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  • 60001. Åkerstedt, Hans
    Velocity shear instabilition associated with plasma motion across a magnetic field1984In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 29, no 1, p. 75-86Article in journal (Refereed)
    Abstract [en]

    Velocity shear instabilition associated with plasma motion across a magnetic fie .a{background-color: #ffff88}.h{background-color: #ffff00}.b{background-color: #ccccff}.i{background-color: #3333ff}.c{background-color: #ffcccc}.j{background-color: #ff3333}.d{background-color: #bbffff}.k{background-color: #00ffff}.e{background-color: #ffdd77}.l{background-color: #ff6600}.f{background-color: #ccffaa}.m{background-color: #33ff00}.g{background-color: #ffccff}.n{background-color: #ff00ff} a:link {color: #0000ff} a.oa:link {color: #00a000} a:visited {color: #4000a0} a.oa:visited {color: #006666} a:hover {color: #ff0000} a.oa:hover {color: #cc33cc} The stability of stationary Vlasov equilibria simulating the motion of plasma across a transverse magnetic field has been analyzed. The stability analysis is based on the Vlasov-Poisson equations together with an assumption of finite larmor radius ordering. Especially the instabilities associated with velocity shear transitions separating plasmas moving in opposite directions have been considered. Two extreme cases of such velocity shear transitions are studied, one where the confining current is only due to ions and one where the containing current is only due to electrons.

  • 60002.
    Åkerstedt, Hans
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Jansson, Ida
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    The stability of a flexibly mounted rotating cylinder in turbulent annular flow2015In: Journal of Fluids and Structures, ISSN 0889-9746, E-ISSN 1095-8622, Vol. 58, p. 152-172Article in journal (Refereed)
    Abstract [en]

    In this paper, we determine the inviscid linear stability with respect to two-dimensional disturbances of a fluid flow between two concentric cylinders. The inner rigid cylinder rotates with the angular velocity Ω0 and is fixed on elastic hinges at each end in the transverse direction. The outer cylinder does not rotate and is rigidly fixed. We assume that the fluid flow has an inner core that rotates as a solid body with angular velocity Ω0/2 and outside the core there are turbulent boundary layers. The velocity profile of the turbulent boundary layers satisfies the viscous Camassa−Holm equations. The perturbed fluid flow is derived from Rayleigh’s equation. The analysis yields an equation of motion of the cylinder equivalent to previous work without boundary layers and a basic flow of constant angular vorticity. The analysis is not restricted to a small gap between the cylinders. The results are compared with the results by Antunes et al. (1996), who consider a similar problem with uniform velocity profile and the limit of small gap. For ρc/ρf<1ρc/ρf<1 the results disagree in that the present analysis shows stability whereas Antunes et al. find instability. For ρc/ρf>1ρc/ρf>1 both theories predict stability and for larger values of ρc/ρfρc/ρf the agreement is good especially for small gap.

  • 60003.
    Åkerstedt, Hans
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, Staffan
    Högberg, Sofie
    Electrostatic filtration of air-borne nano-particles2007In: Journal of Nanostructured Polymers and Nanocomposites, ISSN 1790-4439, Vol. 3, no 4, p. 111-115Article in journal (Other academic)
    Abstract [en]

    Due to potential danger of nanoparticles for human health it is necessary to have control of their motion in air. Mechanical filtering is tricky since the particles are very small and alternative means to guide and collect them, need to be developed. Hence, in this paper clusters of carbon nanotubes are allowed to move through an electrical field generated by two parallel electrodes and it is shown that the tubes are strongly affected by this field and move swiftly towards the electrodes. Once hitting one of them most of the clusters leave towards the other electrode, the procedure is repeated and very few particles are collected. By putting an electrically insulating layer on one plate the particles however get stuck. This implies that the method investigated is an efficient method to collect air-borne carbon nanotubes

  • 60004.
    Åkerstedt, Hans O.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Deposition of charged nano-particles in the human airways including effects from cartilaginous rings2011In: Natural Science, ISSN 2150-4091, E-ISSN 2150-4105, Vol. 3, no 10, p. 885-888Article in journal (Refereed)
    Abstract [en]

    This paper presents a numerical study of the deposition of spherical charged nano-particles caused by convection, Brownian diffusion and electrostatics in a pipe with a cartilaginous ring structure. The model describes the deposition of charged particles in the different generations of the tracheobronchial tree of the human lung. The upper airways are characterized by a certain wall structure called cartilaginous rings which modify the particle deposition when compared to an airway with a smooth wall. The problem is defined by solving Naver-Stokes equations in combination with a convective-diffusion equation and Gauss law for electrostatics. Three non- dimensional parameters describe the problem, the Peclet number Pe = 2ūa/D , the Reynolds number Re = ūa/v and an electrostatic parameter α=α2c0q2/(4ε0κT) . Here U is the mean velocity, a the pipe radius and D the diffusion coefficient due to Brownian motion given by D=κTCu/3πμd , where Cu is the Cunningham-factor Cu=1+λ/d(2.34+1.05exp(-0.39d/λ)) Here d is the particle diameter and λ the mean free path of the air molecules. Results are provided for generations G4-G16 of the human airways. The electrostatic parameter is varied to model different concentrations and charge numbers

  • 60005.
    Åkerstedt, Hans O.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Deposition of submicron charged particles in turbulent pipe flow with an application to trachea2018In: The International Journal of Multiphysics, ISSN 1750-9548, E-ISSN 2048-3961, Vol. 12, no 1, p. 9-25Article in journal (Refereed)
    Abstract [en]

    The paper presents a study of the deposition of submicron charged spherical particles caused by convection, Brownian and turbulent diffusion in a pipe with a smooth wall and with a cartilaginous ring wall structure. The model is supposed to describe deposition of charged particles in generation 0 (trachea) of the human respiratory airways. The flow is modeled with a SST-turbulence model combined with a convective-diffusion equation including electric field migration for the particles, and Poisson’s equation for the determination of the electrostatic potential in terms of the space-charge density of the particles. An approximate analytical solution is derived for the case of a smooth pipe which is used to verify the numerical solutions obtained from using the commercial software Comsol Multiphysics. Numerical results of deposition rates are also provided for the case of a pipe with a cartilaginous ring wall structure.

  • 60006. Åkerstedt, Hans O.
    Finite larmor radius effects on the stability properties of internal modes of a Z-pinch1988In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 37, no 1, p. 117-130Article in journal (Refereed)
    Abstract [en]

    From the Vlasov-fluid model a set of approximate stability equations describing the stability of a cylindrically symmetric z-pinch is derived. The equations are derived in the limit of small gyroradius and include first order kinetic effects such as finite ion Larmor radius effects and resonant ion effects. Neglecting the resonant ion terms, we explicitly solve this set of equations for a constant current density profile leading to a dispersion relation. FLR effects are shown for the case of m = 1 internal mode to be stabilizing and for large wavenumbers k, using a trial function approach, absolute stabilization is found.

  • 60007. Åkerstedt, Hans O.
    Gyrokinetic stability theory of z–pinches1990In: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, Vol. 44, p. 137-149Article in journal (Refereed)
    Abstract [en]

    From the Vlasov-fluid model a set of approximate stability equations describing the stability of the pure z–pinch is derived. The equations are valid for equilibria with small gyroradius compared with the pinch radius, but the perturbation wavenumber k may be of the order of the gyroradius ρi, δ = kρi = 0(1) - so-called gyrokinetic ordering. The equations are used to study the stability of the m = 0 and m = 1 internal modes of the z–pinch. In the limit of zero gyroradius δ → 0 we recover previously obtained results. For δ ≠ 0 we find that increasing δ at first gives a rapidly decreasing growth rate, and for δ ≈ l the growth rate compared with perpendicular MHD is γ/γMHD ≈ 0·09. For larger δ however, the growth rate increases to a quite large value. For the m = O mode we find, provided that drift resonances can be neglected, a stability criterion for δ ≥ 1, which is fulfilled both for the Bennett equilibrium and the constant-current-density equilibrium.

  • 60008. Åkerstedt, Hans O.
    Kinetic effects on the ideal Alfvén continuum1990In: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, Vol. 44, p. 507-516Article in journal (Refereed)
    Abstract [en]

    Kinetic effects such as those of finite Larmor radius (FLR) and. wave–particle resonances on the ideal Alfvén-wave continuum in a z-pinch geometry are investigated. The effect of FLR increases the order of the equations and is therefore treated as a singular perturbation. Similar eigenvalue curves are found as for the case when the non-ideal effect is resistivity, with a bifurcation of a global-mode branch into a boundary-mode branch and an internal-mode branch.

  • 60009. Åkerstedt, Hans O.
    Some kinetic equilibria and their stability1985Doctoral thesis, comprehensive summary (Other academic)
  • 60010.
    Åkerstedt, Hans O.
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Eller, Sebastian
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, Staffan T.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Numerical Investigation of Turbulent Flow through Rectangular and Biconvex Shaped Trash Racks2017In: Engineering, ISSN 1947-3931, E-ISSN 1947-394X, Vol. 9, no 5, p. 412-426, article id 76642Article in journal (Refereed)
    Abstract [en]

    Turbulent flow through a trash rack of bars of rectangular and biconvex shapes is considered. A trash rack is composed of an array of bars fitted into a hydro-electric power station to prevent debris and fish to enter the waterway towards the turbine. The work is directed towards modeling a large number of bars for which the flow turn out to have a periodic structure. It is here shown that this case can be simplified with the flow past a single bar together with periodic boundary conditions. Using this approach the head loss is derived for different angles of attack α and blockages P for two shapes of the rack, a rectangular bar and an aerodynamically shaped biconvex bar. It is found that overall loss of the biconvex bars is in general about 15% of the loss for the rectangular case for small angles of attack. For large angle of attack this difference diminishes. Of interest for the biconvex bars is also a local minimum in the head loss for angles approximately greater than 20˚ and for a blockage P around 0.35. This combination of parameters gives a low loss together with an efficient barrier for debris and fishes.

  • 60011.
    Åkerstedt, Hans O.
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Högberg, Sofie
    Lundström, Staffan
    Sandström, Tomas
    Umeå universitet.
    The effect of cartilaginous rings on particle deposition by convection and Brownian diffusion2010In: Natural Science, ISSN 2150-4091, E-ISSN 2150-4105, Vol. 2, no 7, p. 769-779Article in journal (Refereed)
    Abstract [en]

    A finite volume method for the numerical solution of viscoelastic flows is given. The flow of a differential upper-convected Maxwell (UCM) fluid through a contraction channel has been chosen as a prototype example. The conservation and constitutive equations are solved using the finite volume method (FVM) in a staggered grid with an upwind scheme for the viscoelastic stresses and a hybrid scheme for the velocities. An enhanced-in-speed pressure-correction algorithm is used and a method for handling the source term in the momentum equations is employed. Improved accuracy is achieved by a special discretization of the boundary conditions. Stable solutions are obtained for higher Weissenberg number (We), further extending the range of simulations with the FVM. Numerical results show the viscoelasticity of polymer solutions is the main factor influencing the sweep efficiency.

  • 60012.
    Åkerstedt, Hans O.
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Högberg, Sofie
    Lundström, T. Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    An asymptotic approach of Brownian deposition of nanofibres in pipe flow2013In: Theoretical and Computational Fluid Dynamics, ISSN 0935-4964, E-ISSN 1432-2250, Vol. 27, no 5, p. 561-575Article in journal (Refereed)
    Abstract [en]

    An asymptotic approach is considered for the transport and deposition of nanofibres in pipe flow. Convection and Brownian diffusion are included, and Brownian diffusion is assumed to be the dominant mechanism. The fibre position and orientation are modelled with a probability density function for which the governing equation is a Fokker-Planck equation. The focus is set on dilute fibres concentrations implying that interaction between individual fibres is neglected. At the entrance of the pipe, a fully developed velocity profile is set and it is assumed that the fibres enter the pipe with a completely random orientation and position. A small parameter {Mathematical expression} is introduced, where l is the fibre half-length and a is the pipe radius. The probability density function is expanded for small {Mathematical expression} and the solution turns out to be multi-structured with three areas, consisting of one outer solution and two boundary layers. For the deposition of fibres on the wall, it is found that for parabolic flow, and for the lowest order, the deposition can be obtained with a simplified angle averaged convective-diffusion equation. It is suggested that this simplification is valid also for more complex flows like when the inflow boundary condition yields a developing velocity profile and flows within more intricate geometries than here studied. With the model fibre, deposition rates in human respiratory airways are derived. The results obtained compare relatively well with those obtained with a previously published model

  • 60013.
    Åkerstedt, Hans O.
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Löfgren, Hans B
    Luleå tekniska universitet.
    Free surface magnetohydrodynamic flow with solidification2003In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 22, no 6, p. 581-601Article in journal (Refereed)
    Abstract [en]

    The paper considers an analysis of a liquid metal flow, occurring in the horizontal belt strip casting process. The liquid metal flows over a moving copper belt with a growing solidifying phase beneath the melt. The effect of applying a transverse magnetic field is investigated. A set of three-dimensional shallow water equations is derived. Supercritical flow is assumed and the shallow water equations are solved numerically using a shock-capturing method, which automatically takes care of the possibility of oblique hydraulic jumps.It is shown that non-uniform conditions introduced in the feeding region give a pattern of steady hydraulic jumps, which propagate downstream and are reflected at the sidewalls. The effect of the magnetic field is to brake the flow and damp the standing hydraulic jump pattern. Different feeding methods are compared and it is shown that the magnetic field erases the initial differences in liquid distribution using different feeding techniques

  • 60014.
    Åkerstedt, Hans O.
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Nycander, J.
    Department of Technology, Euratom–NFR Fusion Association, Uppsala University.
    Pavlenko, V.P.
    Department of Technology, Euratom–NFR Fusion Association, Uppsala University.
    Three-dimensional stability of drift vortices1996In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 3, no 1, p. 160-167Article in journal (Refereed)
    Abstract [en]

    The three-dimensional stability of drift vortices that are two-dimensionally stable in the framework of the Hasegawa-Mima equation is studied. The three-dimensionality is caused by a coupling of the vortex to ion-acoustic waves that propagate along the magnetic field. It is concluded that this coupling does not significantly destabilize the vortex. A general stability criterion is derived that guarantees stability if the ratio between the parallel wave number k and the azimuthal mode number m is large enough. It is also found that no instability exists in the limit k→ 0 if the potential vorticity is a strictly decreasing function of r. Such profiles are typical for vortices that are two-dimensionally stable. Some particular profiles where the potential vorticity vanishes outside some radius are also examined. In one case an instability with m=1 is found, but a numerical solution of the eigenvalue problem shows that the maximum growth rate is very small, three orders of magnitude smaller than the angular velocity of the vortex flow.

  • 60015. Åkerstedt, Zandra
    Stress vid inlärning: Inlärningslängd och behållning2014Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    Retreival practice is a learning method that has been shown to yield better retention than other learning methods. Research has also shown that stress has a negative impact on the capacity of working memory and long term memory. A study was, therefore, conducted to examine how stress affects the time needed for studying and the retention of 25 Swahili-Swedish translation equivalents with retrieval practice as learning method. Twentysix subjects learned the words in alternated study-/testperiodes and where then subjected to a final test three days later. Due to previous research about stress it was expected that the subjects in the experimental group would need more time to learn the words and also perform worse on the final test. As expected, the subjects in the experimental group did need more study-/testperiodes than the subjects in the control group, while there where no significant differences in the retention. This might be due to over learning in the experimental group or the fact that the selected stressor, noise, imposed a larger load on the phonetic loop. An explanation to the similarity in retention may be that the perceived stress didn’t reach the stress level, which, according to Yerkes-Dodsons law, would indicate a degrading performance. Keywords: retreival practice, learning, stress, working memory

  • 60016.
    Åkerström, Ingrid
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering.
    Förstärkning av kajkonstruktioner: Förstärkningsalternativ för Piteå Hamn2016Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [sv]

    Detta examensarbete syftar till att undersöka hur man kan förstärka de kajer som inte klarar det allt tyngre gods som anlöper. Alla beräkningar har gjorts för kajen i Piteå Hamn. Den byggdes på 60- och 70-talet och dimensionerades för laster på omkring 50-60 ton. Idag är laster om 70-80 ton inte ovanliga och allt fler lyft på upp mot 100 ton görs. För att även i fortsättningen kunna använda den befintliga kajen behöver förstärkningsförslag tas fram. Kajen i Piteå Hamn är en pålad kaj med betongplatta varvid de två huvudsakliga brott som kan ske är knäckning av pålar eller genomstansning av plattan. Med hjälp av Eurokod kontrolleras om kajen klarar belastningen från en Gottwald kran som vid speciallaster kan lyfta gods om 97 ton med ett bomutligg på 20 m. Den totala dimensionerande lasten från kranen beräknas till 3 210 kN. Beräkningarna visar att båda antagna brott sker och därför måste både pålarna och plattan förstärkas. Förstärkningen av pålarna görs genom att fyra nya stålrörspålar placeras rakt under kranens fyra stödben, två stycken som ytterpålar och två som innerpålar. Pålarna fylls med betong och dimensioneras att klara hela den last som bildas då kranen placeras rakt över dem. Pålarna ska även dimensioneras mot den korrosion, som går på hårt mot stål i synnerhet i skvalpzonen, och mot istryck. Beräkningarna visar att pålar med dimensionerna 400/12,5 mm för den yttre pålen respektive 230/12,5 mm för den inre pålen är tillräckligt för att klara dimensionerad last och ståltjockleken klarar en korrosionshastighet om 0,1 mm/år vilket är antaget värde i undervattenszonen. För att skydda pålarna i skvalpzonen förstärks pålarna med ytterligare ett stålrör utanpå. Dimensionen på detta varierar beroende på om det ska användas till bara korrossionsskydd eller om det även ska skydda mot istryck. Dessa pålar kan installeras i marken genom antingen slagning eller borrning. Genom borrning blir pålarna rakare och exaktheten i placering är bättre men installationskostnaden för dem är mellan 1,5-2 gånger så hög som för slagna pålar. För att förstärka plattan mot genomstansning har kapitäl dimensionerats. Även extra armering kan installeras i plattan, när den ändå öppnas. Kapitälen kan då göras mindre än de som nu dimensionerats. Den förstärkningsmetod som valts har den fördel att fler punkter, än de fyra valda, kan förstärkas och kranen kan på så sätt få ytterligare en placering där tunga lyft kan ske.

  • 60017.
    Åkerström, Paul
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    A Novel Tooling Technology for Hot Forming Processes2017In: 6th International Conference Hot Sheet Metal Forming of High-Performance Steel CHS2: June 4-7 2017, Atlanta, Georgia, USA : proceedings / [ed] Mats Oldenburg, Braham Prakash, Kurt Steinhoff, Warrendale, PA: Association for Iron & Steel Technology, AIST , 2017, p. 243-250Conference paper (Refereed)
  • 60018. Åkerström, Paul
    Material characterisation for simulation of press hardening2004Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The growing effort to reduce vehicle weight and improve passive safety in the automotive industry has drastically increased the demand for ultra high strength steel components. There exist several production methods for such components, where the press hardening technique (hot stamping) is one of the most successful in producing complex components from boron steel. In order to accurately perform numerical Finite Element (FE) simulations of the actual thermo-mechanical forming, it is crucial to use correct material data and models. This work is focusing on two main aspects of the material characterisation as follows. The first is the flow stress of the austenite at elevated temperatures and different strain rates, relevant for the process, which is crucial for correctly predicting the strains in the component and the forming force. During a press hardening cycle, the actual forming is performed at high temperatures and the steel is in the austenitic state. The second, the austenite decomposition into daughter products such as ferrite, pearlite, bainite or martensite is a function of the thermal and mechanical history. To find the mechanical response (flow stress) for the austenite, a method based on multiple overlapping continuous cooling and compression tests (MOCCCT) in combination with inverse modelling has been developed. A validation test (in combination with the compression tests) shows good agreement with the simulated forming force, indicating that the estimated flow stress as a function of temperature, strain and strain rate is accurate in the actual application. The austenite decomposition model is developed and integrated as a material subroutine into the FE-code LS-DYNA. The model is based on the combined nucleation and growth rate equations proposed by Kirkaldy. A separate test to simulate different cooling histories along a boron alloyed steel sheet has been conducted.

  • 60019.
    Åkerström, Paul
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Modelling and simulation of hot stamping2006Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The growing effort to reduce vehicle weight and improve passive safety in the automotive industry has drastically increased the demand for ultra high strength steel components. There are several production technologies for such components. The hot stamping technology (press hardening) is one of the most successful in producing complex components with superior mechanical properties. The hot stamping process can be described by the following steps; punching of blanks, heating to 900C in a furnace to austenitization followed by simultaneous forming and quenching in forming tools. In order to obtain accurate numerical Finite Element (FE) simulations of the actual thermo-mechanical forming, correct material data and models are crucial and mandatory. This work is focusing on three main aspects described below for the numerical simulation of the thermo-mechanical forming of thin boron steel sheets into ultra high strength components. The objective is to predict the shape accuracy, thickness distribution and hardness distribution of the final component with high accuracy. The first aspect is the flow stress of the austenite at elevated temperatures and different strain rates, which is crucial for correctly predicting the strains in the component and the forming force. During a hot stamping cycle, the actual forming is performed at high temperatures and the steel is mainly in the austenitic state. The second aspect is the austenite decomposition into daughter products such as ferrite, pearlite, bainite or martensite that is a function of the thermal and mechanical history. The third aspect is the mechanical material model used, which determine the stress state and consequently the component distortion. To find the mechanical response (flow stress) for the austenite, a method based on multiple overlapping continuous cooling and compression experiments (MOCCCT) in combination with inverse modelling has been developed. A validation test (in combination with the compression tests) shows good agreement with the simulated forming force, indicating that the estimated flow stress as a function of temperature, strain and strain rate is accurate in the actual application. The austenite decomposition model is developed and integrated as a material subroutine into the FE-code LS-DYNA. The model is based on the combined nucleation and growth rate equations proposed by Kirkaldy. A separate test to simulate different cooling histories along a boron alloyed steel sheet has been conducted. Different mixtures of daughter products are formed along the sheet and the corresponding simulation show acceptable good agreement with the experimentally determined temperature histories, hardness profile and volume fractions of the different microconstituents formed in the process. For the mechanical response, a mechanical constitutive model based on the original model proposed by Leblond has been implemented into LS-DYNA. The implemented model account for transformation induced plasticity (local plastic flow in austenite) according to the Greenwood-Johnson mechanism as well as classical plasticity during global yield. Finally, a FE-simulation using the implemented models of the thermo-mechanical forming of a component is compared to the corresponding experiment, including forming force, thickness distribution, hardness distribution and shape accuracy/springback.

  • 60020. Åkerström, Paul
    et al.
    Bergman, Greger
    SSAB HardTech.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Numerical implementation of a constitutive model for simulation of hot stamping2007In: Modelling and Simulation in Materials Science and Engineering, ISSN 0965-0393, E-ISSN 1361-651X, Vol. 15, no 2, p. 105-119Article in journal (Refereed)
    Abstract [en]

    In order to increase the accuracy of numerical simulations of the hot stamping process, an accurate and robust constitutive model is crucial. During the process, a hot blank is inserted into a tool where it is continuously formed and cooled. For the steel grades often used for this purpose, the initially austenitized blank will decompose into different product phases depending on the cooling and mechanical history. As a consequence, the phase proportions change will affect both the thermal and mechanical properties of the continuously formed and cooled blank. A thermo-elastic-plastic constitutive model based on the von Mises yield criterion with associated plastic flow is implemented into the LS-Dyna finite element code. Models accounting for the austenite decomposition and transformation induced plasticity are included in the constitutive model. The implemented model results are compared with experimental dilatation results with and without externally applied forces. Further, the calculated isothermal mechanical response during the formation of a new phase is compared with the corresponding experimental response for two different temperatures.

  • 60021. Åkerström, Paul
    et al.
    Bergman, Greger
    Gestamp R&D.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Salomonsson, Per
    Utveckling av mikrostruktur och mekanisk respons vid presshärdning2007In: Svenska Mekanikdagar 2007: Program och abstracts / [ed] Niklas Davidsson; Elianne Wassvik, Luleå: Luleå tekniska universitet, 2007, p. 98-Conference paper (Other academic)
  • 60022. Åkerström, Paul
    et al.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Austenite decomposition during press hardening of a boron steel: computer simulation and test2006In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 174, no 1-3, p. 399-406Article in journal (Refereed)
    Abstract [en]

    In this work a model to predict the austenite decomposition into ferrite, pearlite, bainite and martensite during arbitrary cooling paths for thin sheet boron steel is used. The model is based on Kirkaldy's rate equations. The basic rate equations has been modified to account for the austenite stabilization effect from the added boron. The model is implemented as part of a material subroutine in the Finite Element Program LS-DYNA 970. Both the obtained simulated volume fractions microconstituents and hardness profiles shows promising agreement to the corresponding experimental observations.

  • 60023.
    Åkerström, Paul
    et al.
    Gestamp Hardtech AB, Luleå.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Numerical simulation of a thermo-mechanical sheet metal forming experiment2008In: Numisheet 2008: Proceedings of the 7th international conference and workshop on numerical simulation of 3D sheet metal forming processes, September 1-5, 2008, Interlaken, Switzerland / [ed] Pavel Hora, Zurich: Institute of Virtual manufacturing, ETH Zurich , 2008, p. 569-574Conference paper (Refereed)
  • 60024. Åkerström, Paul
    et al.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Studies of the thermo-mechanical material response of a boron steel by inverse modelling2004In: Proceedings: 2nd International Conference on Thermal Process Modelling and Computer Simulation : Nancy, France, March 31 - April 2, 2003 / [ed] S. Denis, Les Ulis: EDP Sciences, 2004Conference paper (Refereed)
  • 60025. Åkerström, Paul
    et al.
    Wikman, Bengt
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Material parameter estimation for boron steel from simultaneous cooling and compression experiments2005In: Modelling and Simulation in Materials Science and Engineering, ISSN 0965-0393, E-ISSN 1361-651X, Vol. 13, no 8, p. 1291-1308Article in journal (Refereed)
    Abstract [en]

    In order to increase the accuracy of numerical simulations of the hot stamping process, reliable material data is crucial. Traditionally, the material is characterized by several isothermal compression or tension tests performed at elevated temperatures and different strain rates. The drawback of the traditional methods is the appearance of unwanted phases for some test temperatures and durations. Such an approach is also both time consuming and expensive. In the present work an alternative approach is proposed, which reduces unwanted phase changes and the number of experiments. The isothermal mechanical response is established through inverse modelling of simultaneous cooling and compression experiments. The estimated material parameters are validated by comparison with data from a separate forming experiment. The computed global response is shown to be in good agreement with the experiments.

  • 60026. Åkesson, B.
    et al.
    Karlsson, Lennart
    Prevention of hot cracking in butt welds in steel panels1977In: Metal Construction, ISSN 0307-7896, Vol. 9, no 2, p. 74-77Article in journal (Refereed)
  • 60027. Åkesson, B.
    et al.
    Karlsson, Lennart
    Prevention of hot cracking of butt welds in steel panels by controlled additional heating of the panels1976In: Welding Research International, ISSN 0306-9427, Vol. 6, no 5, p. 35-52Article in journal (Refereed)
    Abstract [en]

    A procedure for controlled additional heating during and after welding to prevent end cracking is devised and analyzed. The search for the optimal distribution and minimal amount of thermal energy needed for the additional heating is formulated as a mathematical programming problem. Two sided irradiation is suggested as a feasible way for transmitting the thermal energy into the panel. A case for application in a Swedish panel production line is studied. A formula for calculating the required additional heating is deduced.

  • 60028. Åkesson, David
    En studie av fri glimmers vattenupptagningsförmåga2009Independent thesis Basic level (university diploma), 5 credits / 7,5 HE creditsStudent thesis
  • 60029. Åkesson, Elin
    Hur ska jag sjunga?: Ett arbete om att vara bred i genre.2013Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
  • 60030. Åkesson, Rebecca
    Sjukgymnastiska behandlingsmetoder vid långvarig Whiplash- Associated Disorders2008Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
  • 60031. Åman, Emelie
    et al.
    Åström, Sofia
    Biodrivmedelsproduktion integrerat i sågverk: En energisystemsanalys2015Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The most commonly used fuels within transport in Sweden today are fossil fuels. Sweden has a national goal to have a fossil free vehicle fleet in 2030. To be able to reach the national goal by 2030 a major adjustment of the transportation sector has to be conducted. This project has investigated biofuel production integrated in sawmills. The integrations have been studied from an economic, resource and environmental perspective.Ethanol and methanol produced from woody biomass via enzymatic hydrolysis and gasification respectively are the biofuels studied in this project. The two biofuels has been studied in two separate case studies. The biofuel production has been integrated into two different sawmills, one that cooperates with nearby industries (cluster) and one that has no cooperation with other industries (stand-alone). Both case studies show that more wood chips than the amount of wood chips available at the studied sawmills are needed to cover the heat demand in each sawmill. The revenues and operating time are the parameters that affect the results most.The ethanol case study shows that an integrated ethanol plant can produce 20 and 17 MW ethanol in the cluster and the stand-alone sawmill, respectively. The efficiency of the cluster is higher than for the stand-alone plant because it produces excess electricity and heat which can be sold to a nearby district heating system. The payback time and internal rate of return for an investment in an ethanol plant is about 12 years and 5% for both integrations. The stand-alone plant has the higher net present value. In the methanol case study the plant in the cluster can produce 58 MW methanol and the stand-alone plant can produce 46 MW methanol. As for the ethanol case the efficiency of the cluster is higher than for the stand-alone plant because it produces excess heat which can be sold to a nearby district heating system. The payback time and internal rate of return for an investment in a methanol plant is about 8 years and 11% for both integrations. The plant in the cluster has the higher net present value. It is economically feasible in today’s society to produce biofuel in plants integrated in sawmills, with profit. For the investigated sawmills a production of biofuel is profitable for all four integrated plant configurations, regarding payback time, internal rate of return and net present value. It is more profitable to produce ethanol or methanol in a plant which is integrated with a sawmill compared to a plant of the same size which is not integrated.

  • 60032. Åman, Emma
    Jämförelse mellan terrängmodeller i NovaPoint 18 och Civil 3D 20142014Independent thesis Basic level (university diploma), 5 credits / 7,5 HE creditsStudent thesis
  • 60033. Åman, Jesper
    Transportsituationen vid NCC Anläggning region Stockholm: en kartläggning1999Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
  • 60034. Ånestrand, Joakim
    Alignment of information classification models for CAD data: from a model and process perspective2010Independent thesis Advanced level (degree of Master (One Year)), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    Information and protection of information is vital to organizations to protect organizational competence. CAD data in form of 2D and 3D models consists of almost all intellectual property in research and development organizations. One way of protecting the information in the 2D and 3D models is to classify the information in the models. By classifying the information security measures can be taken dependent of the classification. It is important for organization to make it the culture to classify information and to use a information classification model with shared terms that is understood by all. The information classification models should also be aligned with business strategies and processes in the organization to be utilized to full extent.

  • 60035. Ångström, Anna
    Resurseffektivisering vid värmebehandlingsprocesser2009Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
  • 60036. Årbrand, Christina
    Så har vi det på jobbet: en kvalitativ studie om vårdbiträdens uppfattningar om sitt arbete2008Independent thesis Advanced level (degree of Master (One Year)), 10 credits / 15 HE creditsStudent thesis
  • 60037.
    Århammar, Cecilia
    et al.
    AB Sandvik Coromant, Sandviken.
    Silvearv, Fredrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Bergman, A.
    Division of Materials Theory, Department of Physics and Astronomy, Uppsala University.
    Norgren, Susanne M
    AB Sandvik Coromant, Sandviken.
    Pedersen, Henrik
    Department of Physics, Chemistry and Biology, Linköping University.
    Ahuja, Rajeev B.
    Division of Materials Theory, Department of Physics and Astronomy, Uppsala University.
    A theoretical study of possible point defects incorporated into α-alumina deposited by chemical vapor deposition2014In: Theoretical Chemistry accounts, ISSN 1432-881X, E-ISSN 1432-2234, Vol. 133, no 2, p. 1-11Article in journal (Refereed)
    Abstract [en]

    The energetics and electronic structure of carbon, chlorine, hydrogen, and sulfur in α-Al2O3 was investigated by first principles and thermodynamical calculations. These species are present in the gas phase during the synthesis of α-Al2O3 by chemical vapor deposition (CVD) but little is known of their solubility in this compound. The heat of formation from standard reference states of the elements varying the chemical potential of each element was calculated. An attempt to model the actual conditions in the CVD process was made, using the species and solid compounds present in a common CVD process as reference states. Our calculations suggest that sulfur from the catalyzing agent H2S will not solve in α-Al2O3 during deposition by CVD. It is found that the neutral chlorine and hydrogen interstitial defects display the lowest heat of formation, 281 and 280 kJ/mol, respectively, at the modeled CVD conditions. This energy is too high in order for neutral defects to form during CVD of α-Al2O3 at any significant amounts. The charged defects and their compensation were studied. Carbon substituting oxygen is found to be energetically favored under the modeled CVD conditions, considering carbon dioxide as competing species to solid solubility in α-Al2O3 at an energy of -128 kJ/mol. However, care needs to be taken when choosing the possible competing carbon-containing phases. Compensation of carbon substituting for oxygen by oxygen vacancies takes place at 110 kJ/mol from standard reference states, graphite, fcc-Al and O2. The carbon solubility in Al2O3 is difficult to measure with standard analysis techniques such as X-ray diffraction and energy dispersive X-ray spectroscopy, but several stable compounds in the Al-C-O are available in the literature

  • 60038. Årnevall, Michael
    Coordinating vehicles, using mirror enhanced target recognition2006Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The long term goal behind this study is to achieve coordinated navigation of two or more robot vehicles, capable of autonomous operations in contaminated and hostile environments. One primary goal is to design and implement two basic functionalities named "Follow Me" and "Run Ahead of Me", which implies coordinated vehicles, capable of autonomous object recognition and tracking, using a time of flight laser range scanner as main sensor. Unfortunately the circumstances, made it impossible to create an applicable target tracking, using the two vehicles available, but a lot was learnt about the limitations when using lasers to recognize och track targets. The project did however, result in some promising motion models, describing the relative position and orientation (pose), of two coordinated vehicles, and because of their generality, the models has proven to work in various contexts. Trails show that target tracking, utilizing a laser range scanner, is possible but the resolution proves to be insufficient. Thus prospect are good to improve the measurements by using mirrors to reflect more laser pulses at the target, and by that receive more stable readings of the objects orientation.

  • 60039. Åsa & Holmqvist, Kristina, Nordgren,
    Motivation till en ökad inlärning inom historieämnet2003Independent thesis Basic level (professional degree), 10 credits / 15 HE creditsStudent thesis
  • 60040.
    Åsander, Sandra
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering.
    Undersökning av kolhalten i ett plastformsstål under tillverkning i ljusbågsugnen2018Independent thesis Basic level (university diploma), 10 credits / 15 HE creditsStudent thesis
    Abstract [sv]

    Detta examensarbete har utförts på Uddeholms AB i Hagfors i syfte med att undersöka möjliga orsaker till uppkolning av smältan under tillverkning av ett lågkolhaltigt krom-manganstål i ljusbågsugnen. Totalt följdes sju charger upp vid nedsmältningen i ljusbågsugnen. Provtagning av stål, slagg och skrot utfördes under processen. Dessutom undersöktes effekten av syrgasfärskning samt grafitelektrodernas inverkan på kolhalten. Resultatet för varje undersökt parameter har utvärderats och kopplats till teorin, med syftet att genom möjliga samband erhålla förslag till en processföring där behovet av syrgasfärskning för reducering av kolhalt i ljusbågsugnen kan undvikas.

    Projektet har behandlat en process där många faktorer kan inverka på uppkolningen av smältan. Dock kan ett samband mellan höga kolhalter och tillsatser av skrotet CrNi-rusor ses. På majoriteten av charger med för hög kolhalt tillsattes detta skrot. Dåliga grafitelektroder uppvisar också en tendens att bidra till uppkolning av stålsmältan.

    Syrgasfärskning tenderar att oxidera större mängder krom och mangan till slaggen, vilket leder till ökade legeringskostnader per tillverkad charge.

    Den främsta anledningen till de för höga kolhalterna har med stor sannolikhet berott på en kombination av tillsatt skrot med för hög kolhalt samt användning av dåliga elektroder under nedsmältning. Vidare uppföljning av elektroderna och kontroll av skrotet för att minimera uppkolning kvarstår för framtida arbete.

  • 60041. Åsberg, Fredrik
    Awareness and presence amplification in virtual socializing2009Independent thesis Advanced level (degree of Master (One Year)), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    The opportunity to do a Master Thesis (30 points) in Virtual Socializing was given as an initiative from the department of the Division of Functional Product Development, Luleå University of Technology, Sweden. The theory, methodology and result of a project focused on a social connectedness experience is presented in this report. The objective was to develop a system that enhance the awareness and presence of others, implemented in a system that can establish a qualitative video-communication allowing eyecontact. The project started with a literature study acting as guidance data throughout the whole project. Defining the problem and implementation of the gained knowledge was stated in a task clarification and translated into a mission statement. Requirements lists and evaluations of desired properties made it possible to generate concepts using scenario-based brainstorming. Reevaluations and improvements of the concepts resulted in a few concepts that focused on an experience using a communication device. The concepts showed a system that identifies a person and recognise the availability for communication, showed in an interface integrated in the communication device.

  • 60042. Åsell, Mikael
    Töjningsutmattning vid fleraxligt spänningstillstånd1999Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
  • 60043.
    Åsenlund, Robin
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering.
    Kartläggning av förvärmningsprocessen för gjutlåda och gjutrör.2018Independent thesis Basic level (university diploma), 10 credits / 15 HE creditsStudent thesis
    Abstract [sv]

    Denna rapport är ett examensarbete utfört på Ovako Bar AB i Smedjebacken, vilket är det avslutade momentet i utbildningen ”Materialteknik med inriktning metallurgi” på Bergsskolan i Filipstad.

     

    Grunden till detta arbete ligger hos ”frysningar” (att stålet slutar rinna från gjutlådan) som företrädesvis sker på yttersträngarna. Igensättning av denna typ orsakas generellt av två faktorer, för låg övertemperatur i förhållande till likvidustemperatur eller otillräcklig förvärmning av gjutlåda samt gjutrör.

     

    Syftet med detta arbete har varit att fokusera på förvärmningen, där målet var att utvärdera hur förvärmningstider ser ut i förhållande till de föreskrivna. Samt utvärdera prestandan hos värmningsutrustningen. Och utvärdera hur temperaturfördelningen ser ut i gjutlådan under gjutning.

     

    Arbetet utfördes med hjälp av termoelement som monterades in i gjutlådan vid 1: a sträng och 3:e sträng för att kunna se hur temperaturfördelning är i gjutlådan. Mätningarna visade tydligt att temperaturen skiljde sig mellan yttersträng (1: a) och centrumsträng (3:e) vilket kan var en orsak till att frysningar sker företrädesvis på yttersträng där temperaturen visat sig vara lägre än centrumsträng.

     

    Vid utvärdering av prestandan på värmningsutrustningen så användes en handpyrometer som möjliggjorde mätning av gjutrören för att ta fram en trendkurva, som visar när temperaturökningen avtar och upphör. Vilket visade brister i tid och prestanda i nuläget.

     

    Med en fullt fungerande förvärmning skulle igensättningsgraden kunna minskas.

  • 60044.
    Åsenlöf, Pernilla
    et al.
    Department of Neuroscience, Physiotherapy, Uppsala University.
    Michaelson, Peter
    Luleå University of Technology, Department of Health Sciences, Health and Rehab. peter.michaelson@ltu.se .
    Grahn, Birgitta
    Department of Orthopedics, Clinical Sciences, Lund University.
    Bergman, Stefan
    Research and Development Center Spenshult, Halmstad.
    Axelson, S.
    Swedish Agency for Health Technology Assessment and Assessment of Social Services.
    Gyllenswärd, Harald
    Swedish Agency for Health Technology Assessment and Assessment of Social Services.
    Bergström, Gunnar
    Institute of Environmental Medicine, Unit of Intervention & Implementation Research, Karolinska Institutet.
    A systematic review of randomized controlled trials studying the preventive effects of physical exercise, manual and behavioural treatments in acute low back pain and neck pain2016In: International Journal of Behavioral Medicine, ISSN 1070-5503, E-ISSN 1532-7558, Vol. 23, no Suppl. 1, p. S187-Article in journal (Refereed)
  • 60045.
    Åsenlöf, Pernilla
    et al.
    Institutionen för neurovetenskap, Uppsala universitet.
    Michaelson, Peter
    Luleå University of Technology, Department of Health Sciences, Health and Rehab.
    Grahn, Birgitta
    Kronoberg occupational rehabilitation service, Växjö, Lunds universitet, Institutionen för rörelseorganens sjukdomar, Kronoberg county council, R&D centre, R&D Welfare of Southern Småland, FoU Välfärd, Kronoberg County.
    Bergman, Stefan
    Högskolan i Halmstad.
    Gyllensvärd, Harald
    SBU.
    Bergström, Gunnar
    Enheten för interventions och implementeringsforskning, Karolinska Institutet.
    Preventiva insatser vid akut smärta från rygg och nacke: Presentation av en SBU-rapport2016In: Fysioterapi, ISSN 1653-5804, no 5, p. 34-42Article in journal (Other academic)
  • 60046. Åsenstam, Jonas
    et al.
    Johansson, Bengt
    Upphovsrätten mot offentlighetsprincipen1999Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
  • 60047. Åslin, Anna
    Stadsrum istället för trafikrum: Strukturella och rumsliga förutsättningar för en hållbar stadsmiljö i de centrala delarna av Teg i Umeå2015Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
  • 60048. Åslund, Birgitta
    Behandlingsmetoder för att öka kroppsmedvetenhet inom strokerehabilitering och dess effekter: en litteraturöversikt och kvalitativ studie2014Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
  • 60049. Åslund, Elina
    et al.
    Eriksson, Emelie
    Tecken som stöd: En metod för alla?2012Independent thesis Basic level (professional degree), 10 credits / 15 HE creditsStudent thesis
  • 60050. Åslund, Elisabet
    Inventering och karakterisering av brännbart avfall i Norrköpingsregionen2000Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
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