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  • 124151. Åkerman, Johan
    et al.
    Escudero, R.
    Leighton, C.
    Kim, S.
    Rabson, D. A.
    Dave, R. W.
    Slaughter, J. M.
    Schuller, I. K.
    Criteria for ferromagnetic-insulator-ferromagnetic tunneling2002In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 240, no 03-jan, p. 86-91Article in journal (Refereed)
    Abstract [en]

    The Rowell criteria, commonly used to identify tunneling in magnetic tunnel junctions (MTJ), are scrutinized. While neither the exponential-thickness dependence of the conductivity nor fits of non-linear transport data are found to be reliable tunneling criteria, the temperature-dependent conductivity does remain a solid criterion. Based on experimental studies of the bias and temperature-dependent resistance and magnetoresistance of MTJs, with and without shorted barriers, a new set of criteria is formulated.

  • 124152. Åkerman, Johan
    et al.
    Guedes, I.
    Leighton, C.
    Grimsditch, M.
    Schuller, I. K.
    Upper bound for the magnetic proximity effect extracted from Brillouin light scattering2002In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 65, no 10Article in journal (Refereed)
    Abstract [en]

    The effective magnetic thickness of Fe films and of Fe/Al and Fe/Pd bilayers is determined using Brillouin light scattering. The magnetic thickness is extracted by fitting the field dependence of the frequencies of two magnon modes. Within experimental errors of about 1 Angstrom, no change in the effective magnetic thickness of the Fe layers was detected. If a net magnetization does exist in Pd when in contact with Fe, it is sufficiently different in nature so as to not modify the Fe spin waves.

  • 124153. Åkerman, Johan
    et al.
    Rao, K. V.
    KTH, Superseded Departments, Materials Science and Engineering.
    Collective dynamics of a highly dilute vortex lattice in YBa2Cu3O7-delta thin films2002In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 65, no 13Article in journal (Refereed)
    Abstract [en]

    The dynamics of highly dilute vortex ensembles in YBa2Cu3O7-delta thin films is investigated by low-field (5-90 Oe) ac susceptibility measurements of the dynamical relaxation rate Q. In a film with relatively weak pinning, flux bundling is found to be effective at all temperatures and down to the lowest fields. In a film with stronger pinning, single-vortex creep dominates only for T<20 K, and small bundle creep is fully developed for T>40 K. The behavior of highly separated vortices is hence surprisingly collective in YBa2Cu3O7-delta. Quantum creep is observed below 11 K, and at about 60 K the elastic vortex description breaks down as the importance of plastic creep gradually increases.

  • 124154. Åkerman, Johan
    et al.
    Rao, K. V.
    Dynamics of dilute vortices in various high-T-C thin films2002In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 91, no 10, p. 7137-7139Article in journal (Refereed)
    Abstract [en]

    We study the dynamics of dilute vortices in c-axis oriented Y-123, Hg-, and Tl-1212 thin films, as well as a-axis oriented Hg-1212 thin films. Results on the Hg- and Tl-1212 thin films indicate that dislocation-mediated plastic flux creep of single vortices dominates at low temperatures and fields. As the temperature (or the field) is increased, increasing vortex-vortex interactions promote a collective behavior, characterized by elastic creep with a nonzero mu exponent. In some samples, effects of thermally assisted quantum creep are visible up to 45 K. In Y-123 thin films, creep is collective down to the lowest temperatures and fields investigated, while quantum creep persists up to 10-11 K.

  • 124155. Åkerman, Johan
    et al.
    Rao, K. V.
    Low-field vortex dynamics in various high-T-c thin films2002In: Pramana (Bangalore), ISSN 0304-4289, E-ISSN 0973-7111, Vol. 58, no 06-maj, p. 985-993Article in journal (Refereed)
    Abstract [en]

    We present a novel ac susceptibility technique for the study of vortex creep in superconducting thin films. With this technique we study the dynamics of dilute vortices in c-axis oriented Y-123, Hg-1212, and TI-1212 thin films, as well as a-axis oriented Hg-1212 thin films. Results on the Hg-1212 and Tl-1212 thin films indicate that dislocation-mediated plastic flux creep of single vortices dominates at low temperatures and fields. As the temperature (or the field) is increased, the increasing vortex-vortex interactions promote a collective behavior, which can be characterized by elastic creep with a non-zero It exponent. Also, in some of these samples effects of thermally assisted quantum creep are visible up to 45 K in some of these samples. In Y-123 thin films, creep is found to be collective down to the lowest temperatures and fields investigated, while the quantum creep persists only up to 10-11 K.

  • 124156. Åkerman, Johan
    et al.
    Roshchin, I. V.
    Slaughter, J. M.
    Dave, R. W.
    Schuller, I. K.
    Origin of temperature dependence in tunneling magnetoresistance2003In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 63, no 1, p. 104-110Article in journal (Refereed)
    Abstract [en]

    We present detailed measurements of the differential resistance (dV/dI) of state-of-the-art FM/AlOx/FM magnetic tunnel junctions (MTJ) as a function of applied bias and temperature. Temperature effects are particularly significant in physical quantities involving narrow features such as those at low-voltage bias. We show that the temperature evolution of the tunneling characteristics and, in particular, the pronounced rounding of the dV/dI curves with increasing temperature can be well explained by thermal smearing of the tunneling electron energy distribution.

  • 124157. Åkerman, Johan
    et al.
    Slaughter, J. M.
    Dave, R. W.
    Schuller, I. K.
    Tunneling criteria for magnetic-insulator-magnetic structures2001In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 79, no 19, p. 3104-3106Article in journal (Refereed)
    Abstract [en]

    The bias and temperature dependent resistance and magnetoresistance of magnetic tunnel junctions with and without intentional shorts through the insulating barrier were studied. Based on the experimental results, a set of quality criteria was formulated that enables the identification of barrier shorts. While the temperature and bias dependencies of the junction resistance and of the fitted barrier parameters are very sensitive to the presence of such shorts, the same dependencies of the magnetoresistance are surprisingly insensitive. Finally, junctions with a shorted barrier exhibit a dramatic increase in noise level and junction instability.

  • 124158.
    Åkerman, Johan
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Ström, Valter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Rao, K. V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Dahlberg, E. Dan
    Separation of exchange anisotropy and magnetocrystalline anisotropy in Co/CoO bilayers by means of ac susceptibility measurements2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 76, no 14Article in journal (Refereed)
    Abstract [en]

    In Co/CoO bilayers separation of the unidirectional anisotropy energy from crystalline anisotropy is accomplished with the temperature variation of the ac susceptibility from room temperature to 4.2 K. Measurements above the blocking temperature of the CoO layer give a clear measure of crystalline anisotropy in the Co. At 4.2 K 85% of the total anisotropy is from K-ud while the anisotropy of the Co layer only contributes about 15%. The small crystalline anisotropy energy does not account for the large difference between the exchange anisotropy determined by ac susceptibility and hysteresis loops.

  • 124159. Åkerman, Johan
    et al.
    Yun, S. H.
    Karlsson, U. O.
    Rao, K. V.
    KTH, Superseded Departments, Materials Science and Engineering.
    Re-entrant behavior of low-field flux creep in c-axis-oriented HgBa2CaCu2O6+delta thin films2001In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 6418, no 18Article in journal (Refereed)
    Abstract [en]

    The temperature, ac and dc field, and current dependent activation energy U(T,H)[(J(c0)/J(c))(mu)-1]/mu governing low-field flux creep in epitaxial c-axis-oriented HgBa2CaCu2O6+delta thin films has been determined from measurements of the frequency-dependent in-phase ac susceptibility. Above 35 K three different thermally activated flux creep regimes can be identified: (i) dislocation-mediated plastic flux creep, described by U(T,H) = U-0(1 - t(4))H-1/2 and mu = 0, (ii) elastic collective flux creep which decreases with temperature and has a weaker field dependence of H-0.22 above a field-dependent temperature Tc.(H) where A acquires finite values, and (iii) reappearance of dislocation-mediated plastic flux creep which rapidly increases as T-c is approached. It is argued that the re-entrant plastic-elastic-plastic vortex creep behavior is driven by the underlying temperature and field dependence of the shear modulus c(66). T-cm(H) marks a line in the H-T plane where the increasing c(66) promotes long-range correlations in the dilute vortex phase and creep becomes collective. At high H and T, c(66) again decreases and plastic creep reappears as the ordered phase starts to melt. Evidence for thermally assisted quantum creep is observed up to temperatures as high as T-0 = 35 K.

  • 124160.
    Åkerman, Jonas
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Environmental Strategies.
    Climate impact of international travel by Swedish residents2012In: Journal of Transport Geography, ISSN 0966-6923, E-ISSN 1873-1236, Vol. 25, p. 87-93Article in journal (Refereed)
    Abstract [en]

    International travel is increasing rapidly, but emissions from this segment are not included in the Kyoto Protocol and seldom in national statistics. The aim of this paper is to estimate the volume of international travel by the Swedish population, and the emissions of greenhouse gas associated with that travel. Data used are from a national travel survey for 2006 including 27,000 telephone interviews. It is shown that international travel by the Swedish population amounted to 37 billion passenger-km, or 4100 km per capita in 2006. This corresponds to 22.5% of total travel by Swedes, an increase from 16.6% in 1994. Emissions of greenhouse gases from international travel by the Swedish population in 2006 amounted to 8.5 million tons of carbon dioxide equivalents, of which 92% stems from air travel. This is approx. 11% of total Swedish emissions including international transport. It is concluded that a continued growth of international travel at current pace hardly may be reconciled with long-term climate targets.

  • 124161. Åkerman, Jonas
    Potential för ny fordonsteknikoch förnybara drivmedel inomvägtransportsektorn2012Report (Other (popular science, discussion, etc.))
  • 124162.
    Åkerman, Jonas
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Environmental Strategies.
    Sustainable air transport - on track in 20502005In: Transportation Research Part D: Transport and Environment, ISSN 1361-9209, E-ISSN 1879-2340, Vol. 10, no 2, p. 111-126Article in journal (Refereed)
    Abstract [en]

    In this paper three 'Images' of air travel on a sustainable path in 2050 are outlined. The method used is backcasting. A stabilization of the carbon dioxide concentration in the atmosphere at 450 parts per million is assumed to constitute a sustainable level. It is concluded that a refinement of the conventional turbofan aircraft is probably not sufficient to reach even the less demanding target level, even though a 40% reduction of fuel intensity may be achieved. The introduction of more radical aircraft configurations, e.g. the flying wing, probably has the greatest potential for reducing fuel consumption, but the development of such aircraft is uncertain. A less risky strategy is to opt for a high-speed propeller aircraft with a cruise speed which is 20-25% lower than for a conventional turbofan aircraft. This configuration entails a 56% cut of carbon dioxide emissions per passenger-kilometre compared to 2000. If this technology trajectory is combined with a development characterized by a weakened emphasis on economic growth together with less hectic lifestyles, it may be possible to reach even the most demanding of the target levels. Air travel per capita in 2050 would then be slightly higher than in 2000. If only the less demanding target level is to be reached global air travel per capita may be about 110% higher than in 2000.

  • 124163.
    Åkerman, Jonas
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Environmental Strategies.
    The role of high-speed rail in mitigating climate change: The Swedish case Europabanan from a life cycle perspective2011In: Transportation Research Part D: Transport and Environment, ISSN 1361-9209, E-ISSN 1879-2340, Vol. 16, no 3, p. 208-217Article in journal (Refereed)
    Abstract [en]

    In this paper a life cycle perspective is used to analyse Europabanan, a proposed high-speed rail track in Sweden. The life cycle emissions reductions are found to be 550,000 tons of CO2-equivalents per annum by 2025/2030 with almost 60% of this coming from a shift from truck to rail freight and 40% from a shift from air and road travel to high-speed rail travel. In contexts similar to Sweden, it is thus an important issue whether a large increase is required in freight rail capacity anyway, since high-speed rail investments may not be justified for the passenger markets alone. The study also indicates that a substantial share of emissions due to construction of the new railway could be counterbalanced through the reduced need for building and maintaining roads and airports, and for manufacturing cars.

  • 124164.
    Åkerman, Jonas
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Environmental Strategies.
    Transport systems meeting long-term climate targets: A backcasting approach2011Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Future transport systems consistent with long-term climate targets are examined in this thesis, using a systems perspective covering the entire transport system. Aviation is given particular attention, as expansion of this mode is difficult to reconcile with climate targets. The aim is to provide scientific decision support for current transport policy-making, especi-ally regarding structures with high inertia, e.g. urban structure, roads, railways, fuel produc-tion systems and vehicle fleets. An additional aim is to widen the perception of possible transport futures consistent with meeting climate targets, and to support a wider discussion in society on this topic. Papers I and III are backcasting studies which encompass the whole transport system. Paper III outlines an image of future Swedish transport by 2050, in which energy use per capita is reduced by 60%. This reduction is consistent with a 42% reduction in total global greenhouse gas emissions. Paper IV shows that total air travel by Swedes generates about 8.7 million tons of CO2-equivalents annually. This corresponds to about 12% of total Swedish emissions. Considering the rapid growth in emissions, aviation is key to achieving overall climate targets. Paper V indicates that building high-speed tracks between Stockholm, Gothenburg and Malmö may yield emissions reductions of about 550,000 tons of CO2-equivalents annually, if a life-cycle perspective is considered for all modes. However, this reduction is contingent on continuing growth of transport volumes, which seems diffi-cult to reconcile with the images in Papers II and III. This might consequently be a „second best‟ solution if a more radical break in transport growth is deemed unlikely due to external drivers.

    The overall conclusion from this thesis is that improved vehicle technology and low carbon fuels are necessary, but not solely sufficient, to achieve long-term targets consistent with limiting global warming to two degrees. The growth in volume, especially of car and air travel and truck transport, must also be curbed. However, total travel volume can be maintained at 2005 levels if substantial modal shifts to cycling and public transport are achieved. Moreover, if conscious measures are taken regarding urban planning and the use of communications technology to replace travel, functional accessibility may increase considerably. The trend-breaking development needed to achieve climate targets requires a combination of different policy measures. Pricing of external effects, e.g. greenhouse gas emissions, is a key strategy and would involve ending aviation‟s exemption from carbon tax and value-added tax. Other possible pricing measures include differentiated charges for car travel in urban areas, km-charges on trucks and increased fuel taxes. However, to gain acceptance for pricing measures and maintain a well-functioning society, better alternatives with a lower climate impact are needed. Increased road capacity in urban areas usually increases car travel. Therefore, to achieve the targets set, strict prioritisation of investments in public transport, cycling and ICT infrastructure is needed, especially since public resources are limited. Another conclusion is that, for transport policies to be effective and not lead to sub-optimisations, it is necessary to consider the wider system delimitations explored in this thesis.

  • 124165.
    Åkerman, Jonas
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Environmental Strategies Research (fms).
    Finnveden, Göran
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Environmental Strategies Research (fms).
    Höjer, Mattias
    KTH, School of Computer Science and Communication (CSC), Centres, School of Architecture and the Built Environment (ABE), Centres, Centre for Sustainable Communications, CESC.
    Nässén, Jonas
    Larsson, Jörgen
    Elofsson, Anna
    Forskare: Så ser vi på alternativen till flygskatt2017In: Dagens SamhälleArticle in journal (Other (popular science, discussion, etc.))
    Abstract [sv]

    Sverige och andra länder behöver driva på för att etablera starka internationella styrmedel för att få ner utsläppen från flyget. Detta kommer dock att ta många år. Med tanke på den snabba ökningen av utsläpp krävs därför också temporära styrmedel om Parismålen ska kunna nås.

  • 124166.
    Åkerman, Jonas
    et al.
    KTH, School of Architecture and the Built Environment (ABE).
    Hedegaard Sörensen, Claus
    Technical University of Denmark.
    Isaksson, Karolina
    Swedish National Road and Transport Research Institute.
    Kressler, Florian
    Austria Tech.
    Macmillen, James
    University of Oxford.
    Deliverable 5 How to manage barriers to formation and implementation of policy packages in transport: Grant Agreement No:No TREN/FP7TR/233681/"OPTIC”2011Report (Other academic)
    Abstract [en]

    The aim of this study has been to explore success factors and barriers to the formation and implementation of single policy measures and policy packages in transport, and to identify strategies to manage such barriers. As a first step, we developed a typology of barriers and success factors for policy formation and implementation. Secondly, we carried out an empirical analysis of barriers and success factors in four cases of policy packaging: Urban Congestion Charging; National Heavy Vehicle Fees; Aviation in the European Emissions Trading System and The EU’s First Railway Package. The third and final task was to identify more general strategies to manage barriers in policy formation and implementation. A main conclusion in this report is that a conscious application of these strategies may contribute significantly to successful formation and implementation of even controversial policies or policy packages.

  • 124167.
    Åkerman, Jonas
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Environmental Strategies.
    Höjer, Mattias
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Environmental Strategies. School of Computer Science and Communication (CSC), Centres, KTH, School of Architecture and the Built Environment (ABE), Centres, Centre for Sustainable Communications, CESC.
    How much transport can the climate stand? Sweden on a sustainable path in 20502006In: Energy Policy, ISSN 0301-4215, E-ISSN 1873-6777, Vol. 34, no 14, p. 1944-1957Article in journal (Refereed)
    Abstract [en]

    In this paper an image of a sustainable transport system for Sweden in 2050 is outlined. The emissions per capita in this image may be generalized to a global population of 9 billions, and still be consistent with a stabilization of the carbon dioxide concentration at 450ppm (parts per million). Swedish transport energy use per capita is 4.6MWh in the image, compared to 12.5MWh at present. The aim is, first, to widen the perspective of sustainable transport futures and, second, to provide a basis for present decisions in areas characterized by a high inertia, e.g. regarding infrastructure and the built-up environment. All transport generated by the lifestyles of Swedish residents are included. The reduction of energy use in the image is primarily achieved by an introduction of energy efficient vehicles and a conscious combination of IT-services and urban planning. The latter aims at increasing functional accessibility while reducing commuting. A prioritization of leisure travel to structurally-enforced travel gives the possibility to increase leisure travel per capita by one third. However, this is contingent on a 50% reduction of per capita car travel in cities. Given the set-up target, it may be concluded that the need for new arterial road capacity in cities often is negligible, even with a considerable population increase.

  • 124168.
    Åkerman, Jonas
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering.
    Höjer, Mattias
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering.
    Gunnarsson-Östling, Ulrika
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering.
    En Stockholmsregion utan klimatpåverkan från transport- och bebyggelsesektorn 2050: framtidsbilder och strategier i närtid2018Report (Other academic)
  • 124169. Åkerman, S.
    et al.
    Lindeberg, Tony
    KTH, School of Computer Science and Communication (CSC), Computational Biology, CB.
    Roland, P.
    Surface Model Generation and Segmentation of the Human Celebral Cortex for the Construction of Unfolded Cortical Maps1996In: Proc. 2nd International Conference on Functional Mapping of the Human Brain: HBM'96, published in Neuroimage, volume 3, number 3, 1996, p. S126-S126Conference paper (Refereed)
    Abstract [en]

    Representing the shape of the human cerebral cortex arises as a basic subproblem in several areas of brain science, such as when describing the anatomy of the cortex and when relating functional measurements to cortical regions. 

    Most current methods for building such representions of the cortical surface are either based on contours from two-dimensional cross sections or landmarks that have been obtained manually.

    In this article, we outline a methodology for semi-automatic contruction of a solely surface based representation of the human cerebral cortex in vivo for subsequent generation of  (unfolded) two-dimensional brain maps.

    The method is based on input data in the form of three-dimensional NMR images, and comprises the following main steps:

    • suppression of disturbing fine-scale structures by linear and non-linear scale-space techniques,
    • generation of a triangulated surface representation based on either iso-surfaces or three-dimensional edge detection,
    • division of the surface model into smaller segments based on differential invariants computed from the image data.

    When constructing an unfolded (flattened) surface representation, the instrinsic curvature of the cortex means that such a unfolding cannot be done without introducing distortions. To reduce this problem, we propose to cut the surface into smaller parts, where a ridge detector acts as guideline, and then unfold each patch individually, so as to obtain low distortions.

    Having a solely surface based representation of the cortex and expressing the image operations using multi-scale differential invariants in terms of scale-space derivatives as done in this work is a natural choice both in terms of conceptual and algorithmic simplicity. Moreover, explicitly handling the multi-scale nature of the data is necessary to obtain robust results.

  • 124170.
    Åkermark, Anne-Marie
    KTH, Superseded Departments, Machine Design.
    Environmentally adapted product development from the designers perspective1999Licentiate thesis, comprehensive summary (Other scientific)
  • 124171.
    Åkermark, Anne-Marie
    KTH, Superseded Departments, Machine Design.
    The Crucial Role of the Designer in EcoDesign2003Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    This goal of this thesis is to present an overview of thedesigner’s situation in Swedish companies in relation toenvironmental work in the product development process. Theoverall aim is to describe the designer’s situation andhis/her potential to minimize the environmental impact ofproducts.

    Awareness of the environmental problems has led to a newapproach to product development, and today EcoDesign is beingintegrated in many Swedish companies. However, if products areto be improved from an environmental aspect, it is not onlynecessary to integrate the issue in the product developmentprocess, but designers themselves must also be given thepossibility of actually designing environmentally friendlyproducts. This makes the designer a key actor in EcoDesign.

    There are different levels of support for designers duringthe various stages of product development. The most commonenvironmental work within Swedish companies is to perform LifeCycle Assessment (LCA) or to introduce an environmentalmanagement system (EMS), most commonly ISO 14000. In order tooptimize a product from an environmental view, it is necessarynot only to know how the product affects the environment butalso how to evaluate different solutions and concepts. In orderto do this, LCA can be a support but it does not lead to thefinding of new solutions or concepts. There is control of theproduct’s content and restrictions on substances thatcannot be used. The focus of the product’s environmentalimpact is not only in the manufacturing and use phase but alsoin the end-of-life phase. This focus is due to a new type oflegislation, the producer responsibility law. The overall aimof this regulation is to increase reuse and recycling. To makethis economically possible, the present efforts to adaptproducts to recycling must be vastly increased.

    Finally, in order to integrate the environmental issue inthe everyday work of designers it is necessary to have thefollowing conditions:

        Environmental expertise available

        An evaluation tool in order to confirm and choosealternatives

        Education and information for designers to spreadknowledge and to motivate them

        Constant stress on the importance of environmentaladaptation

        Documentation routines and guidelines connected to theproduct development process to ensure that the issue isconsidered at every step

    Integrating the issue in the product development process canclearly make significant environmental product improvements.For all companies, however, the overall aim is to make as muchmoney as possible, and it is evident that any major productchanges also have to lead to an increased customer benefit. Ifsignificant product changes are necessary for environmentalimprovements, further driving forces, such as governmentalregulations, may be needed in order to develop suchproducts.

    Keywords:EcoDesign, designers, environmental productdevelopment, design for environment.

  • 124172.
    Åkermark, Torbjörn
    KTH, Superseded Departments, Physics.
    Molecular or atomic oxygen as the transported species in oxidation of silicon2000In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 147, no 5, p. 1882-1887Article in journal (Refereed)
    Abstract [en]

    There is a general belief that the oxidation of silicon is due to the transport of molecular oxygen through the growing oxide. However, several recent experimental results have shown that atomic oxygen is also a possible candidate fur the transported species. This paper discusses which of these two candidates is the most likely transported species, considering both traditional arguments and these based on more recent experimental results. This discussion shows that the experimental results traditionally used as arguments for molecular oxygen being the transported species are equally valid for atomic oxygen as the transported species. However more recent experimental results strongly support atomic oxygen, not molecular oxygen, as the transported species. The conclusion is that atomic oxygen is the more likely candidate For the transported species in the oxidation of silicon.

  • 124173.
    Åkermark, Torbjörn
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Doremus, R. H.
    Comment on "oxidation of alloys containing aluminum and diffusion in Al2 O3 " [J. Appl. Phys. 95, 3217 (2004)]2005In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 97, no 11, p. 116109-Article in journal (Refereed)
    Abstract [en]

    The introduction of AlO as the diffusing species can be seen as an attempt to bridge the gap between the two scientific communities: those working on the oxidation of metals and those working on the oxidation of silicon. The attempt is, however, not successful and would have been more successful if the Wagner theory [O. Wagner, Z. Phys. Chem. Abt. B 21, 25 (1993)] would have been used to evaluate the mechanisms. There is also a lack of agreement with the two-stage oxidation experiment, oxidation first in O-16 and then in O-18. The experimental O-18 profile in the oxides formed cannot be explained by the diffusion of AlO, so it is unlikely that AlO is the diffusing species during oxidation.

  • 124174.
    Åkermark, Torbjörn
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Emmoth, Birger
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Bergsåker, Henric
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Laser annealing in combination with mass spectroscopy, a technique to study deuterium on tokamak carbon samples, a tool for detritiation2006In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 359, no 3, p. 220-226Article in journal (Refereed)
    Abstract [en]

    In this study, a method is presented based on mass spectroscopy to measure the a real density of deuterium on a graphite surface exposed to tokamak discharges. The studied sample was cut from a bumper limiter exposed in the TEXTOR tokamak and annealed by a 1 J Excimer laser (KrF). The energy used was 400 mJ cm(-2), which is below the threshold for ablation. 1 J cm(-2). The release of HD and D, was measured by a mass spectroscopy set-up and no other species released from the sample were detected in this experiment. The amount of D released from the sample after 20 laser pulses was measured to 7 x 10(16) D atoms per cm(-2) (for this particular sample) and most of the hydrogen at the surface was released in the first pulse, as checked by nuclear reaction analysis (NRA) techniques, which gave changes of the amount of deuterium before and after laser annealing. The sensitivity in this experiment was 5 x 10(14) atoms per cm(-2) for HD and 5 x 10(13) atoms per cm(-2) for D-2.

  • 124175.
    Åkermo, Malin
    KTH, Superseded Departments, Aeronautical Engineering.
    Compression moulding of thermoplastic composite sandwich components1999Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    The thesis investigates various aspects of compressionmoulding of sandwich components with thermoplastic-basedcomposite faces and thermoplastic or thermoset core.Compression moulding of glass/polypropylene (PP) faces toexpanded PP core is initially experimentally investigated usinga statistical experimental design approach. The influence ofvarious processing parameters is evaluated in terms of theshear, flexural and transverse tensile properties of mouldedcomponents. Results indicate that as long as the faces remainmelted when stacked with the core, the face-core bond strengthis always greater than the tensile strength of the core. Ingeneral properties seem to improve with reduced mouldingpressure and time.

    Bonding of thermoplastic composite faces to a thermosetclosed cell foam core is then modelled assuming that there is acorrelation between the amount of matrix penetrating into corecells at the surface and the bond strength. The model is usedin a parametric study showing that matrix penetration into corecells is initially a rapid process, but flow is interruptedbefore cells are filled owing to increasing pressure of the airtrapped in the cells. The model indicates that increasedmoulding pressure and reduced face temperature increase matrixpenetration into core cells. Initial experimental resultssupport model predictions.

    Compression moulding of thermoplastic composite faces to athermoplastic honeycomb core is also modelled focusing on theface-core bonding. A process model is developed from modelsdescribing the temperature distribution in the sandwich and theflow of the melted part of the core wall during moulding. Modelpredictions are compared to experimental data in terms of faceand core surface temperature and degree of face-coreinterfacial contact with reasonably good agreement. The modelis also used in a parametric study investigating the influenceof the major process variables on the face-core bondstrength.

    Finally, the process economy of compression moulding ofthermoplastic composite and sandwich components is modelled andcompared to predicted component costs for compression mouldingof sheet moulding compound and stamping of metals. A range ofcomponent sizes, complexities and flexural rigidities isinvestigated. The results imply that thermoplastic compositecomponents are mainly cost competitive for small to mediumsized or highly complex components and for short productionseries. Sandwich components seem to offer enhanced processeconomy for components requiring high flexural rigidity,however, still for short to intermediate production series.

    Keywords: sandwich, thermoplastic, composite,compression moulding, modelling, bonding, heat transfer, matrixflow, cost, testing

  • 124176.
    Åkermo, Malin
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Thermoforming of closed cell polymer foam and its residual compressive mechanical properties2008In: 8th International Conference on Sandwich Structures, FEUP edições (Faculdade de Engenharia da Universidade do Porto Edicoes), 2008Conference paper (Refereed)
  • 124177.
    Åkermo, Malin
    et al.
    KTH, Superseded Departments, Aeronautical and Vehicle Engineering.
    Astrom, B. T.
    Modeling compression molding of all-thermoplastic honeycomb core sandwich components. Part A: Model development2000In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 21, no 2, p. 245-256Article in journal (Refereed)
    Abstract [en]

    The compression molding process is studied with the aim of modeling the instantaneous degree of face-core bonding in all-thermoplastic sandwich components during molding. The theory of bonding is briefly discussed and it is concluded that for most thermoplastic materials, processing is performed at temperatures where full bond strength is seemingly immediately established at positions in full face-core contact. A two-dimensional model is developed to predict increase in contact area due to flow of melted core material during molding. Further, heat transfer during processing is modeled in order to determine the extent of melted core material. The two models are coupled into one process model and a numerical example is presented illustrating the predicted behavior of polypropylene-based sandwich components in compression molding. The process model suggests that the face-core bond strength may be significantly increased through flow of the melted core wall.

  • 124178.
    Åkermo, Malin
    et al.
    KTH, Superseded Departments, Aeronautical and Vehicle Engineering.
    Astrom, B. T.
    Modeling compression molding of all-thermoplastic honeycomb core sandwich components. Part B: Model verification2000In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 21, no 2, p. 257-267Article in journal (Refereed)
    Abstract [en]

    The compression molding process for manufacturing of all-thermoplastic honeycomb core sandwich components is studied with the aim of verifying a previously proposed model describing the instantaneous face-core bonding during molding. Experiments are performed with glass/polypropylene (PP) faces and PP honeycomb core and experimental data are compared to model predictions in terms of temperature and degree of face-core interfacial contact with reasonable good agreement. Model predictions show that the face-core interface bond strength may increase with up to 200% when molding at an interface temperature slightly above the crystal melting temperature of the matrix for only a few seconds. The influence of the major process parameters, including molding temperature, molding pressure and core viscosity, on the predicted bond strength is investigated using the model.

  • 124179.
    Åkermo, Malin
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Larberg, Ylva
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallander, Per
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Forming of advanced components out of pre-stacked crossplied UD prepreg2011In: 18th International Conference on Composite Materials, ICCM-18, 2011Conference paper (Refereed)
  • 124180.
    Åkermo, Malin
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Larberg, Ylva
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Sjölander, Jens
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallnader, Per
    Saab.
    Influence of interply friction on the forming of stacked prepreg2013In: Proceedings of 19th International Conference on Composite Materials (ICCM19), Curran Associates, Inc., 2013, p. 919-928Conference paper (Refereed)
  • 124181.
    Åkerstedt, Josefin
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Subvalent Cluster Compounds and Synthesis in Alternative Reaction Media2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    With the aim of finding alternative reaction media for the synthesis of subvalent main group and transition metal cluster compounds, traditionally made through solid state reactions or in superacidic media, different alternative reaction media have been explored in this work. Room-temperature ionic liquids are amongst the more unconventional reaction media used. The syntheses performed have been aimed at both anionic and cationic cluster and the main tools used for characterization have been different X-ray diffraction and spectroscopic techniques.

    Selected ionic liquids have along with dichloromethane been shown to work as alternative reaction media for room temperature synthesis of the Bi5[GaCl4]3 salt. The salt containing the subvalent naked bismuth polycation Bi5 3+ was isolated from reduction reactions of BiCl3 in Ga/GaCl3-dichloromethane respectively Ga/GaCl3-ioinc liquid media. Three different classes of ionic liquids based on phosphonium-, imidazolium- and pyrrolidinium- salts have been used in synthesis. Homopolyatomic clusters from the lighter Group 15 element arsenic have also been studied. Solutions from the oxidative and reductive reaction routes of arsenic and AsCl3 in Lewis acidic toluene media were studied by EXAFS spectroscopy. The results were evaluated using molecular dynamics simulations of arsenic clusters. A discussion on how the calculated As4 cluster model relates to the experimental data resulted from this study.

    In terms of homopolyatomic anionic clusters the [K+(2,2,2-crypt)]2Ge9 2- compound containing the naked Ge9 2- anionic cluster has been isolated. The crystallographic investigation of [K+(2,2,2-crypt)]2Ge9 2- shows Zintl cluster anion Ge9 2- to be tricapped trigonal-prismatic with a symmetry very close to D3h. A chemical bonding analysis reveals two local minima of D3h symmetry and the cluster interaction scheme to be based on highly delocalised bonding.

    Ligand supported transition metal clusters from tungsten and palladium have also been prepared. Reduction of WCl6 in a reaction mixture of ionic liquid and co-solvent toluene resulted in tritungsten decachloride; W3Cl10(MeCN)3, being formed. Furthermore, palladium sandwich compounds; [Pd2(Ga2Cl7)(C7H8)2], [Pd2(GaCl4)(C9H12)2]∙C9H12 and [Pd2(Ga2Cl7)(C6H5Cl)2] have been prepared using GaCl3-arene reaction media.

  • 124182.
    Åkerstedt, Josefin
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Inorganic Chemistry.
    Tailored Reaction Media for the Synthesis of Subvalent Cluster Compounds2010Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Different synthetic approaches and modifications to reaction media have been applied in order to find new routes to the synthesis of main-group clusters and transition metal compounds. The focus has been on the Group 15 element bismuth and the transition metal palladium. The reactions performed have mainly been Lewis acid-base reactions and the characterization tools used X-ray diffraction and Raman spectroscopy.

    The Bi5[GaCl4]3 salt, previously known form synthesis using other reaction routes, has been isolated from GaCl3-dichloromethane media. The salt containing the subvalent naked bismuth polycation Bi53+ was isolated from the reduction of bismuth(III) chloride. Quantum chemical calculations on the interaction energies of the isolated bismuth cluster show the dichloromethane-cluster interaction energy to be higher than that between benzene and the cluster.

    Ionic liquids have, in addition to dichloromethane, been shown to work as alternative reaction media for room-temperature synthesis of the Bi5[GaCl4]3 salt. Three different classes of ionic liquids have been used; phosphonium-, imidazolium- and pyrrolidinium-based salts. The ionic liquids used have been treated with Lewis acids in order to promote cluster formation.

    In this work three new palladium sandwich compounds have also been isolated, using GaCl3-arene reaction media; [Pd2(Ga2Cl7)(C7H8)2], [Pd2(GaCl4)(C9H12)2]∙C9H12 and [Pd2(Ga2Cl7)(C6H5Cl)2]. Quantum chemical calculations on these palladium sandwiches show the chlorobenzene sandwiching ligands unexpectedly interacting more strongly with the dipalladium unit than the methyl substituted arenes.

  • 124183.
    Åkerstedt, Josefin
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Inorganic Chemistry (closed 20110630).
    Gorlov, Mikhail
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Organic Chemistry.
    Fischer, Andreas
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Inorganic Chemistry (closed 20110630).
    Kloo, Lars
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Inorganic Chemistry (closed 20110630).
    Synthesis and characterization of binuclear palladium(I) compounds and the influence of competing arenes2010In: Journal of Organometallic Chemistry, ISSN 0022-328X, E-ISSN 1872-8561, Vol. 695, no 10-11, p. 1513-1517Article in journal (Refereed)
    Abstract [en]

    The binuclear palladium(I) compounds, [Pd-2(Ga2Cl7)(2)(C7H8)(2)] (1), [Pd-2(GaCl4)(2)(C9H12)(2)]center dot C9H12 (2) and [Pd-2(Ga2Cl7)(2)(C6H5Cl)(2)] (3), have been prepared from palladium(II) chloride in gallium(III) chloride-arene reaction media. All isolated crystalline solids (1, 2 and 3) have been structurally characterized by single crystal X-ray diffraction and Raman spectroscopy. The results form quantum chemical calculations on the interaction energies of the arenes and the dipalladium unit of these compounds is also presented.

  • 124184.
    Åkerstedt, Josefin
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Gorlov, Mikhail
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Kloo, Lars
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Room-Temperature Synthesis of the Bi-5[GaCl4](3) Salt From Three Different Classes of Ionic Liquids2013In: Journal of cluster science, ISSN 1040-7278, E-ISSN 1572-8862, Vol. 24, no 1, p. 157-164Article in journal (Refereed)
    Abstract [en]

    Following the development in the synthesis of subvalent cluster compounds, we report on the use of three different classes of room-temperature ionic liquids for the synthesis of the pentabismuth-tris(tetragallate) salt, Bi-5[GaCl4](3), characterized by X-ray diffraction. The Bi-5[GaCl4](3) salt was prepared by reduction of BiCl3 using gallium metal in ionic liquid reaction media containing a strong Lewis acid, GaCl3. The ionic liquids; trihexyltetradecyl phosphonium chloride [Th-Td-P+]Cl-, 1-dodecyl-3-methylimidazolium chloride [Dod-Me-Im(+)]Cl- and N-butyl-N-methylpyrrolidinium chloride [Bu-Me-Pyrr(+)]Cl- from three of the main classes of ionic liquids were used in synthesis. Reactions using ionic liquids composed of the trihexyltetradecyl phosphonium cation [Th-Td-P+] and the anions; tetrafluoroborate [BF4 (-)], bis(trifluoro-methyl sulfonyl) imide [(Tf)(2)N-] and hexafluorophosphate [PF6 (-)] were also investigated.

  • 124185.
    Åkerstedt, Josefin
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Inorganic Chemistry (closed 20110630).
    Gorlov, Mikhail
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Organic Chemistry.
    Kloo, Lars
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Inorganic Chemistry (closed 20110630).
    Synthesis of Bi5[GaCl4]3 from room-temperature ionic liquid reaction mediaManuscript (preprint) (Other academic)
  • 124186.
    Åkerstedt, Josefin
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Inorganic Chemistry.
    Ponou, Simeon
    Kloo, Lars
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Inorganic Chemistry.
    Lidin, Sven
    Structural Investigation of a Fully Ordered closo-Ge(9)(2-) Cluster in the Compound [K(+)(2,2,2-crypt)](2)Ge(9)(2-)2011In: European Journal of Inorganic Chemistry, ISSN 1434-1948, E-ISSN 1099-1948, no 26, p. 3999-4005Article in journal (Refereed)
    Abstract [en]

    [K(+)(2,2,2-crypt)] (2)Ge(9)(2-) containing the deltahedral Zintl anion Ge(9)(2-) has been isolated from a solution of K(4)Ge(9) in ethylenediamine in the presence of 2,2,2-crypt, intended as an intermediate in mixed-cluster synthesis. The subsequent crystallographic characterisation shows the closo-Ge(9)(2-) cluster unit to be tricapped trigonal-prismatic with a symmetry very close to D(3h). Each Ge(9)(2-) unit is surrounded by two 2,2,2-crypt units, each with a chelated K(+) cation, viz. [K(+)(2,2,2-crypt)]. The structure characterisation of this ordered Ge(9)(2-) cluster is surprisingly enough the first one reported. A chemical bonding analysis reveals two local minima of D(3h) symmetry, with that of lowest energy being highly similar to that resulting from the crystallographic analysis of low-temperature data. The cluster interaction scheme is based on highly delocalised bonding.

  • 124187.
    Åkerstedt, Josefin
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Rosdahl, Jan
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Persson, Per
    Umeå University.
    Kloo, Lars
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Arsenic Clusters in Solution: An Experimental and A Priori Theoretical EXAFS StudyManuscript (preprint) (Other academic)
  • 124188.
    Åkerstedt, Josefin
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Ruck, M.
    Kloo, Lars
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Long, J. R.
    Tungsten Chloride W3Cl10(CH3CN)3 fromRoom-temperature Synthesis in Ionic liquid using an Organic Co-solventManuscript (preprint) (Other academic)
  • 124189.
    Åkerstedt, Josefin
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Inorganic Chemistry.
    Zaffaroni, Riccardo
    Kloo, Lars
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Inorganic Chemistry.
    Dichloromethane as solvent for the synthesis of polycationic clusters at room temperature: a link to standard organometallic chemistry2010In: Dalton Transactions, ISSN 1477-9226, E-ISSN 1477-9234, Vol. 39, no 35, p. 8132-8134Article in journal (Refereed)
    Abstract [en]

    Dichloromethane (DCM), CH2Cl2 has been identified as an alternative reaction medium for room-temperature synthesis of subvalent homopolyatomic clusters of the group 15 element bismuth.

  • 124190.
    Åkerstrand, Hampus
    KTH, School of Industrial Engineering and Management (ITM), Industrial Economics and Management (Dept.), Economics.
    Do Housing Prices Affect Loan Supply?: Evidence from Sweden During the Post-Crisis Period2018Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Financial intermediaries are paramount for modern society. During the last decade, however, our reliance on these institutions have been meticulously debated, especially in the aftermath of the financial crisis. This thesis contributes to this debate with a novel perspective on loan supply changes in light of the recent events in the Swedish real estate market. More specifically, it investigates what influence housing prices have on the supply of commercial and industrial loans. This is done by estimating dynamic panel data models using a quarterly panel containing balance sheet data for 68 Swedish monetary financial institutions, during the post-financial crisis period of 2009-2017. The results indicate that housing prices do not have a significant effect on commercial and industrial loan supply. However, these loans are to a considerable degree dependent on the institutes’ earlier levels of commercial and industrial loans.

  • 124191.
    Åkervik, Espen
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Feedback Control of Spatially Evolving Flows2007Licentiate thesis, comprehensive summary (Other scientific)
    Abstract [en]

    In this thesis we apply linear feedback control to spatially evolving flows in order to minimize disturbance growth. The dynamics is assumed to be described by the linearized Navier--Stokes equations. Actuators and sensor are designed and a Kalman filtering technique is used to reconstruct the unknown flow state from noisy measurements. This reconstructed flow state is used to determine the control feedback which is applied to the Navier--Stokes equations through properly designed actuators. Since the control and estimation gains are obtained through an optimization process, and the Navier--Stokes equations typically forms a very high-dimensional system when discretized there is an interest in reducing the complexity of the equations. One possible approach is to perform Fourier decomposition along (almost) homogeneous spatial directions and another is by constructing a reduced order model by Galerkin projection on a suitable set of vectors. The first strategy is used to control the evolution of a range of instabilities in the classical family of Falkner--Skan--Cooke flows whereas the second is applied to a more complex cavity type of geometry.

  • 124192.
    Åkervik, Espen
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Global stability and feedback control of boundary layer flows2008Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    In this thesis the stability of generic boundary layer flows is studied from a global viewpoint using optimization methods. Global eigenmodes of the incompressible linearized Navier-Stokes equations are computed using the Krylov subspace Arnoldi method. These modes serve as a tool both to study asymptotic stability and as a reduced basis to study transient growth. Transient growth is also studied using adjoint iterations. The knowledge obtained from the stability analysis is used to device systematic feedback control in the Linear Quadratic Gaussian framework. The dynamics is assumed to be described by the linearized Navier-Stokes equations. Actuators and sensors are designed and a Kalman filtering technique is used to reconstruct the unknown flow state from noisy measurements. This reconstructed flow state is used to determine the control feedback which is applied to the Navier-Stokes equations through properly designed actuators. Since the control and estimation gains are obtained through an optimization process, and the Navier-Stokes equations typically forms a very high-dimensional system when discretized there is an interest in reducing the complexity of the equations. A standard method to construct a reduced order model is to perform a Galerkin projection of the full equations onto the subspace spanned by a suitable set of vectors, such as global eigenmodes and balanced truncation modes.

  • 124193.
    Åkervik, Espen
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Hoepffner, Jérôme
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Marxen, Olaf
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Steady solutions of the Navier-Stokes equations by selective frequency damping2006In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 18, no 6, p. 068102-Article in journal (Refereed)
    Abstract [en]

    A new method, enabling the computation of steady solutions of the Navier-Stokes equations in globally unstable configurations, is presented. We show that it is possible to reach a steady state by damping the unstable (temporal) frequencies. This is achieved by adding a dissipative relaxation term proportional to the high-frequency content of the velocity fluctuations. Results are presented for cavity-driven boundary-layer separation and a separation bubble induced by an external pressure gradient.

  • 124194.
    Åkervik, Espen
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Ehrenstein, Uwe
    IRPHÉ, Université de Provence.
    Gallaire, Francois
    Laboratoire J.A. Dieudonné.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Global two-dimensional stability measures of the flat plate boundary-layer flow2008In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 27, no 5, p. 501-513Article in journal (Refereed)
    Abstract [en]

    The stability of the two-dimensional flat plate boundary-layer is studied by means of global eigenmodes. These eigenmodes depend both on the streamwise and wall-normal coordinate, hence there are no assumptions on the streamwise length scales of the disturbances. Expanding the perturbation velocity field in the basis of eigenmodes yields a reduced order model from which the stability characteristics of the flow, i.e. the initial condition and forcing function leading to the largest energy growth, are extracted by means of non-modal analysis. In this paper we show that, even when performing stability analysis using global eigenmodes, it is not sufficient to consider only a few of the least damped seemingly relevant eigenmodes. Instead it is the task of the optimization procedure, inherent in the non-modal analysis, to decide which eigenmodes are relevant. We show that both the optimal initial condition and the optimal forcing structure have the form of upstream tilted structures. Time integration reveals that these structures gain energy through the so called Orr mechanism, where the instabilities extract energy from the mean shear. This provides the optimal way of initiating Tollmien-Schlichting waves in the boundary layer. The optimal initial condition results in a localized Tollmien-Schlichting wavepacket that propagates downstream, whereas the optimal forcing results in a persistent Tollmien-Schlichting wave train.

  • 124195.
    Åkervik, Espen
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Haepffner, Jerome
    Ehrenstein, Uwe
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Model reduction and control of a cavity-driven separated boundary layer2008In: IUTAM Symposium On Flow Control And Mems / [ed] Morrison, JF; Birch, DM; Lavoie, P, 2008, Vol. 7, p. 147-155Conference paper (Refereed)
    Abstract [en]

    The control of a globally unstable boundary-layer flow along a two-dimensional cavity is considered. When perturbed by the worst-case initial condition, the flow exhibits a large transient growth associated with the development of a wave packet along the cavity shear layer followed by a global cycle related to the least stable global eigenmodes. The flow simulation procedure is coupled to a measurement feedback controller, which senses the wall shear stress at the downstream lip of the cavity and actuates at the upstream lip. A reduced model for the control optimization is obtained by a projection on the least stable global eigenmodes. The LQG controller is run in parallel to the Navier-Stokes time integration. It is shown that the controller is able to damp out the global oscillations.

  • 124196.
    Åkervik, Espen
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hoepffner, Jérôme
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Uwe, Eherenstein
    IRPH́E, Université de Provence.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Optimal growth, model reduction and control in a separated boundary-layer flow using global eigenmodes2007In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 579, p. 305-314Article in journal (Refereed)
    Abstract [en]

    Two-dimensional global eigenmodes are used as a projection basis both for analysing the dynamics and building a reduced model for control in a prototype separated boundary-layer flow. In the present configuration, a high aspect ratio smooth cavity-like geometry confines the separation bubble. Optimal growth analysis using the reduced basis shows that the sum of the highly non-normal global eigenmodes are able to describe a localized disturbance. Subject to this worst-case initial condition, a large transient growth associated with the development of a wavepacket along the shear layer followed by a global cycle related to the two unstable global eigenmodes is found. The flow simulation procedure is coupled to a measurement feedback controller, which senses the wall shear stress at the downstream lip of the cavity and actuates at the upstream lip. A reduced model for the control optimization is obtained by a projection on the least stable global eigenmodes, and the resulting linear-quadratic-gaussian controller is applied to the Navier--Stokes time integration. It is shown that the controller is able to damp out the global oscillations.

  • 124197.
    Åkervik, Mattias
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Network Gaming: Performance and Traffic Modeling2006Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    There are several different types of games that are played in multiplayer mode over networks. The type of network games that, from a network’s perspective, are the most demanding is real-time based multiplayer games. Users of such games both assume and require that game play interaction happens in near real-time and these games often support a large number of simultaneous players. Most networks are specialized to either voice traffic (such as the first and second generation of mobile networks) or data traffic (such as wired data networks). It is not clear that the requirements for such real time games can always be met on either type of network. The core of this thesis investigates the performance requirements real-time multiplayer games place on packet switched data networks and the connection between network impairments and game quality degradation. Traffic generated by network games distinguishes itself from other traffic both regarding its general characteristics and the requirements it places on the network. Understanding these traffic characteristics, requirements, and what consequences failures to support such requirements entail are of great importance when designing new networks in order to guarantee suitable quality of service for such real-time games.

  • 124198. Åkeson, M.
    et al.
    Nordberg, Markus
    KTH, School of Industrial Engineering and Management (ITM), Production Engineering.
    Ehlerding, Anneli
    KTH, School of Industrial Engineering and Management (ITM), Production Engineering.
    Nilsson, L. -E
    Östmark, H.
    Strömbeck, P.
    Picosecond laser pulses improves sensitivity in standoff explosive detection2011In: Proc SPIE Int Soc Opt Eng, 2011Conference paper (Refereed)
    Abstract [en]

    Portendo has in collaboration with FOI, the Swedish Defence Research Agency, developed a world-leading technique of trace detection of explosives at standoff distance using Raman spectroscopy. The technology is further developed in order to enhance the sensitivity of the method and to be able to extend the field of applications. Raman scattering is a well-established technique able to detect substances down to single micrograms at standoff distances, however, one of the obstacles limiting the detection possibilities is interfering fluorescence, originating either from the substance itself or from the surrounding material. One main challenge for this technology is thus to either omit the excitation of the fluorescent process altogether or to be able to separate the two processes and only detect the Raman signal. Due to the difference in the temporal behavior of the two processes - Raman scattering occurs in the order of femtoseconds while fluorescence typically has a lifetime in the order of nanoseconds - one way to theoretically separate them is to limit the measurement to as short time as possible, cutting off most of the emitted fluorescence. The improvement depends on how much of the fluorescence can be omitted without decreasing the Raman signal. Experimentally, we have verified this improvement in signal to noise ratio when using a laser with picosecond pulses instead of nanosecond pulses, which has resulted in an improvement in SNR of up to 7 times for bulk ANFO. These results verify the predicted signal enhancement and suggest higher sensitivity for standoff detection in future systems.

  • 124199.
    Åkesson, Anna
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Hydraulic Engineering.
    Hydraulic- hydromorphologic analysis as an aid for improving peak flow predictions2010Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Conventional hydrological compartmental models have been shown to exhibit a high degree of uncertainty for predictions of peak flows, such as the design floods for design of hydropower infrastructure. One reason for these uncertainties is that conventional models are parameterised using statistical methods based on how catchments have responded in the past. Because the rare occurrence of peak flows, these are underrepresented during the periods used for calibration. This implies that the model has to be extrapolated beyond the discharge intervals where it has been calibrated.

    In this thesis, hydromechanical approaches are used to investigate the properties of stream networks, reflecting mechanisms including stage dependency, damming effects, interactions between tributaries (network effects) and the topography of the stream network. Further, it is investigated how these properties can be incorporated into the streamflow response functions of compartmental hydrological models.

    The response of the stream network was shown to vary strongly with stage in a non-linear manner, an effect that is commonly not accounted for in model formulation. The non-linearity is particularly linked to the flooding of stream channels and interactions with the flow on flood-plains.

    An evaluation of the significance of using physically based response functions on discharge predictions in a few sub-catchments in Southern Sweden show improvements (compared to a conventional model) in discharge predictions – particularly when modelling peak discharges.

    An additional benefit of replacing statistical parameterisation methods with physical parameterisation methods is the possibility of hydrological modelling during non-stationary conditions, such as the ongoing climate change.

  • 124200.
    Åkesson, Anna
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Peakflow response of stream networks: implications of physical descriptions of streams and temporal change2015Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Through distributed stream network routing, it has quantitatively been shown that the relationship between flow travel time and discharge varies strongly nonlinearly with stream stage and with catchment-specific properties.

    Physically derived distributions of water travel times through a stream network were successfully used to parameterise the streamflow response function of a compartmental hydrological model. Predictions were found to improve compared to conventional statistically based parameterisation schemes, for most of the modelled scenarios, particularly for peakflow conditions.

    A Fourier spectral analysis of 55-110 years of daily discharge time series from 79 unregulated catchments in Sweden revealed that the discharge power spectral slope has gradually increased over time, with significant increases for 58 catchments. The results indicated that the catchment scaling function power spectrum had steepened in most of the catchments for which historical precipitation series were available. These results suggest that (local) land-use changes within the catchments may affect the discharge power spectra more significantly than changes in precipitation (climate change).

    A case study from an agriculturally intense catchment using historical (from the 1880s) and modern stream network maps revealed that the average stream network flow distance as well as average water levels were substantially diminished over the past century, while average bottom slopes increased. The study verifies the hypothesis that anthropogenic changes (determined through scenario modelling using a 1D distributed routing model) of stream network properties can have a substantial influence on the travel times through the stream networks and thus on the discharge hydrographs.

    The findings stress the need for a more hydrodynamically based approach to adequately describe the variation of streamflow response, especially for predictions of higher discharges. An increased physical basis of response functions can be beneficial in improving discharge predictions during conditions in which conventional parameterisation based on historical flow patterns may not be possible - for example, for extreme peak flows and during periods of nonstationary conditions, such as during periods of climate and/or land use change.

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