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  • 151.
    Bergh, Jöran
    et al.
    Chalmers University of Technology, Department of Mathematics.
    Burenkov, Victor
    Department of Differential Equations and Functional Analysis, Russian Peoples' Friendship University.
    Persson, Lars-Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    On some sharp reversed Hölder and Hardy type inequalities1994Inngår i: Mathematische Nachrichten, ISSN 0025-584X, E-ISSN 1522-2616, nr 169, s. 19-29Artikkel i tidsskrift (Fagfellevurdert)
  • 152.
    Berglund, Tomas
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, EISLAB.
    Brodnik, Andrej
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Datavetenskap.
    Jonsson, Håkan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Datavetenskap.
    Mrozek, Kent
    Staffansson, Mats
    Luleå tekniska universitet.
    Söderkvist, Inge
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Minimum curvature variation B-splines: validation of a path-planning model2004Rapport (Annet vitenskapelig)
  • 153.
    Berglund, Tomas
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, EISLAB.
    Brodnik, Andrej
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Datavetenskap. University of Primorska, Slovenia. .
    Jonsson, Håkan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Datavetenskap.
    Staffansson, Mats
    Luleå tekniska universitet.
    Söderkvist, Inge
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Planning smooth and obstacle-avoiding b-spline paths for autonomous mining vehicles2010Inngår i: IEEE Transactions on Automation Science and Engineering, ISSN 1545-5955, E-ISSN 1558-3783, Vol. 7, nr 1, s. 167-172Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We study the problem of automatic generation of smooth and obstacle-avoiding planar paths for efficient guidance of autonomous mining vehicles. Fast traversal of a path is of special interest. We consider four-wheel four-gear articulated vehicles and assume that we have an a priori knowledge of the mine wall environment in the form of polygonal chains. Computing quartic uniform B-spline curves, minimizing curvature variation, staying at least at a proposed safety margin distance from the mine walls, we plan high speed paths.

  • 154.
    Berglund, Tomas
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, EISLAB.
    Erikson, Ulf
    Luleå tekniska universitet.
    Jonsson, Håkan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Datavetenskap.
    Mrozek, Kent
    Navigator AB.
    Söderkvist, Inge
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Automatic generation of smooth paths bounded by polygonal chains2001Inngår i: CIMCA 2001: 2001 international conference on computational intelligence for modelling, control & automation : 9-11 July 2001, Las Vegas, Nevada, USA : proceedings / [ed] M. Mohammadian, CIMCA , 2001, s. 528-535Konferansepaper (Fagfellevurdert)
  • 155. Berglund, Tomas
    et al.
    Jonsson, Håkan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Datavetenskap.
    Söderkvist, Inge
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    An obstacle-avoiding minimum variation B-spline problem2003Inngår i: Proceedings: 2003 International Conference on Geometric Modeling and Graphics, GMAG 2003 ; 16 - 18 July 2003, London, England, Los Alamitos, Calif: IEEE Communications Society, 2003, s. 156-161Konferansepaper (Fagfellevurdert)
    Abstract [en]

    We study the problem of computing a planar curve, restricted to lie between two given polygonal chains, such that the integral of the square of arc-length derivative of curvature along the curve is minimized. We introduce the minimum variation B-spline problem, which is a linearly constrained optimization problem over curves, defined by B-spline functions only. An empirical investigation indicates that this problem has one unique solution among all uniform quartic B-spline functions. Furthermore, we prove that, for any B-spline function, the convexity properties of the problem are preserved subject to a scaling and translation of the knot sequence defining the B-spline.

  • 156.
    Berglund, Tomas
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, EISLAB.
    Jonsson, Håkan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Datavetenskap.
    Söderkvist, Inge
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    The problem of computing an obstacle-avoiding minimum variation B-spline2003Rapport (Annet vitenskapelig)
    Abstract [en]

    We study the problem of computing a planar curve restricted to lie between two given polygonal chains such that the integral of the square of arc- length derivative of curvature along the curve is minimized. We introduce the Minimum Variation B-spline problem which is a linearly constrained optimization problem over curves defined by B-spline functions only. An empirical investigation indicates that this problem has one unique solution among all uniform quartic B-spline functions. Furthermore, we prove that, for any B-spline function, the convexity properties of the problem are preserved subject to a scaling and translation of the knot sequence defining the B-spline.

  • 157.
    Berglund, Tomas
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, EISLAB.
    Strömberg, Thomas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Jonsson, Håkan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Datavetenskap.
    Söderkvist, Inge
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Epi-convergence of minimum curvature variation B-splines2003Rapport (Annet vitenskapelig)
    Abstract [en]

    We study the curvature variation functional, i.e., the integral over the square of arc-length derivative of curvature, along a planar curve. With no other constraints than prescribed position, slope angle, and curvature at the endpoints of the curve, the minimizer of this functional is known as a cubic spiral. It remains a challenge to effectively compute minimizers or approximations to minimizers of this functional subject to additional constraints such as, for example, for the curve to avoid obstacles such as other curves. In this paper, we consider the set of smooth curves that can be written as graphs of three times continuously differentiable functions on an interval, and, in particular, we consider approximations using quartic uniform B- spline functions. We show that if quartic uniform B-spline minimizers of the curvature variation functional converge to a curve, as the number of B-spline basis functions tends to infinity, then this curve is in fact a minimizer of the curvature variation functional. In order to illustrate this result, we present an example of sequences of B-spline minimizers that converge to a cubic spiral.

  • 158.
    Berglund, Tomas
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, EISLAB.
    Synnergren, Per
    Luleå tekniska universitet.
    Söderkvist, Inge
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Estimation of curl in paper: an industrial application combining shape measurement and least squares modeling2000Inngår i: Proceedings / International Conference on Trends in Optical Nondestructive Testing: Lugano, Switzerland, May 3-6, 2000 / [ed] Pramod K. Rastogi; D. Inaudi, Lugano: Ecole Polytechnique Fédérale de Lausanne , 2000, s. 23-33Konferansepaper (Fagfellevurdert)
    Abstract [en]

    If a sheet of paper is subjected to humidity changes and have structural variations through its thickness such as gradients of fibre orientation, density and filler content, the sheet will curl and hence assume a cylindrical shape. Curl is a quality problem that makes the paper less suitable for printing. We propose a method to measure curl that can be used for automated analysis of the paper quality. The shape of the curled paper is measured from the perspective difference in a stereoscopic camera system, which is viewing an irregular pattern that is projected onto the specimen. The perspective difference is calculated by a correlation algorithm, a technique often referred to as digital speckle photography. The most interesting quality parameters are the magnitude of curl, which is defined as the inverse of the radius of curvature and also the orientation of the curled paper. These parameters are estimated by performing a least squares fit of a cylindrical shape to the three-dimension...

  • 159.
    Bergquist, Bjarne
    et al.
    Luleå tekniska universitet, Institutionen för ekonomi, teknik och samhälle, Industriell Ekonomi.
    Albing, Malin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Statistical methods - does anyone really use them?2006Inngår i: Total quality management and business excellence (Online), ISSN 1478-3363, E-ISSN 1478-3371, Vol. 17, nr 8, s. 961-972Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Students taking courses in quality management at Luleå University of Technology receive extensive education in statistical methods. To improve the education and to understand what kind of competence students need when they graduate, a survey was preformed examining how and to what extent the methods Statistical Process Control, Capability Analysis and Design of Experiments are used by organisations hiring the alumni. The result shows that the students employed in the Swedish industrial sector witness a modest use of statistical methods, while use of statistical methods in other sectors hiring the alumni is uncommon. Lack of competence and resources within the organizations are stated as hindrances to expanded use. Conclusions from the study are that implementation techniques must be emphasized in the curriculum and that different types of courses should be given - practical, hands-on courses for engineers, managers and others working in organizations. Furthermore, courses offered at universities must have a strong focus on practical problems such as difficulties randomizing experiments and that graphical methods should be favoured.

  • 160.
    Bergström, Lars
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Development of three-dimensional disturbances in pipe poiseuille flow1992Licentiatavhandling, med artikler (Annet vitenskapelig)
  • 161.
    Bergström, Lars
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Evolution of laminar disturbances in pipe Poiseuille flow1993Inngår i: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 12, nr 6, s. 749-768Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    An experimental study on the evolution of localized disturbances in pipe Poiseuille flow has been performed. The aim was to investigate the relevance of the theoretical findings on the algebraic transient growth mechanism. For non-axisymmetric disturbances the amplitudes and the propagation velocities were measured in the laminar regime. The initial disturbances were induced radially through the pipe wall and had a distribution in the azimuthal direction corresponding to a periodicity of one and five, respectively. The amplitude of the resulting streamwise velocity perturbation was measured at different radial and axial positions by means of laser doppler velocimetry. In the laminar regime, the results showed that the disturbance peaks are actually amplified but will eventually decay as predicted in the theory. The peak of an initial disturbance of periodicity five showed a rapid amplification to a maximum before the decay sets in. The initial disturbance of periodicity gave a more slowly varying disturbance peak level over the investigated interval. The propagation speed of the front of the disturbances was in the range of 0.86-0.9 times the centreline velocity. The propagation speed of the disturbance peaks was in the range of 0.51 to 0.80 times the centreline velocity and dependent on the azimuthal distribution of the initial disturbance.

  • 162.
    Bergström, Lars
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Initial algebraic growth of small angular dependent disturbances in pipe Poiseuille flow1992Inngår i: Studies in applied mathematics (Cambridge), ISSN 0022-2526, E-ISSN 1467-9590, Vol. 87, nr 1, s. 61-79Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The evolution of small, angular dependent velocity disturbances in laminar pipe flow is studied. In particular, streamwise independent perturbations are considered. To fully describe the flow field, two equations are required, one for the radial and the other for the streamwise velocity perturbation. Whereas the former is homogeneous, the latter has the radial velocity component as a forcing term. The normal modes of the system are determined and analytical solutions for eigenfunctions, damping rates and phase velocities are calculated. Then, the initial value problem is treated and the time development of the disturbances is determined.

  • 163.
    Bergström, Lars
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Interactions of three components and subcritical self-sustained amplification of disturbances in plane Poiseuille flow1999Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 11, nr 3, s. 590-601Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A low-dimensional nonlinear model for the normal velocity (v) and normal vorticity (eta) disturbance development in plane Poiseuille flow is studied. The study is restricted to the interactions of a pair of oblique components of the form e(/(alpha x+/-beta z)) and the component of the form e(/2 beta z) where alpha and beta are streamwise and spanwise wave numbers, respectively. The disturbances considered are also assumed to be highly elongated in the streamwise direction. Owing to the non-normal properties of the basic equations, the eta disturbance is first transiently amplified. Then, if the Reynolds number (R) and the initial disturbance are sufficiently large, the nonlinear interactions lead to a self-sustained process of disturbance amplification at subcritical R. For large R (R greater than or similar to 5000). the threshold disturbance amplitude scales like R-3. The, results also strongly indicate that the nonlinear feedback from eta to v is crucial for the establishment of the self-sustained process.

  • 164.
    Bergström, Lars
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Nonlinear behaviour of transiently amplified disturbances in pipe Poiseuille flow1995Inngår i: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 14, nr 6, s. 719-735Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The present work is a continuation of previous investigations of the author and deals with the nonlinear behaviour of transiently amplified disturbances in pipe Poiseuille flow. The disturbances are expanded in terms of a small amplitude parameter, and a spectral formulation, based on the eigenfunctions of the linear problem, is used to solve the resulting evolution equations. For the corresponding linear problem, analytical solutions are known. Here the author shows that the nonlinear problem also allows an analytical treatment. The analysis concentrates on the interaction of the dominating azimuthal wave number components of axially independent disturbances, since they exhibit largest growth in linear theory. The analysis is restricted to the first modes of the components, since these are found to dominate the disturbance magnitude at the peak amplification. In particular, the nonlinear correction for the component with an azimuthal wave number equal to one is considered. Including the nonlinear correction, the author reduces the linearly obtained amplification of the streamwise disturbance energy density and finds that the peak position of the disturbance energy density occurs earlier than for linear disturbance. This result is consistent with the experimental findings on transient growth for subcritical disturbances. Moreover, the reduced amplification is in agreement with recent results from direct numerical simulations of axially dependent disturbances.

  • 165.
    Bergström, Lars
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Nonmodal growth of three-dimensional disturbances on plane Couette-Poiseuille flows2005Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 17, nr 1Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The time development of three-dimensional disturbances superimposed on a variety of mean flow profiles representing plane Couette-Poiseuille flow is investigated numerically. Specifically, with y representing the wall normal coordinate, the mean flow profiles U(y) are represented by U(y) = A(1-y2) + By, where B = 1 ......

  • 166.
    Bergström, Lars
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Optimal growth of small disturbances in pipe Poiseuille flow1993Inngår i: Physics of Fluids. A, Fluid Dynamics (1989-1993), ISSN 0899-8213, Vol. 5, nr 11, s. 2710-2720Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A theoretical study is made of initial algebraic growth for small angular-dependent disturbances in pipe Poiseuille flow. The analysis is based on the homogeneous equation for the pressure for which the eigenvalue problem is solved numerically. In the limit of small streamwise wave numbers asymptotic results for the eigenvalues are derived. On the basis of the modes of the system, which are all damped, the initial value problem is considered and in particular the largest possible growth of the disturbance energy density is determined following the ideas of Butler and Farrell [Phys. Fluids A 4, 1637 (1992)]. The results show that a large amplification of the disturbance energy is possible. The largest amplification is obtained for disturbances with a small streamwise wave number and with an azimuthal wave number of one. The energy growth is then only due to the growth of the streamwise disturbance component. However, for disturbances of shorter wavelength, the energy growth is also substantial and not only concentrated to the streamwise velocity component. The wall shear corresponding to disturbances with the largest energy growth also shows a large amplification and the dependence of wave numbers and the Reynolds number is the same as for the energy. However, the wall pressure of a long wavelength disturbance of the largest growth just decays from its initial value, but for disturbances of shorter wavelength, it is also amplified

  • 167.
    Bergström, Lars
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Self-sustained amplification of disturbances in pipe Poiseuille flow1999Inngår i: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 18, nr 4, s. 635-647Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The nonlinear development of disturbances in pipe Poiseuille flow is studied with a low-dimensional model. The basic system from which the model is derived governs disturbances closely related to the radial velocity and radial vorticity disturbances. The analysis is restricted to the interaction of the two first harmonics of streamwise elongated disturbances since they are the most transiently amplified ones in linear theory. In the resulting dynamical system a nonlinear feedback from the normal vorticity disturbance (which is transiently amplified according to linear theory) to the radial velocity disturbance is present. Above a threshold of the initial amplitude, the feedback leads to a self-sustained amplification of the disturbances continuing for all times.

  • 168.
    Bergström, Lars
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    The effect of the Earth's rotation on the transient amplification of disturbances in pipe flow2003Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 15, nr 10, s. 3028-3035Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In pipe flow, for large Reynolds numbers, A. A. Draad and F. T. M. Nieuwstadt \ref[J. Fluid Mech. 361 (1998), 297--308 showed that the small Coriolis force due to the Earth's rotation may affect the mean flow profile of liquids substantially. In this paper, the development of small disturbances superimposed on a laminar mean flow affected by the Coriolis force is investigated analytically. The investigation is focused on the time development and the transient growth of streamwise-independent disturbances since they are the most amplified disturbances without the Coriolis effect included. The results show that the modification of the parabolic mean flow caused by the Coriolis force significantly affects the transient disturbance amplification when the Reynolds number $(R)$ is high. For example, with $R=9000$ and the Ekman number $\rm Ek=5.23$, due to the Coriolis effect, the peak value of the transient disturbance amplification becomes about two-thirds of the peak value obtained in the case where the mean flow is unaffected by the Coriolis force. When the Reynolds number is decreased, the reduction of the transient growth becomes smaller.

  • 169.
    Bergström, Lars
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    The initial-value problem for three-dimensional disturbances in plane Poiseuille flow of helium II2008Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 598, s. 227-244Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The time development of small three-dimensional disturbances in plane Poiseuille flow of helium II is considered. The study is conducted by considering the interaction of a normal fluid field and a superfluid field. The interaction is caused by a mutual friction forcing between the two flow fields. Specifically, the stability of the normal fluid affected by the mutual forcing is considered. Compared to the ordinary fluid case where the mutual forcing is not present, the presence of the mutual forcing implies a substantial increase of the transient growth of the disturbances. The increase of the transient growth occurs because the mutual forcing reduces the damping of the disturbances. The phase of transient growth becomes thereby more prolonged and higher levels of amplification are reached. There is also a minor effect on the transient growth caused by the modification of the mean flow owing to the mutual forcing. The strongest transient growth occurs for streamwise elongated disturbances, i.e. disturbances with streamwise wavenumber α = 0. When α increases beyond zero, the transient amplification quickly becomes reduced. Striking differences compared to the ordinary fluid case are that the largest transient amplification does not occur when the spanwise wavenumber (β) is close to two and that the peak level of the disturbance energy density amplification does not depend on the square of the Reynolds number.

  • 170.
    Bergström, Lars
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Transient growth of disturbances in laminar pipe flow1994Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Transient growth of disturbances is studied theoretically in pipe Poiseuille flow and experimentally both in pipe and plane Poiseuille flow. The theoretical results, based on the initial value problem, show that a large transient amplification occurs for small, angular dependent disturbances in pipe Poiseuille flow although all modes are damped. The largest growth is obtained for disturbances with zero streamwise wave number and with an azimuthal periodicity of one. Only the streamwise disturbance component is then amplified and the energy density growth is proportional to the Reynolds number squared. For disturbances of finite axial wave number, also the radial and azimuthal disturbance components are amplified from their initial values but the growth is still dominated by the streamwise disturbance. When the azimuthal wave number increases the disturbance peak occurs at shorter times and the amplification decreases. However, for small times, larger azimuthal wave numbers dominate the growth. Asymptotic results for the most amplified disturbances show that two classes of modes can be distinguished and it is necessary to include modes from both classes to get amplification. The asymptotically derived propagation speeds of the two classes are in the ranges <2/3 times the centerline velocity (Ucl) and <0.818Ucl, respectively. If the nonlinear interaction of axially independent disturbances is considered, the results indicate that the amplification of the most linearly amplified disturbance is reduced. Laser Doppler measurements of the streamwise disturbance velocity due to radially induced initial disturbances of azimuthal periodicities (n) of one and five have also been conducted. The results show that the disturbance velocity in fact exhibits a transient growth. The disturbance with n=1 gives a slowly varying peak-value while the peak owing to an initial disturbance with n=5 shows a more rapid amplification to a maximum before it decays. The propagation speeds of the disturbance peaks are in the range theoretically expected for transiently amplified disturbances. By hot-wire measurements of the streamwise disturbance velocity due to two radially induced jet-like disturbances, the spatial structure of an evolving disturbance is investigated in detail. Especially the positive disturbance velocity exhibits an amplification followed by a decay. The disturbance also spreads and becomes streak-like and elongated in the streamwise direction. The spread implies that an integrated disturbance quantity grows to a peak and eventually decays. The propagation speed and the radial location of the disturbances agree with the theoretical findings for algebraically growing disturbances. In plane Poiseuille flow the symmetry properties of an evolving disturbance are investigated by means of hot-wire anemometry. The initial streamwise disturbance is designed to be symmetric with respect to the normal direction. Despite this, an antisymmetric structure develops in the region where the main amplification occurs as indicated by the theory.

  • 171.
    Bergström, Lars
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Transient growth of small disturbances in a Jeffrey fluid flowing through a pipe2003Inngår i: Fluid Dynamics Research, ISSN 0169-5983, E-ISSN 1873-7005, Vol. 32, nr 1-2, s. 29-44Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The hydrodynamic stability of small disturbances in a non-Newtonian fluid flowing through a circular pipe is studied analytically. Specifically, the time development of the disturbances and the transient disturbance amplification are studied. The non-Newtonian fluid is modelled by a two-constant Jeffrey model characterized by the relaxation time $\lambda$ and a constant $K$ representing the ratio of relaxation to retardation times. All the investigations are carried out for streamwise-independent disturbances since they are the most amplified ones in a Newtonian fluid and can be treated analytically. The eigenvalue problem and the initial value problem for the disturbances are studied. Compared to the Newtonian case, the results show that, dependent on $K$ and $\lambda$, a reduction or an increase in the disturbance transient growth may occur.

  • 172.
    Bergström, Lars
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Transient properties of a developing laminar disturbance in pipe Poiseuille flow1995Inngår i: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 14, nr 5, s. 601-615Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Hot-wire measurements of the velocity disturbance from point-like disturbances in a fully developed laminar pipe Poiseuille flow are performed. Two jets are induced radially into the pipe by diametrically opposed loud speakers. The resulting streamwise disturbance velocity component is investigated over the Reynolds number range 1550-2000. Close to the position of the initial disturbance, the disturbance amplitudes and gradients are large. After a few pipe radii the disturbance is reduced and consists of two small regions of positive disturbance with a larger region of negative disturbance in between. A downstream interval then follows in which the amplitudes of the regions with positive disturbance exhibit an amplification followed by a decay. The peaks of the two amplified regions are located at 0.6 radii and 0.4 radii, respectively, and propagate with velocities of 0.65 times the centreline velocity (Ucl) and 0.88 Ucl, respectively. Subsequently, the disturbance obtains an almost constant level, while it becomes elongated in the streamwise direction. The dominating part of the disturbance is then confined to a region around 0.5 radii from the centerline and the propagation velocity of the region with largest amplitude is in the range 0.7-0.8 Ucl. An integrated disturbance quantity is defined by integrating the square of the disturbance amplitude over the radius and time in a downstream position. In a position a few radii downstream, this quantity has decayed to about 10-20 per cent of the value obtained one pipe radius downstream. It is then amplified by a factor of about five before it finally decays

  • 173.
    Bergström, Lars
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Alfredsson, P. Henrik
    Royal Institute of Technology, Stockholm.
    Symmetry properties of developing three-dimensional laminar disturbances in plane Poiseuille flow1994Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 6, nr 4, s. 1618-1620Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The evolution of a finite-amplitude point-like laminar disturbance in plane Poiseuille flow is investigated using hot-wire anemometry. In contrast to earlier experiments, the initial disturbance was introduced simultaneously through both the upper and lower wall of the channel, resulting in an antisymmetric disturbance in the normal velocity. Although the initial disturbance mainly generated symmetrical streamwise velocity modes, the subsequent development of the perturbation showed a marked tendency for antisymmetry.

  • 174.
    Bergström, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Activity: SIAM Conference on Geometric Design and Computing2007Konferansepaper (Annet (populærvitenskap, debatt, mm))
  • 175.
    Bergström, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Computational methods for on-line shape inspection2009Licentiatavhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    This licentiate thesis describes computational methods that solve problems occurring in industrial on-line shape quality inspection of produced items. These items are measured and compared with their corresponding CAD object. The meaning of on-line is that the inspection is done on-line in the production line, i.e. the items are not removed from the line. In practice this means that the inspection must be done very fast, both the measurement and the data analysis. The measurement is done using an optical non-contact method based on projection of fringes.The presented methods are mainly based on finding a transformation, a rotation and a translation, of the measurement values which consists of a point cloud representing the measured surface. This transformation is calculated using the iterative closest point (ICP) method such that the point cloud fits the corresponding surface of the CAD object properly. The method for finding this transformation is adapted for reiterated use, i.e. it makes use of the fact that the same CAD object is used several times for different measurements. A search tree making it possible to do this fast is proposed.When dealing with real measurements obtained from optical methods undesired measurement errors will occur, caused by reflections, dirt on lenses or other likely matters in the industrial environment. The iteratively re-weighted least squares (IRLS) method for different robust functions are used in combination with ICP for handling these errors, in order to do a correct surface matching. This result in much higher matching accuracy and almost no additional computations are needed.

  • 176.
    Bergström, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Computational methods for shape verification of free-form surfaces2011Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [sv]

    Beräkningsmetoder för formverifiering av friformsytor utgör huvudinnehållet i denna doktorsavhandling. En gemensam egenskap för dessa metoder är att de möjliggör formverifiering online direkt i produktionslinan. Av den anledningen måste metoderna vara snabba och robusta. Ett av problemen som uppkommer i formverifieringen av friformsytor är registrering. Det är problemet med att matcha datapunkter i 3D-rymden, som representerar den uppmätta ytan, med ett CAD-objekt genom att ansätta en stelkropps transformation. En metod för att utföra registreringen snabbt och robust är utvecklad. Metoden är en utveckling av ”the iterative closest point method, ICP”. Vi förprocessar CAD-objektet genom att skapa en datastruktur för att möjliggöra snabb närmsta-punkt sökning. Initialt läggs mycket tid på att skapa datastrukturen för att de enskilda registreringarna skall gå snabbt. Den robusta registreringen baserar sig på teorier från robust statistik genom att tillämpa ”iteratively re-weighted least squares” i kombination med ICP metoden. Detta resulterar i en snabb registreringsmetod som är okänslig för avvikande data. Metoden med registreringen används i en tillämpning för att hitta avvikelser mellan formen för ett objekt och dess ideala form. Den ideala formen är känd och ges av ett CAD-objekt. En optisk formmätningsmetod, projicerade fransar med en enda mönsterdetektering, används för att skapa datapunkter av den uppmätta ytan. Denna metod är snabb och okänslig för vibrationer men datapunkterna kan innehålla fel i vissa regioner, vilket hanteras av registreringen. Ett inversproblem som uppkommer i många optiska formmätningsmetoder är fasuppvikning. Vi introducerar en uppvikningsmetod med regularisering genom att använda information från ett CAD-objekt. Formmätningsmetoden som vi använder oss av här baserar sig på två-våglängds holografi. Vår fasuppvikningsmetod funkar oberoende av diskoninuiteter men mätobjektet får inte avvika alltför mycket i form jämfört med CAD-objektet. En metod för att snabbt få fram den behövda forminformationen från CAD-objektet är också utvecklad. För att få fram lämplig forminformation från datapunkter kan en parametrisk kurva eller yta, t.ex. NURBS, anpassas till dessa punkter. Ett delproblem som uppstår vid NURBS-anpassning vid användandet av Gauss-Newton metoden är studerad. Beräkningsaspekter för att få fram en sökriktning är diskuterade. Vi behandlar också metoder för NURBS anpassning som baserar sig på en teknik för separabla icke-linjära minstakvadratproblem. Denna teknik använder sig av variabelprojektioner för att separera beräkningarna av de linjära parametrarna från beräkningarna av de icke-linjära parametrarna.

  • 177.
    Bergström, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Evaluation of NURBS Surfaces for Regular Structured Parameter Values2015Inngår i: Journal of Computing and Information Science in Engineering, ISSN 1530-9827, E-ISSN 1944-7078, Vol. 15, nr 1, artikkel-id 11005Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The evaluation of surface points and derivatives of NURBS surfaces for parameter values that are regularly distributed in a rectangular structure is considered. Because of the regularity, parts of the computations can be done on just a small portion of all parameter values and computed data is stored and reused for many other parameter values. Hence, the evaluation of NURBS surfaces can be performed faster when the regularity is used. We are making a complexity analysis of the number of floating point operations, which is required for the evaluations. To get knowledge about how the evaluations perform in practice, we are doing a numerical experiment where we are measuring the runtime to obtain the output both by using ordinary evaluation of the NURBS surface and by making use of the regular structure. Making use of the regularity gives significantly faster output.

  • 178.
    Bergström, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Perspective depth extraction of points on a surface under an instantaneous rigid body transformation2012Inngår i: Journal of Computing and Information Science in Engineering, ISSN 1530-9827, E-ISSN 1944-7078, Vol. 12, nr 2Artikkel i tidsskrift (Fagfellevurdert)
  • 179.
    Bergström, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Reliable updates of the transformation in the iterative closest point algorithm2016Inngår i: Computational optimization and applications, ISSN 0926-6003, E-ISSN 1573-2894, Vol. 63, nr 2, s. 543-557Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The update of the rigid body transformation in the iterative closest point (ICP) algorithm is considered. The ICP algorithm is used to solve surface registration problems where a rigid body transformation is to be found for fitting a set of data points to a given surface. Two regions for constraining the update of the rigid body transformation in its parameter space to make it reliable are introduced. One of these regions gives a monotone convergence with respect to the value of the mean square error and the other region gives an upper bound for this value. Point-to-plane distance minimization is then used to obtain the update of the transformation such that it satisfies the used constraint.

  • 180.
    Bergström, Per
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Edlund, Ove
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Robust registration of point sets using iteratively reweighted least squares2014Inngår i: Computational optimization and applications, ISSN 0926-6003, E-ISSN 1573-2894, Vol. 58, nr 3, s. 543-561Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Registration of point sets is done by finding a rotation and translation that produces a best fit between a set of data points and a set of model points. We use robust M-estimation techniques to limit the influence of outliers, more specifically a modified version of the iterative closest point algorithm where we use iteratively re-weighed least squares to incorporate the robustness. We prove convergence with respect to the value of the objective function for this algorithm. A comparison is also done of different criterion functions to figure out their abilities to do appropriate point set fits, when the sets of data points contains outliers. The robust methods prove to be superior to least squares minimization in this setting.

  • 181.
    Bergström, Per
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Edlund, Ove
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Robust registration of surfaces using a refined iterative closest point algorithm with a trust region approach2017Inngår i: Numerical Algorithms, ISSN 1017-1398, E-ISSN 1572-9265, Vol. 74, nr 3, s. 755-779Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The problem of finding a rigid body transformation, which aligns a set of data points with a given surface, using a robust M-estimation technique is considered. A refined iterative closest point (ICP) algorithm is described where a minimization problem of point-to-plane distances with a proposed constraint is solved in each iteration to find an updating transformation. The constraint is derived from a sum of weighted squared point-to-point distances and forms a natural trust region, which ensures convergence. Only a minor number of additional computations are required to use it. Two alternative trust regions are introduced and analyzed. Finally, numerical results for some test problems are presented. It is obvious from these results that there is a significant advantage, with respect to convergence rate of accuracy, to use the proposed trust region approach in comparison with using point-to-point distance minimization as well as using point-to-plane distance minimization and a Newton- type update without any step size control.

  • 182.
    Bergström, Per
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Edlund, Ove
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Söderkvist, Inge
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Efficient computation of the Gauss-Newton direction when fitting NURBS using ODR2012Inngår i: BIT Numerical Mathematics, ISSN 0006-3835, E-ISSN 1572-9125, Vol. 52, nr 3, s. 571-588Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We consider a subproblem in parameter estimation using the Gauss-Newton algorithm with regularization for NURBS curve fitting. The NURBS curve is fitted to a set of data points in least-squares sense, where the sum of squared orthogonal distances is minimized. Control-points and weights are estimated. The knot-vector and the degree of the NURBS curve are kept constant. In the Gauss-Newton algorithm, a search direction is obtained from a linear overdetermined system with a Jacobian and a residual vector. Because of the properties of our problem, the Jacobian has a particular sparse structure which is suitable for performing a splitting of variables. We are handling the computational problems and report the obtained accuracy using different methods, and the elapsed real computational time. The splitting of variables is a two times faster method than using plain normal equations.

  • 183.
    Bergström, Per
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Edlund, Ove
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Söderkvist, Inge
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Repeated surface registration for on-line use2011Inngår i: The International Journal of Advanced Manufacturing Technology, ISSN 0268-3768, E-ISSN 1433-3015, Vol. 54, nr 5-8, s. 677-689Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We consider the problem of matching sets of 3D points from a measured surface to the surface of a corresponding computer-aided design (CAD) object. The problem arises in the production line where the shape of the produced items is to be compared on-line with its pre-described shape. The involved registration problem is solved using the iterative closest point (ICP) method. In order to make it suitable for on-line use, i.e., make it fast, we pre-process the surface representation of the CAD object. A data structure for this purpose is proposed and named Distance Varying Grid tree. It is based on a regular grid that encloses points sampled from the CAD surfaces. Additional finer grids are added to the vertices in the grid that are close to the sampled points. The structure is efficient since it utilizes that the sampled points are distributed on surfaces, and it provides fast identification of the sampled point that is closest to a measured point. A local linear approximation of the surface is used for improving the accuracy. Experiments are done on items produced for the body of a car. The experiments show that it is possible to reach good accuracy in the registration and decreasing the computational time by a factor 700 compared with using the common kd-tree structure.

  • 184.
    Bergström, Per
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Fergusson, Michael
    Xtura AB, Kungsbacka.
    Folkesson, Patrik
    Xtura AB, Kungsbacka.
    Runnemalm, Anna
    University West, Department of Engineering Sciences, Trollhättan.
    Ottosson, Mattias
    University West, Department of Engineering Sciences, Trollhättan.
    Andersson, Alf
    Chalmers University of Technology, Department of Product and Production Development.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Automatic in-line inspection of shape based on photogrammetry2016Inngår i: SPS16, Lund: SPS16 , 2016, , s. 9Konferansepaper (Fagfellevurdert)
    Abstract [en]

    We are describing a fully automatic in-line shape inspection system for controlling the shape of moving objects on a conveyor belt. The shapes of the objects are measured using a full-field optical shape measurement method based on photogrammetry. The photogrammetry system consists of four cameras, a flash, and a triggering device. When an object to be measured arrives at a given position relative to the system, the flash and cameras are synchronously triggered to capture images of the moving object. From the captured images a point-cloud representing the measured shape is created. The point-cloud is then aligned to a CAD-model, which defines the nominal shape of the measured object, using a best-fit method and a feature-based alignment method. Deviations between the point-cloud and the CAD-model are computed giving the output of the inspection process. The computational time to create a point-cloud from the captured images is about 30 seconds and the computational time for the comparison with the CAD-model is about ten milliseconds. We report on recent progress with the shape inspection system.

  • 185.
    Bergström, Per
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Khodadad, Davood
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hällstig, Emil
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Dual-wavelength digital holography: single-shot shape evaluation using speckle displacements and regularization2014Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 53, nr 1, s. 123-131Artikkel i tidsskrift (Fagfellevurdert)
  • 186.
    Bergström, Per
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Khodadad, Davood
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hällstig, Emil
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Single shot shape evaluation using dual-wavelength holographic reconstructions and regularization2014Inngår i: Fringe 2013: 7th International Workshop on Advanced Optical Imaging and Metrology / [ed] Wolfgang Osten, Berlin: Encyclopedia of Global Archaeology/Springer Verlag, 2014, s. 103-108Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The aim of this work is to evaluate the shape of a free form object using single shot digital holography. The digital holography results in a gradient field and wrapped phase maps representing the shape of the object. The task is then to find a surface representation from this data which is an inverse problem. To solve this inverse problem we are using regularization with additional shape information from the CAD-model of the measured object.

  • 187.
    Bergström, Per
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Rosendahl, Sara
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Shape verification using dual-wavelength holographic interferometry2011Inngår i: Optical Engineering: The Journal of SPIE, ISSN 0091-3286, E-ISSN 1560-2303, Vol. 50, nr 10, artikkel-id 101503Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In automotive industry there is an interest of controlling the shape of a large number of identical components on-line in the manufacturing process. We propose a method to do this by capturing a digital hologram of the object and then using information from its computer aided design (CAD) model to calculate the shape and determine the agreement between the manufactured object and the CAD-model. The holographic recording of the object is done using dual wavelengths with a synthetic wavelength of approximately 400 μm. The optical measurement results in a wrapped phase map with the phase values in the interval [−π, π]. Each phase interval represents a depth distance on the object of about 0.2 mm. The phase unwrapping is done iteratively using information from the CADmodel. This implies that it is possible to measure large discontinuities on the surface of the measured object. The method also gives a point-to-point correspondence between the measurement and the CAD-model which is vital for tolerance control.

  • 188.
    Bergström, Per
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Rosendahl, Sara
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Shape verification aimed for manufacturing process control2011Inngår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 49, nr 3, s. 403-409Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We describe a method to verify the shape of manufactured objects by using their design model. A non-contact measuring method that consists of a stereo-camera system and a single projected fringe pattern is used. The method acquires one image from each camera. Additional shape information from the design model is also used. This surface-measurement method gives an accuracy of about 45 µm. Deviations from the design model within ±1.6 mm can be correctly detected. The measured surface representation is matched to the design model using the ICP-method. Fast performance has been considered adapting the method for on-line use.

  • 189.
    Bergström, Per
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Söderkvist, Inge
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Fitting NURBS using separable least squares techniques2012Inngår i: International Journal of Mathematical Modelling and Numerical Optimisation, ISSN 2040-3607, E-ISSN 2040-3615, Vol. 3, nr 4, s. 319-334Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We consider the problem of fitting a non-uniform rational B-spline (NURBS) curve to a set of data points by determining the control points and the weights using techniques aimed for solving separable least squares problems. The main technique under consideration is the variable projection method which utilises that the NURBS model depends linearly on its control points but non-linearly on the weights. The variable projection method can be used with the Gauss-Newton algorithm but also with Newton's algorithm. We investigate the efficiency of the different algorithms when fitting NURBS and observe that the variable projection methods do not perform as well as reported for its use on, e.g., exponential fitting problems.

  • 190.
    Blahota, I.
    et al.
    Institute of Mathematics and Computer Sciences, College of Nyíregyháza, Nyíregyháza, Hungary.
    Nagy, K.
    Institute of Mathematics and Computer Sciences, College of Nyíregyháza, Nyíregyháza, Hungary.
    Persson, Lars-Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper. UiT the Arctic University of Norway, Narvik, Norway.
    Tephnadze, G.
    School of IT, Engineering and Mathematics, IV, University of Georgia, Tbilisi, Georgia.
    A sharp boundedness result for restricted maximal operators of Vilenkin–Fourier series on martingale Hardy spaces2019Inngår i: Georgian Mathematical Journal, ISSN 1072-947X, E-ISSN 1572-9176, Vol. 26, nr 3Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The restricted maximal operators of partial sums with respect to bounded Vilenkin systems are investigated. We derive the maximal subspace of positive numbers, for which this operator is bounded from the Hardy space H p {H-{p}} to the Lebesgue space L p {L-{p}} for all 0 < p ≤ 1 {0<p\leq 1}. We also prove that the result is sharp in a particular sense. 

  • 191.
    Blahota, I.
    et al.
    Institute of Mathematics and Computer Sciences, University of Nyìıregyhàza.
    Persson, Lars-Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Tephnadze, George
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Two-sided Estimates of the Lebesgue Constants with respect to Vilenkin Systems and Applications2018Inngår i: Glasgow Mathematical Journal, ISSN 0017-0895, E-ISSN 1469-509X, Vol. 60, nr 1, s. 17-34Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper, we derive two-sided estimates of the Lebesgue constants for bounded Vilenkin systems, we also present some applications of importance, e.g., we obtain a characterization for the boundedness of a subsequence of partial sums with respect to Vilenkin–Fourier series of H1 martingales in terms of n's variation. The conditions given in this paper are in a sense necessary and sufficient.

  • 192.
    Blahota, István
    et al.
    Institute of Mathematics and Computer Sciences, College of Nyíregyháza.
    Persson, Lars-Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Tephnadze, George
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    On the Nörlund means of Vilenkin-Fourier series2015Inngår i: Czechoslovak Mathematical Journal, ISSN 0011-4642, E-ISSN 1572-9141, Vol. 65, nr 4, s. 983-1002Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We prove and discuss some new (H p ,L p )-type inequalities of weighted maximal operators of Vilenkin-Nörlund means with non-increasing coefficients {q k : k ⩾ 0}. These results are the best possible in a special sense. As applications, some well-known as well as new results are pointed out in the theory of strong convergence of such Vilenkin-Nörlund means. To fulfil our main aims we also prove some new estimates of independent interest for the kernels of these summability results.In the special cases of general Nörlund means t n with non-increasing coefficients analogous results can be obtained for Fejér and Cesàro means by choosing the generating sequence {q k : k ⩾ 0} in an appropriate way.

  • 193.
    Blahota, István
    et al.
    Institute of Mathematics and Computer Sciences, University of Nyíregyháza.
    Tephnadze, George
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    A note on maximal operators of Vilenkin-Nörlund means2016Inngår i: Acta Mathematica Academiae Paedagogicae Nyíregyháziensis, ISSN 1786-0091, E-ISSN 1786-0091, Vol. 32, nr 12, s. 203-213Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper we prove and discuss some new (Hp;Lp)-type inequalities of weighted maximal operators of Vilenkin - Nörlund means with non-increasing coeffcients. These results are the best possible in a special sense. As applications, both some well-known and new results are pointed out in the theory of strong convergence of Vilenkin - Nörlund means with non-increasing coeffcients

  • 194.
    Blahota, István
    et al.
    Institute of Mathematics and Computer Sciences, College of Nyíregyháza.
    Tephnadze, George
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    On the (C, α)-means with respect to the Walsh system2014Inngår i: Analysis Mathematica, ISSN 0133-3852, E-ISSN 1588-273X, Vol. 40, nr 3, s. 161-174Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In our main result we prove strong convergence theorems for Cesàro means (C, α) on the Hardy spaces H1/(1+α), where 0 < α < 1. © 2014 Akadémiai Kiadó, Budapest, Hungary.

  • 195.
    Blahota, István
    et al.
    Institute of Mathematics and Computer Sciences, College of Nyíregyháza.
    Tephnadze, George
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Strong convergence theorem for Vilenkin-Fejér means2014Inngår i: Publicationes mathematicae (Debrecen), ISSN 0033-3883, E-ISSN 2064-2849, Vol. 85, nr 1-2, s. 181-196Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    As main result we prove strong convergence theorems of Vilenkin- Fejér means when 0 < p ≤ 1/2.

  • 196.
    Blanksvärd, Thomas
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Häggström, Jens
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Nilimaa, Jonny
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Sabourova, Natalia
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Täljsten, Björn
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Elfgren, Lennart
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Carolin, Anders
    Paulsson, Björn
    UIC, Trafikverket.
    Tu, Yongming
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Test to failure of a steel truss bridge: Calibration of assessment methods2014Inngår i: Bridge Maintenance, Safety, Management and Life Extension: proceedings of the Seventh International Conference of Bridge Maintenance, Safety and Management, 7-11 July 2014, Shanghai, China / [ed] Airong Chen; Dan M. Frangopol; Xin Ruan, London: CRC Press, Taylor & Francis Group , 2014, s. 1076-1081Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The steel truss railway bridge at Åby River was built in 1957 with a span of 32 m (105 feet). In 2012 it was replaced by a new steel beam bridge and the old bridge was placed beside the river. It was tested to failure to study its remaining load-carrying capacity in September 2013. The test was carried out by Luleå University of Technology by commission from Trafikverket as a part of the European Research Project MAINLINE (www.mainline-project.eu). In this paper some preliminary results are given. Two hydraulic jacks, anchored by cables to the bedrock, pulled the bridge downwards. The bridge remained elastic up to about three times the original design load and the load could then be almost doubled with substantial yielding deformations before a buckling failure appeared in the top girders for a load of ca. 11 MN (1000 short tons) for a midpoint deflection of ca. 0, 2 m (8 inches). No brittle or fatigue failure in any of the joints appeared and the bridge proved to behave in a ductile way with a substantial hidden capacity.

  • 197.
    Blumenau, A.T.
    et al.
    Max-Planck-Institut für Eisenforschung GmbH.
    Eberlein, T.A.G.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Frauenheim, T.
    Theoretische Physik, Universität Paderborn.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    The effect of charge on kink migration at 90° partial dislocations in SiC2005Inngår i: Physica status solidi. A, Applied research, ISSN 0031-8965, E-ISSN 1521-396X, Vol. 202, nr 5, s. 877-882Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    SiC bipolar devices show a degradation under forward-biased operation which has been linked with a recombination enhanced motion of one of the two glide dislocations having either Si or C core atoms. We have carried out calculations of the core structures and dynamics of partial dislocations in 3C and 2H-SiC using the density functional based codes DFTB and AIMPRO. After in earlier theoretical work we reported on the structure, energetics and electronic activity of both of the Shockley partials, and on the formation and migration barriers of kinks, in this work we present first results on the effect of charge on the disloction kinks. The calculations give insights into the device degradation mechanism.

  • 198.
    Blumenau, A.T.
    et al.
    University of Exeter.
    Elsner, J.
    University of Exeter.
    Jones, R.
    University of Exeter.
    Heggie, M.I.
    University of Sussex.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Frauenheim, T.
    Universität Paderborn.
    Briddon, P.R.
    University of Newcastle Upon Tyne.
    Dislocations in hexagonal and cubic GaN2000Inngår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 12, s. 10223-10233Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The structure and electronic activity of several types of dislocations in both hexagonal and cubic GaN are calculated using first-principles methods. Most of the stoichiometric dislocations investigated in hexagonal GaN do not induce deep acceptor states and thus cannot be responsible for the yellow luminescence. However, it is shown that electrically active point defects, in particular gallium vacancies and oxygen-related defect complexes, can be trapped at the stress field of the dislocations and may be responsible for this luminescence. For cubic GaN, we find the ideal stoichiometric 60° dislocation to be electrically active and the glide set to be more stable than the shuffle. The dissociation of the latter is considered

  • 199.
    Blumenau, A.T.
    et al.
    Theoretische Physik, Universität Paderborn.
    Fall, C.J.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Heggie, M.I.
    CPES, University of Sussex, Falmer, Brighton.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Frauenheim, T.
    Theoretische Physik, Universität Paderborn.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Straight and kinked 90° partial dislocations in diamond and 3C-SiC2002Inngår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 14, nr 48, s. 12741-12747Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Density-functional based calculations are used to investigate low energy core structures of 90° partial dislocations in diamond and 3C-SiC. In both materials dislocation glide is analysed in terms of kink formation and migration and the fundamental steps to kink migration are investigated. We find the C terminated core structure in SiC to be more mobile than the Si core. However, the Si partial is electrically active and this opens the possibility of recombination-enhanced glide under ionizing conditions or an enhanced mobility in doped material.

  • 200.
    Blumenau, A.T.
    et al.
    School of Physics, University of Exeter.
    Fall, C.J.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Frauenheim, T.
    Department of Physics, Faculty of Science, Universität Paderborn.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Structure and motion of basal dislocations in silicon carbide2003Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 68, nr 17, s. 174108-1Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    30° and 90° Shockley partial dislocations lying in {111} and basal planes of cubic and hexagonal silicon carbide, respectively, are investigated theoretically. Density-functional-based tight-binding total-energy calculations are used to determine the core structure and energetics of the dislocations. In a second step their electronic structure is investigated using a pseudopotential method with a Gaussian basis set. Finally, the thermal activation barriers to glide motion of 30° and 90° Shockley partials are calculated in terms of a process involving the formation and migration of kinks along the dislocation line. The mechanism for enhanced dislocation movement observed under current injection conditions in bipolar silicon carbide devices is discussed.

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