Parkinson’s disease (PD). The current state-of-the-art  computational model of the network can replicate both transient and persistent beta oscillations, but fails to capture the beta-band phase alignment between the two nuclei as seen in human  recordings. This was particularly evident during simulations of the PD condition, where STN or GPe were receiving additional stimulation in order to induce pathological levels of beta-band  activity. Here I show that by manipulating the percentage of the neurons in either population that receives stimulation it is possible to increase STN-GPe phase difference heterogeneity.  Furthermore, a similar effect can be achieved by adjusting synaptic transmission delays between the two populations. Quantifying the difference between human recordings and network  simulations, I provide the set of parameters for which the model produces the greatest correspondence with experimental results.

    The full text will be freely available from 2019-12-25 09:00
  • Public defence: 2019-12-18 09:30 T2, Huddinge
    Moustaid, Elhabib
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Biomedical Engineering and Health Systems, Health Informatics and Logistics.
    Perspectives on Modeling and Simulation of Urban Systems with Multiple Actors and Subsystems2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Cities are the spaces of the interaction between social, physical, political, and economic entities, which makes planning and intervening in such systems difficult. Urban systems are complex adaptive systems in that their behaviours are often the result of the interaction of their components. The growth of urban systems is driven by mass urbanization. Their complexity is the result of interactions between its constituent systems and components.

    Simulations and models as tools of exploration of urban systems face many challenges to be useful tools for intervening. Throughout the past decades, the use of simulation models focused on providing tools for managing functions and systems within metropolitan and urban environments. The cognizance of the complexity of these environments and the maturity of complexity science as a field of studying complex systems allow for the application of complexity science methods to study urban systems not only as physical systems but as social systems too.

    As learning from simulations and models can occur both at their construction and their use, this thesis focused on model and simulation building, running, and final use. The thesis takes into account two main aspects of urban systems. First, urban systems are often multi-stakeholder, that is systems where multiple stakeholders are intervening at the same time, and sometimes without clear boundaries and agency over sub-parts of the system. Second, urban systems can have a multi-subsystem structure, where each subsystem often have their objectives and affecting the rest of the system in unfamiliar ways.

    The thesis investigates through a multicase study, with three case studies, five main themes in simulation modeling that relate to increasing validity and usefulness of models for urban complex systems. Those themes are as follows; (1) the ability of simulation to be tools that capture complexity in ways that are similar to the real target systems, (2) the effects of the inclusion of experts in simulation models construction on the models, (3) the ways quantitative and qualitative ways of modeling can together make simulations and models more useful, (4) the value of simulation modeling to study connections in systems that are multi-system and multi-stakeholder, and (5) the ability to learn from models under the model building journey.

    The study cases included are modeling of a city pedestrian network, a metropolitan emergency care provision, and urban mental health dynamics. The case studies provided a diversity of system granularity. The methods used for each of the case studies have also been different in able to study different levels of inclusion of expert knowledge, data, and theoretical models.

    Besides its contribution to each of the case studies, with new models and simulation approaches, the thesis contributes to the five themes it investigated. It showed simulation modeling to be able to exhibit multiple elements of complexity. It also showed the ability of expert knowledge to help models become more useful and valid either by increasing their realism or level of representation. This result is achieved by the contextualization of the expert knowledge in the case of pedestrian modeling, and its full exploration in the mental health modeling. Furthermore, the thesis shows ways in which simulation and modeling can find and investigate bridges between urban subsystems. The outcomes suggest that simulation modeling can be a useful tool for exploring different kinds of complexity in urban systems as multi-actor and multi-system systems. Models can mirror the complexity of urban systems in their structure. They can also be ways of exploring non-intuitive behaviors and dynamics. Expert knowledge, in particular, is shown throughout the thesis to be able to help simulations achieve more validity and usefulness.

  • Public defence: 2019-12-18 10:00 F3, Stockholm
    Pålsson, Sara
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Numerical Analysis, NA.
    Boundary integral methods for fast and accurate simulation of droplets in two-dimensional Stokes flow2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Accurate simulation of viscous fluid flows with deforming droplets creates a number of challenges. This thesis identifies these principal challenges and develops a numerical methodology to overcome them. Two-dimensional viscosity-dominated fluid flows are exclusively considered in this work. Such flows find many applications, for example, within the large and growing field of microfluidics; accurate numerical simulation is of paramount importance for understanding and exploiting them.

    A boundary integral method is presented which enables the simulation of droplets and solids with a very high fidelity. The novelty of this method is in its ability to accurately handle close interactions of drops, and of drops and solid boundaries, including boundaries with sharp corners. The boundary integral method is coupled with a spectral method to solve a PDE for the time-dependent concentration of surfactants on each of the droplet interfaces. Surfactants are molecules that change the surface tension and are therefore highly influential in the types of flow problems which are considered herein.

    A method’s usefulness is not dictated by accuracy alone. It is also necessary that the proposed method is computationally efficient. To this end, the spectral Ewald method has been adapted and applied. This yields solutions with computational cost O(N log N ), instead of O(N^2), for N source and target points.

    Together, these innovations form a highly accurate, computationally efficient means of dealing with complex flow problems. A theoretical validation procedure has been developed which confirms the accuracy of the method.

  • Public defence: 2019-12-19 10:00 Room nr: B4:1026 Code: FB42, Stockholm
    Zakomirnyi, Vadim
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology. Siberian Federal University : Krasnoyarsk , RU.
    Multicomponent Resonant Nanostructures: Plasmonic and Photothermal Effects2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In recent decades, plasmonic nanoparticles have attracted considerable attention due to their ability to localize electromagnetic energy at a scale much smaller than the wavelength of optical radiation. The study of optical plasmon waveguides (OPWs) in the form of chains of nanoparticles is important for modern photonics. However, the widespread use of OPWs is limited due to the suppression of the resonance properties of classical plasmon materials under laser irradiation. The study of the influence of nanoparticle heating on the optical properties of waveguides and the search for new materials capable of stable functioning at high temperatures is an important task.

    In this thesis, the processes occurring during heating of plasmon nanoparticles and OPWs are studied. For this purpose, a model was developed that takes into account the heat transfer between the particles of an OPW and the environment. The calculations used temperature-dependent optical constants. As one of possible ways to avoid thermal destabilization of plasmon resonanses, new materials for OPWs formed by nanoparticles were proposed. I show that titanium nitride is a promising thermally stable material, that might be useful for manufacturing of OPWs and that works in high intensity laser radiation.

    Another hot topic at present is the study of periodic structures of resonant nanoparticles. Periodic arrays of nanoparticles have a unique feature: the manifestation of collective modes, which are formed due to the hybridization of a localized surface plasmon resonance or a Mie resonance and the Rayleigh lattice anomaly. Such a pronounced hybridization leads to the appearance of narrow surface lattice resonances, the quality factor of which is hundreds of times higher than the quality factor of the localized surface plasmon resonance alone. Structures that can support not only electric, but also magnetic dipole resonances becomes extremely important for modern photonics on chip systems. An example of a material of such particles is silicon. Using the method of generalized coupled dipoles, I studied the optical response of arrays of silicon nanoparticles. It is shown that under certain conditions, selective hybridization of only one of the dipole moments with the Rayleigh anomaly occurs.

    To analyze optical properties of intermediate sized particles with N = 103-105 atoms and diameter of particle d < 12 nm an atomistic approach, where the polarizabilities can be obtained from the atoms of the particle, could fill an important gap in the description of nanoparticle plasmons between the quantum and classical extremes. For this purpose I introduced an extended discrete interaction model where every atom makes a difference in the formation optical properties of nanoparticles within this size range. In this range are first principal approaches not applicable due to the high number of atoms and classical models based on bulk material dielectric constants are not available due to high influence from quantum size effects and corrections to the dielectric constant. To parametrize this semi-empirical model I proposed a method based on the concept of plasmon length. To evaluate the accuracy of the model, I performed calculations of optical properties of nanoparticles with different shapes: regular nanospheres, nanocubes and nanorods. Subsequently, the model was used to calculate hollow nanoparticles (nano-bubbles).

  • Public defence: 2019-12-19 10:00 D3, Stockholm
    Högström, Johan
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Sustainability Assessment and Management.
    Lost in translation?: A process-oriented view on municipal spatial planning practice2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    As urban areas are developing and becoming increasingly important for responding to sustainability goals and challenges, it is necessary to understand the conditions for local spatial planning practices who play a key role in shaping urban environments. This thesis aims to explore and analyze municipal spatial planning practices and spatial planning processes in the Stockholm region, Sweden. More specifically, the thesis aims to contribute to the development of municipal spatial planning practice by creating knowledge of the organization and application of the municipal spatial planning process. Based on the results of the four included studies, planning processes at different levels are used to organize embedded processes of contextualization, translation, concretization and formalization, which all play a key role for governing the municipal spatial planning process. Planners engage with processes of contextualization, translation, concretization and formalization to pursue an understanding of how to shape urban environments across unique planning situations. The use of these four processes are of importance for the cross-level interaction in local planning practice. To strengthen the cross-level interplay in municipal spatial planning, it is necessary to pay attention to the relevance of strategic plans, and the influential role of protocol (e.g. the project manuals) in shaping conditions for process participation. Moreover, the cross-level interplay can be strenghthened by designing a process which ties the different levels together and creates conditions for learning and capacity building.

  • Public defence: 2019-12-19 10:15 F3, Stockholm
    Montecchia, Matteo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Numerical and modelling aspects of large-eddy and hybrid simulations of turbulent flows2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this study, the explicit algebraic sub-grid scale (SGS) model (EAM) has been extensively validated in wall-resolved large-eddy simulations (LES) of wall-bounded turbulent flows at different Reynolds numbers and a wide range of resolutions. Compared to eddy-viscosity based models, the formulation of the EAM is more consistent with the physics and allows to accurately capture SGS anisotropy,which is relevant especially close to walls.The present work aims to extend the validation of the EAM to larger Reynolds numbers using codes with different orders of numerical accuracy.The first simulations, performed by using a pseudo-spectral code, show that the use of the EAM, compared to the dynamic Smagorinsky model (DSM), leads to significant improvements in the prediction of the first-and second order statistics of turbulent channel flow.These improvements are observed from relatively low to  reasonably high Reynolds numbers and with coarse grids.The evaluation of the EAM was continued by implementing and testing of the EAM in the general-purpose finite-volume code OpenFOAM.Several tests of LES of turbulent channel flow have shown thatthe use of the Rhie and Chow (R&C) interpolation in OpenFOAM induces significant numerical dissipation.A new custom-built solver has been utilized in order to minimize the dissipation without generating significant adverse effects. The use of the EAM, together with the new solver, gives a substantially improved prediction of the mean velocity profiles as compared to predictions using the DSM, resulting in roughly 50% reduction in the grid point requirements to achieve a given degree of accuracy. In periodic hill flow, LES with the EAM agreed reasonably well with the reference dataat different bulk Reynolds numbers and reduced the misprediction of the first- and second order statistics observed in LES with DSM.The reduction of the R&C filter dissipation was also shown to be beneficial for the prediction of the mean quantities. An analysis of the skin friction along the lower wall reveals spanwise-elongated, almost axi-symmetric vortical structures generated by the Kelvin-Helmholtz instability. The structures introduced a significant amount of anisotropy.The last part of the study involved the development of a novel hybrid RANS-LES model where explicit algebraic Reynolds stress modelling is applied in both RANS and LES regions.Validations have been conducted on turbulent channel and periodic hill flows at different Reynolds numbers.The explicit algebraic Reynolds stress model for improved-delayed-detached-eddy simulation (EARSM-IDDES) gives reasonable predictions of the mean quantities and Reynolds stresses in both the geometries considered.The use of EARSM-IDDES, compared to the k-omega SST-IDDES model, improves the estimation of the quantities close to the wall.The present work has proven that the use of EAM in wall-resolved LES of wall-bounded flows in simple and complex geometries leads to a substantial reduction of  computational requirements both in high-accuracy and general-purpose codes, compared to the use of eddy-viscosity models.In hybrid simulations the EARSM-IDDES shows a clear potential in capturing the physics of wall-bounded flows.

  • Public defence: 2020-01-09 10:00 F3, Stockholm
    Aguilar, Xavier
    KTH, School of Electrical Engineering and Computer Science (EECS), Computer Science, Computational Science and Technology (CST).
    Performance Monitoring, Analysis, and Real-Time Introspection on Large-Scale Parallel Systems2020Doctoral thesis, monograph (Other academic)
    Abstract [en]

    High-Performance Computing (HPC) has become an important scientific driver. A wide variety of research ranging for example from drug design to climate modelling is nowadays performed in HPC systems. Furthermore, the tremendous computer power of such HPC systems allows scientists to simulate problems that were unimaginable a few years ago. However, the continuous increase in size and complexity of HPC systems is turning the development of efficient parallel software into a difficult task. Therefore, the use of per- formance monitoring and analysis is a must in order to unveil inefficiencies in parallel software. Nevertheless, performance tools also face challenges as a result of the size of HPC systems, for example, coping with huge amounts of performance data generated.

    In this thesis, we propose a new model for performance characterisation of MPI applications that tackles the challenge of big performance data sets. Our approach uses Event Flow Graphs to balance the scalability of profiling techniques (generating performance reports with aggregated metrics) with the richness of information of tracing methods (generating files with sequences of time-stamped events). In other words, graphs allow to encode ordered se- quences of events without storing the whole sequence of such events, and therefore, they need much less memory and disk space, and are more scal- able. We demonstrate in this thesis how our Event Flow Graph model can be used as a trace compression method. Furthermore, we propose a method to automatically detect the structure of MPI applications using our Event Flow Graphs. This knowledge can afterwards be used to collect performance data in a smarter way, reducing for example the amount of redundant data collected. Finally, we demonstrate that our graphs can be used beyond trace compression and automatic analysis of performance data. We propose a new methodology to use Event Flow Graphs in the task of visual performance data exploration.

    In addition to the Event Flow Graph model, we also explore in this thesis the design and use of performance data introspection frameworks. Future HPC systems will be very dynamic environments providing extreme levels of parallelism, but with energy constraints, considerable resource sharing, and heterogeneous hardware. Thus, the use of real-time performance data to or- chestrate program execution in such a complex and dynamic environment will be a necessity. This thesis presents two different performance data introspec- tion frameworks that we have implemented. These introspection frameworks are easy to use, and provide performance data in real time with very low overhead. We demonstrate, among other things, how our approach can be used to reduce in real time the energy consumed by the system.

    The approaches proposed in this thesis have been validated in different HPC systems using multiple scientific kernels as well as real scientific applica- tions. The experiments show that our approaches in performance character- isation and performance data introspection are not intrusive at all, and can be a valuable contribution to help in the performance monitoring of future HPC systems.

  • Public defence: 2020-01-31 14:00 K1, Stockholm
    Winberg-Wang, Helen
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering, Chemical Engineering.
    Water density impact on water flow and mass transport in rock fractures2020Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    One way of taking care of spent nuclear fuel is to place it in a geological repository. In Sweden, a three-barrier system is planned. The system is based on encapsulating the fuel in copper canisters. These are surrounded by bentonite clay and buried under 500 m of bedrock. As a part of the safety assessment, the Q-equivalent model is used to quantify the possible release of radioactive material. This model also describes the rate at which corrosive agents carried by seeping water in rock fractures can reach the canisters, which may affect the longevity of the canisters.

    The aim of this thesis was originally to develop an experimental, phys- ical model to visualize and validate the Q-equivalent model. However, the overarching theme of this work has been to study the effect of minor density differences that might be overlooked in experiments, both concentration- dependent and density-difference induced by light absorption.

    In the initial diffusion and flow-experiment and associated calculations and simulations, it was found that simple Q-equivalent can describe and quantify the mass transport in both parallel and variable aperture fractures. However, this is the case only if the density difference between seeping water and clay pore water is insignificant. It was found in experiments with dyes used to visualise the flow and diffusion patterns that even minimal density differences could significantly alter the flow pattern. Density differences can result from concentration gradients or be induced by light absorption. TheQ-equivalent model was extended to account for density-induced flow. The importance of density-induced flow due to concentration gradients at the setting of a long-term repository for nuclear waste was evaluated. It was found that concentration gradients are able to induce rapid vertical up- or downward flow. This could increase the overall mass transport of radioactive material up to the biosphere or carry it downward to larger depths.