| 1. |
- Tornberg, Anna, et al.
(författare)
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A prehistory of violence : Evidence of violence related skull trauma in southern Sweden, 2300-1100 BCE
- 2022
-
Ingår i: Life and Afterlife in the Nordic Bronze Age : Proceedings of the 15th Nordic Bronze Age Symposium held in Lund 11th to 15th June 2019 - Proceedings of the 15th Nordic Bronze Age Symposium held in Lund 11th to 15th June 2019. - 0065-1001. - 9789189415430 - 9789189415447 ; :37, s. 99-118
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Konferensbidrag (refereegranskat)abstract
- Warriors and warfare have become common themes within Bronze Age archaeology the past 10 -20 years. Recent reporting of Neolithic and Bronze Age massacres and battlefields in Germany supports endemic violence in these regions. But what about in southern Scandinavia? This paper explores the evidence of violence related skull trauma from a pooled sample of 257 individuals from 40 different localities in southern Sweden. The results show that there is a relatively large difference in the frequency of skull trauma depending on burial type. Due to the common practice of Early Bronze Age reburials in Late Neolithic gallery graves, the high frequency of trauma in gallery graves and barrows is probably linked to increased violence rates in the Early Bronze Age. The majority of cases are caused by blunt force, and up to 13% of the individuals were affected. Most of the traumata were healed, especially among males. It is probable that the high levels of blunt force skull trauma in southern Sweden mirrors a society with endemic warfare during the Early Bronze Age.
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| 2. |
- Axelsson, Mette, et al.
(författare)
-
Mat vid diabetes. : En systematisk översikt med utvärdering av effekter samt hälsoekonomiska och etiska aspekter.
- 2022
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- SlutsatserTyp 1- och typ 2-diabetes Det finns ett samband mellan att äta medelhavskost och lägre risk att dö i förtid oavsett orsak (måttlig tillförlitlighet). Det finns ett samband mellan att äta en större andel2 fibrer eller baljväxter och lägre risk att dö i förtid oavsett orsak (måttlig tillförlitlighet). Det kan även finnas ett samband mellan att äta en större andel nötter och lägre risk att dö i förtid oavsett orsak (låg tillförlitlighet) samt lägre risk att insjukna i hjärt- och kärlsjukdom (låg tillförlitlighet). Det finns ett samband mellan att dricka mer2 kaffe och lägre risk att dö i förtid oavsett orsak och lägre risk att dö i förtid i kranskärlssjukdom (måttlig tillförlitlighet) samt möjligen en lägre risk att dö i förtid i hjärt- och kärlsjukdom (låg tillförlitlighet). Det råder generell brist på studier med lång uppföljningstid som jämför inverkan av olika slags kostråd på överlevnad, diabeteskomplikationer, diabetesremission3, livskvalitet och biverkningar. Tillförlitligheten av befintliga resultat är dessutom mycket låg för de flesta koster, kostbehandlingar, livsmedel och näringsämnen som har utvärderats. Effekter på hälsa och relaterade mått kan i dessa fall inte bedömas.2. Begreppet ”större andel” eller ”mer” avser inte nödvändigtvis att äta eller dricka mer totalt utan att öka mängden av ett visst livsmedel genom att byta ut annan mat eller dryck.Typ 2-diabetes Det kan finnas ett samband mellan att äta en större andel mättat fett och högre risk för att dö i förtid av hjärt- och kärlsjukdom (låg tillförlitlighet). Det kan även finnas ett samband mellan att äta en större andel enkelomättat fett och lägre risk att dö i förtid oavsett orsak (låg tillförlitlighet). En behandling med en initial period av kraftigt minskat energiintag med hjälp av lågenergipulver (VLED) med efterföljande övergång till mat för viktstabilitet jämfört med vanlig kostbehandling har gynnsamma effekter på livskvalitet (enligt EQ-5D), långtidsblodsocker (HbA1c) och vikt upp till 12 månader (måttlig tillförlitlighet)4. Vidare kan metoder där VLED ingår ha gynnsamma effekter på diabetesremission5 och midjeomfång upp till 12 månader (låg tillförlitlighet) och långtidsblodsocker (HbA1c) upp till 24 månader (låg tillförlitlighet). Intensiv livsstilsbehandling därlågfettkost kombineras med fysisk aktivitet och minskat energiintag har gynnsamma effekter jämfört med vanlig kostbehandling på långtidsblodsocker (HbA1c), vikt, kroppsmasseindex (BMI), midjeomfång och vissa blodfetter upp till 12 månader (måttlig tillförlitlighet)3. Viktminskningen kan kvarstå upp till omkring 10 år (låg tillförlitlighet). Behandlingen kan leda till bättre fysisk livskvalitet upp till 8 år (låg tillförlitlighet) medan effektskillnaden i psykisk livskvalitet under samma tid kan vara obefintlig eller försumbar (låg tillförlitlighet). Jämförelsen påvisar ingen förändrad risk att dö i förtid oavsett orsak eller att dö eller insjukna av kardiovaskulära orsaker efter omkring 10 år (låg tillförlitlighet). I det hälsoekonomiska perspektivet är intensiv livsstilsbehandling mer resurskrävande än vanlig kostbehandling, och beräkningar visar små eller inga vinster i kvalitetsjusterade levnadsår (QALYs) på individnivå. Energirestriktion i samband med intensiv livsstilsbehandling med ketogen kost eller med högproteinkost (20 E%) i kombination med fysisk aktivitet jämfört med vanlig kostbehandling kan ge en viktminskning upp till 11 månader (låg tillförlitlighet) men det saknas studier som kan visa om vikten kan bibehållas på längre sikt. Det saknas studier som undersökt kliniskt viktiga utfall som dödlighet, kardiovaskulära sjukdomar, livskvalitet och diabetesremission.3. Gäller endast vid typ 2-diabetes.4. Utgår från individer med en medelkroppsvikt på cirka 100 kg och medel-HbA1c på 60 mmol/mol.5. Resultaten för utfallet diabetesremission (att uppnå normala blodsockervärden) gäller när en diabetesdiagnos sattes för mindre än 6 år sedan eller för mindre än 3 år sedan. Definitionen för diabetesremission var ett HbA1c på mindre än 48 mmol/mol och att samtidigt vara fri från blodsockersänkande läkemedel.Graviditetsdiabetes Det saknas studier om kost vid graviditetsdiabetes med tillräcklig tillförlitlighet för att kunna bedöma effekterna.
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| 3. |
- Broms, Anna, et al.
(författare)
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A barrier method for contact avoiding particles in Stokes flow
- 2024
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Ingår i: Journal of Computational Physics. - : Elsevier. - 0021-9991 .- 1090-2716. ; 497, s. 112648-112648
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Tidskriftsartikel (refereegranskat)abstract
- Rigid particles in a Stokesian fluid experience an increasingly strong lubrication resistance as particle gaps narrow. Numerically, resolving these lubrication forces comes at an intractably large cost, even for moderate system sizes. Hence, it can typically not be guaranteed that artificial particle collisions and overlaps do not occur in a dynamic simulation, independently of the choice of method to solve the Stokes equations. In this work, the potentially large set of non-overlap constraints, in terms of the Euclidean distance between boundary points on disjoint particles, are efficiently represented via a barrier energy. We solve for the minimum magnitudes of repelling contact forces and torques between any particle pair in contact to correct for overlaps by enforcing a zero barrier energy at the next time level, given a contact-free configuration at a previous instance in time. Robustness for the method is illustrated using a multiblob method to solve the mobility problem in Stokes flow, applied to suspensions of spheres, rods and boomerang shaped particles. Collision free configurations are obtained at all instances in time, and considerably larger time-steps can be taken than without the technique. The effect of the contact forces on the collective order of a set of rods in a background flow that naturally promote particle interactions is also illustrated.
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| 4. |
- Broms, Anna, et al.
(författare)
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A locally corrected multiblob method with hydrodynamically matched grids for the Stokes mobility problem
- 2023
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Ingår i: Journal of Computational Physics. - : Elsevier. - 0021-9991 .- 1090-2716. ; 487, s. 112172-112172
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Tidskriftsartikel (refereegranskat)abstract
- Inexpensive numerical methods are key to enabling simulations of systems of a large number of particles of different shapes in Stokes flow and several approximate methods have been introduced for this purpose. We study the accuracy of the multiblob method for solving the Stokes mobility problem in free space, where the 3D geometry of a particle surface is discretised with spherical blobs and the pair-wise interaction between blobs is described by the RPY-tensor. The paper aims to investigate and improve on the magnitude of the error in the solution velocities of the Stokes mobility problem using a combination of two different techniques: an optimally chosen grid of blobs and a pair-correction inspired by Stokesian dynamics. Different optimisation strategies to determine a grid with a given number of blobs are presented with the aim of matching the hydrodynamic response of a single accurately described ideal particle, alone in the fluid. It is essential to obtain small errors in this self-interaction, as they determine the basic error level in a system of well-separated particles. With an optimised grid, reasonable accuracy can be obtained even with coarse blob-resolutions of the particle surfaces. The error in the self-interaction is however sensitive to the exact choice of grid parameters and simply hand-picking a suitable geometry of blobs can lead to errors several orders of magnitude larger in size. The pair-correction is local and cheap to apply, and reduces the error for moderately separated particles and particles in close proximity. Two different types of geometries are considered: spheres and axisymmetric rods with smooth caps. The error in solutions to mobility problems is quantified for particles of varying inter-particle distances for systems containing a few particles, comparing to an accurate solution based on a second kind BIE-formulation where the quadrature error is controlled by employing quadrature by expansion (QBX).
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| 5. |
- Broms, Anna, et al.
(författare)
-
A Method of Fundamental Solutions for Large-Scale 3D Elastance and Mobility Problems
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The method of fundamental solutions (MFS) is effective for the 3D Laplace and Stokes Dirichlet BVPs in the exterior of a collection of simple objects. Here, we present new formulations for the related elastance and mobility problems in the same geometries. The mobility problem computes rigid body velocities, given net forces and torques on the particles, while the elastance problem is the counterpart for Laplace and solves the problem of conductors with known net charges to determine unknown constant potentials. The new formulations allow for application of one-body preconditioning of the resulting systems, and for large suspensions with moderate lubrication forces, sources on inner proxy-surfaces give accuracy on par with a well-resolved boundary integral formulation. The performance of the well-conditioned mobility solver is demonstrated for a suspension of 10000 nearby ellipsoids. Using a discretization with $2.59\times 10^7$ total degrees of freedom in the preconditioned system, GMRES converges to five digits of accuracy in less than two hours on a workstation and the scaling is linear in the number of particles.
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| 6. |
- Broms, Anna, et al.
(författare)
-
A reflection enhanced method of fundamental solutions for Laplace and Stokes boundary value problems in 2D
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Two elliptic PDEs with Dirichlet boundary conditions are considered in 2D for a collection of simple objects: the exterior Laplace and Stokes boundary value problems. We present a novel, cost-effective, accurate and singularity-free solution technique based on the method of fundamental solutions. For circular objects, controllable accuracy is obtained for close-to-touching neighbours, with sources on inner proxy-boundaries complemented as needed by a small set of extra singularities in near-contact regions. The locations of the extra sources are deduced from the fractal obtained by repeated inversion of circles in circles, sometimes referred to as Indra’s pearls. For Stokes, results for coarsely resolved closely interacting circular particles, undergoing rigid body motion, are compared to results from a well-resolved boundary integral equation equipped with a special quadrature method. A careful parameter study is made for the locations of the additional sources and their singularity types to reach a target accuracy of 10^{-6} for circles of unequal radii, down to particle separations of a thousandth of the particle radii. For well-separated objects, a one-body preconditioning strategy allows for acceleration with the fast multipole method.
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| 7. |
- Broms, Anna, 1993-
(författare)
-
Accuracy, efficiency and robustness for rigid particle simulations in Stokes flow
- 2024
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The thesis concerns simulation techniques for systems of nano- to micro-scaled rigid particles immersed in a viscous fluid, ubiquitous in nature and industry. With negligible fluid inertia, the set of PDEs known as the Stokes equations can be used to model the hydrodynamics. For a dynamic study, the PDEs have to be solved at any given instance of time, provided the particle configuration and any non-hydrodynamic interactions. The resulting particle velocities can then be used to update the particle coordinates, and the equations repeatedly solved anew. For any simulation result of a physical system to be reliable, it is crucial to control different error contributions, with two error types here particularly in focus: those related to solving the Stokes equations and those related to the update in time.The PDEs can be recast as boundary integral equations (BIEs) that hold on the particle surfaces. Hydrodynamic interactions are challenging: they are simultaneously long-ranged and expensive to resolve both in time and space for closely interacting particles. The latter is caused by strong lubrication forces resulting from bodies in relative motion. We approach two alternative and related techniques to BIEs that allow for more cost-effective simulations, namely the rigid multiblob method and the method of fundamental solutions. The former is a regularisation technique that allows for generally shaped particles in large systems, both with and without thermal fluctuations. We make two improvements: the basic error level is tied to the discretisation and set by solving a small optimisation problem off-line for each given particle shape, and the accuracy for closely interacting particles is improved by pair-corrections. With the method of fundamental solutions, we present a technique with linear or close to linear scaling in the number of particles, depending on if a so-called resistance or mobility problem is solved. For circles and spheres, the accuracy can be controlled to a target level independently of the particle separations. This is done by the introduction of a small set of image points for every pair of particles close to contact that well manage to represent lubrication forces. In the model, particles can neither touch nor overlap, and our work on time-stepping is tied to the problem of contact avoiding. We develop a new strategy that guarantees contact free simulations in 3D, essential for studying the system of particles over long time spans. Controlled accuracy in solutions to the Stokes equations can together with robust timestepping allow for simulations that can complement physical experiments of particle systems for a better understanding of their behaviour, to drive the development in fields such as materials science, biomedical engineering and environmental engineering.
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| 8. |
- Broms, Anna, et al.
(författare)
-
Accurate close interactions of Stokes spheres using lubrication-adapted image systems
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Stokes flows with near-touching rigid particles induce near-singular lubrication forces under relative motion, making their accurate numerical treatment challenging. With the aim of controlling the accuracy with a computationally cheap method, we present a new technique that combines the method of fundamental solutions (MFS) with the method of images. For rigid spheres, we propose to represent the flow using Stokeslet sources on interior spheres, augmented by lines of sources adapted to each near-contact to resolve lubrication. The source strengths are found via a least-squares solve at contact-adapted boundary collocation nodes. Results for coarsely resolved spheres undergoing rigid body motion are compared to reference solutions determined with a well-resolved boundary integral formulation equipped with a special quadrature method. With less than 60 additional sources per particle per contact, we show controlled accuracy to three digits in the relative surface velocities for separations between the particles down to a thousandth of the particle radius. Computed forces and torques are more accurate than surface velocities, by a few orders of magnitude. For fixed spheres in a given background flow, the proxy-surface discretization alone gives controlled accuracy. A one-body preconditioning strategy allows for acceleration with the fast multipole method that combined yield close to linear scaling in the number of particles. This is demonstrated by solving problems of up to 2000 spheres on a workstation using 700 unknown proxy-sources per particle.
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| 9. |
- af Klinteberg, Ludvig, et al.
(författare)
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A fast integral equation method for solid particles in viscous flow using quadrature by expansion
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Boundary integral methods are advantageous when simulating viscous flow around rigid particles, due to the reduction in number of unknowns and straightforward handling of the geometry. In this work we present a fast and accurate framework for simulating spheroids in periodic Stokes flow, which is based on the completed double layer boundary integral formulation. The framework implements a new method known as quadrature by expansion (QBX), which uses surrogate local expansions of the layer potential to evaluate it to very high accuracy both on and off the particle surfaces. This quadrature method is accelerated through a newly developed precomputation scheme. The long range interactions are computed using the spectral Ewald (SE) fast summation method, which after integration with QBX allows the resulting system to be solved in M log M time, where M is the number of particles. This framework is suitable for simulations of large particle systems, and can be used for studying e.g. porous media models.
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| 10. |
- af Klinteberg, Ludvig, et al.
(författare)
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A fast integral equation method for solid particles in viscous flow using quadrature by expansion
- 2016
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Ingår i: Journal of Computational Physics. - : Academic Press. - 0021-9991 .- 1090-2716. ; 326, s. 420-445
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Tidskriftsartikel (refereegranskat)abstract
- Boundary integral methods are advantageous when simulating viscous flow around rigid particles, due to the reduction in number of unknowns and straightforward handling of the geometry. In this work we present a fast and accurate framework for simulating spheroids in periodic Stokes flow, which is based on the completed double layer boundary integral formulation. The framework implements a new method known as quadrature by expansion (QBX), which uses surrogate local expansions of the layer potential to evaluate it to very high accuracy both on and off the particle surfaces. This quadrature method is accelerated through a newly developed precomputation scheme. The long range interactions are computed using the spectral Ewald (SE) fast summation method, which after integration with QBX allows the resulting system to be solved in M log M time, where M is the number of particles. This framework is suitable for simulations of large particle systems, and can be used for studying e.g. porous media models.
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| 11. |
- af Klinteberg, Ludvig, et al.
(författare)
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Adaptive Quadrature by Expansion for Layer Potential Evaluation in Two Dimensions
- 2018
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Ingår i: SIAM Journal on Scientific Computing. - : Society for Industrial and Applied Mathematics. - 1064-8275 .- 1095-7197. ; 40:3, s. A1225-A1249
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Tidskriftsartikel (refereegranskat)abstract
- When solving partial differential equations using boundary integral equation methods, accurate evaluation of singular and nearly singular integrals in layer potentials is crucial. A recent scheme for this is quadrature by expansion (QBX), which solves the problem by locally approximating the potential using a local expansion centered at some distance from the source boundary. In this paper we introduce an extension of the QBX scheme in two dimensions (2D) denoted AQBX—adaptive quadrature by expansion—which combines QBX with an algorithm for automated selection of parameters, based on a target error tolerance. A key component in this algorithm is the ability to accurately estimate the numerical errors in the coefficients of the expansion. Combining previous results for flat panels with a procedure for taking the panel shape into account, we derive such error estimates for arbitrarily shaped boundaries in 2D that are discretized using panel-based Gauss–Legendre quadrature. Applying our scheme to numerical solutions of Dirichlet problems for the Laplace and Helmholtz equations, and also for solving these equations, we find that the scheme is able to satisfy a given target tolerance to within an order of magnitude, making it useful for practical applications. This represents a significant simplification over the original QBX algorithm, in which choosing a good set of parameters can be hard.
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| 12. |
- af Klinteberg, Ludvig, et al.
(författare)
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An explicit Eulerian method for multiphase flow with contact line dynamics and insoluble surfactant
- 2014
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Ingår i: Computers & Fluids. - : Elsevier BV. - 0045-7930 .- 1879-0747. ; 101, s. 50-63
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Tidskriftsartikel (refereegranskat)abstract
- The flow behavior of many multiphase flow applications is greatly influenced by wetting properties and the presence of surfactants. We present a numerical method for two-phase flow with insoluble surfactants and contact line dynamics in two dimensions. The method is based on decomposing the interface between two fluids into segments, which are explicitly represented on a local Eulerian grid. It provides a natural framework for treating the surfactant concentration equation, which is solved locally on each segment. An accurate numerical method for the coupled interface/surfactant system is given. The system is coupled to the Navier-Stokes equations through the immersed boundary method, and we discuss the issue of force regularization in wetting problems, when the interface touches the boundary of the domain. We use the method to illustrate how the presence of surfactants influences the behavior of free and wetting drops.
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| 13. |
- af Klinteberg, Ludvig
(författare)
-
Computational methods for microfluidics
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is concerned with computational methods for fluid flows on the microscale, also known as microfluidics. This is motivated by current research in biological physics and miniaturization technology, where there is a need to understand complex flows involving microscale structures. Numerical simulations are an important tool for doing this.The first paper of the thesis presents a numerical method for simulating multiphase flows involving insoluble surfactants and moving contact lines. The method is based on an explicit interface tracking method, wherein the interface between two fluids is decomposed into segments, which are represented locally on an Eulerian grid. The framework of this method provides a natural setting for solving the advection-diffusion equation governing the surfactant concentration on the interface. Open interfaces and moving contact lines are also incorporated into the method in a natural way, though we show that care must be taken when regularizing interface forces to the grid near the boundary of the computational domain.In the second paper we present a boundary integral formulation for sedimenting particles in periodic Stokes flow, using the completed double layer boundary integral formulation. The long-range nature of the particle-particle interactions lead to the formulation containing sums which are not absolutely convergent if computed directly. This is solved by applying the method of Ewald summation, which in turn is computed in a fast manner by using the FFT-based spectral Ewald method. The complexity of the resulting method is O(N log N), as the system size is scaled up with the number of discretization points N. We apply the method to systems of sedimenting spheroids, which are discretized using the Nyström method and a basic quadrature rule.The Ewald summation method used in the boundary integral method of the second paper requires a decomposition of the potential being summed. In the introductory chapters of the thesis we present an overview of the available methods for creating Ewald decompositions, and show how the methods and decompositions can be related to each other.
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| 14. |
- af Klinteberg, Ludvig, et al.
(författare)
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Error estimation for quadrature by expansion in layer potential evaluation
- 2017
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Ingår i: Advances in Computational Mathematics. - : Springer. - 1019-7168 .- 1572-9044. ; 43:1, s. 195-234
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Tidskriftsartikel (refereegranskat)abstract
- In boundary integral methods it is often necessary to evaluate layer potentials on or close to the boundary, where the underlying integral is difficult to evaluate numerically. Quadrature by expansion (QBX) is a new method for dealing with such integrals, and it is based on forming a local expansion of the layer potential close to the boundary. In doing so, one introduces a new quadrature error due to nearly singular integration in the evaluation of expansion coefficients. Using a method based on contour integration and calculus of residues, the quadrature error of nearly singular integrals can be accurately estimated. This makes it possible to derive accurate estimates for the quadrature errors related to QBX, when applied to layer potentials in two and three dimensions. As examples we derive estimates for the Laplace and Helmholtz single layer potentials. These results can be used for parameter selection in practical applications.
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| 15. |
- af Klinteberg, Ludvig, et al.
(författare)
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Estimation of quadrature errors in layer potential evaluation using quadrature by expansion
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In boundary integral methods it is often necessary to evaluate layer potentials on or close to the boundary, where the underlying integral is difficult to evaluate numerically. Quadrature by expansion (QBX) is a new method for dealing with such integrals, and it is based on forming a local expansion of the layer potential close to the boundary. In doing so, one introduces a new quadrature error due to nearly singular integration in the evaluation of expansion coefficients. Using a method based on contour integration and calculus of residues, the quadrature error of nearly singular integrals can be accurately estimated. This makes it possible to derive accurate estimates for the quadrature errors related to QBX, when applied to layer potentials in two and three dimensions. As examples we derive estimates for the Laplace and Helmholtz single layer potentials. These results can be used for parameter selection in practical applications.
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| 16. |
- af Klinteberg, Ludvig
(författare)
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Fast and accurate integral equation methods with applications in microfluidics
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is concerned with computational methods for fluid flows on the microscale, also known as microfluidics. This is motivated by current research in biological physics and miniaturization technology, where there is a need to understand complex flows involving microscale structures. Numerical simulations are an important tool for doing this.The first, and smaller, part of the thesis presents a numerical method for simulating multiphase flows involving insoluble surfactants and moving contact lines. The method is based on an interface decomposition resulting in local, Eulerian grid representations. This provides a natural setting for solving the PDE governing the surfactant concentration on the interface.The second, and larger, part of the thesis is concerned with a framework for simulating large systems of rigid particles in three-dimensional, periodic viscous flow using a boundary integral formulation. This framework can solve the underlying flow equations to high accuracy, due to the accurate nature of surface quadrature. It is also fast, due to the natural coupling between boundary integral methods and fast summation methods.The development of the boundary integral framework spans several different fields of numerical analysis. For fast computations of large systems, a fast Ewald summation method known as Spectral Ewald is adapted to work with the Stokes double layer potential. For accurate numerical integration, a method known as Quadrature by Expansion is developed for this same potential, and also accelerated through a scheme based on geometrical symmetries. To better understand the errors accompanying this quadrature method, an error analysis based on contour integration and calculus of residues is carried out, resulting in highly accurate error estimates.
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| 17. |
- af Klinteberg, Ludvig, et al.
(författare)
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Fast Ewald summation for free-space Stokes potentials
- 2017
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Ingår i: Research in the Mathematical Sciences. - : Springer. - 2197-9847. ; 4:1
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Tidskriftsartikel (refereegranskat)abstract
- We present a spectrally accurate method for the rapid evaluation of free-space Stokes potentials, i.e., sums involving a large number of free space Green’s functions. We consider sums involving stokeslets, stresslets and rotlets that appear in boundary integral methods and potential methods for solving Stokes equations. The method combines the framework of the Spectral Ewald method for periodic problems (Lindbo and Tornberg in J Comput Phys 229(23):8994–9010, 2010. doi: 10.1016/j.jcp.2010.08.026 ), with a very recent approach to solving the free-space harmonic and biharmonic equations using fast Fourier transforms (FFTs) on a uniform grid (Vico et al. in J Comput Phys 323:191–203, 2016. doi: 10.1016/j.jcp.2016.07.028 ). Convolution with a truncated Gaussian function is used to place point sources on a grid. With precomputation of a scalar grid quantity that does not depend on these sources, the amount of oversampling of the grids with Gaussians can be kept at a factor of two, the minimum for aperiodic convolutions by FFTs. The resulting algorithm has a computational complexity of $$O(N \log N)$$ O ( N log N ) for problems with N sources and targets. Comparison is made with a fast multipole method to show that the performance of the new method is competitive.
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| 18. |
- af Klinteberg, Ludvig, et al.
(författare)
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Fast Ewald summation for Stokesian particle suspensions
- 2014
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Ingår i: International Journal for Numerical Methods in Fluids. - : John Wiley & Sons. - 0271-2091 .- 1097-0363. ; 76:10, s. 669-698
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Tidskriftsartikel (refereegranskat)abstract
- We present a numerical method for suspensions of spheroids of arbitrary aspect ratio, which sediment under gravity. The method is based on a periodized boundary integral formulation using the Stokes double layer potential. The resulting discrete system is solved iteratively using generalized minimal residual accelerated by the spectral Ewald method, which reduces the computational complexity to O(N log N), where N is the number of points used to discretize the particle surfaces. We develop predictive error estimates, which can be used to optimize the choice of parameters in the Ewald summation. Numerical tests show that the method is well conditioned and provides good accuracy when validated against reference solutions.
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| 19. |
- af Klinteberg, Ludvig, et al.
(författare)
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Quadrature error estimates for layer potentials evaluated near curved surfaces in three dimensions
- 2022
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Ingår i: Computers and Mathematics with Applications. - : Elsevier BV. - 0898-1221 .- 1873-7668. ; 111, s. 1-19
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Tidskriftsartikel (refereegranskat)abstract
- The quadrature error associated with a regular quadrature rule for evaluation of a layer potential increases rapidly when the evaluation point approaches the surface and the integral becomes nearly singular. Error estimates are needed to determine when the accuracy is insufficient and a more costly special quadrature method should be utilized.& nbsp;The final result of this paper are such quadrature error estimates for the composite Gauss-Legendre rule and the global trapezoidal rule, when applied to evaluate layer potentials defined over smooth curved surfaces in R-3. The estimates have no unknown coefficients and can be efficiently evaluated given the discretization of the surface, invoking a local one-dimensional root-finding procedure. They are derived starting with integrals over curves, using complex analysis involving contour integrals, residue calculus and branch cuts. By complexifying the parameter plane, the theory can be used to derive estimates also for curves in R3. These results are then used in the derivation of the estimates for integrals over surfaces. In this procedure, we also obtain error estimates for layer potentials evaluated over curves in R2. Such estimates combined with a local root-finding procedure for their evaluation were earlier derived for the composite Gauss-Legendre rule for layer potentials written in complex form [4]. This is here extended to provide quadrature error estimates for both complex and real formulations of layer potentials, both for the Gauss-Legendre and the trapezoidal rule.& nbsp;Numerical examples are given to illustrate the performance of the quadrature error estimates. The estimates for integration over curves are in many cases remarkably precise, and the estimates for curved surfaces in R-3 are also sufficiently precise, with sufficiently low computational cost, to be practically useful.
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| 20. |
- Bagge, Joar, 1991-
(författare)
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Accurate quadrature and fast summation in boundary integral methods for Stokes flow
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis concerns accurate and efficient numerical methods for the simulation of fluid flow on the microscale, known as Stokes flow or creeping flow. Such flows are important, for example, in understanding the swimming of microorganisms, spreading of dust particles, as well as in developing new nano-materials, and microfluidic devices that can be used for on-the-fly analysis of blood samples, among other things.Flow on the microscale is dominated by viscous forces, meaning that a fluid such as water or air will behave as a very viscous fluid, like e.g. honey. The equations governing the flow, known as the Stokes equations, are linear PDEs, which permits the use of boundary integral methods (BIMs). In these methods, the PDE is reformulated as a boundary integral equation, thus reducing the dimensionality of the computational problem from three dimensions to two dimensions. The boundary integral formulation is well-conditioned, so that high accuracy can be achieved.We consider two main challenges related to BIMs. The first challenge is that the integrals in the formulation contain integrands that vary rapidly for evaluation points close to the boundary, and cannot be accurately resolved using a standard method for numerical integration. Therefore, special quadrature methods are needed. We consider two such methods: quadrature by expansion (QBX) and the “line extrapolation/interpolation method” (also known as the Hedgehog method). In particular, we consider these methods applied to simulations involving rigid rodlike particles and surrounding walls.The second challenge is that discretizing the boundary integral formulation leads to a dense linear system, which requires O(N2) operations to solve iteratively, where N is the number of unknowns. This becomes too expensive for large systems. A fast summation method, such as the Spectral Ewald (SE) method considered in this thesis, reduces the number of operations required, for example to O(N log N). The SE method can also be used for problems with periodic boundary conditions in any number of the spatial directions (arbitrary periodicity).We also consider an application of these methods to a flow problem involving an inertial spheroid in a parabolic flow profile, and analyze the lateral drift of this spheroidal particle.The numerical methods studied in this thesis enable fast and accurate computer simulations of e.g. suspensions of rigid particles in three-dimensional Stokes flow, including surrounding walls and arbitrary periodicity.
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| 21. |
- Bagge, Joar, 1991-, et al.
(författare)
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Accurate quadrature methods with application to Stokes flow with particles in confined geometries
- 2017
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Ingår i: Proceedings of the Eleventh UK Conference on Boundary Integral Methods (UKBIM 11). - Nottingham : Nottingham Trent University. ; , s. 15-24
-
Konferensbidrag (refereegranskat)abstract
- Boundary integral methods are attractive for simulating Stokes flow with particles or droplets due to the reduction in dimensionality and natural handling of the geometry. In many problems walls are present, and it becomes necessary to evaluate singular or nearly singular layer potentials over the wall. In this paper we show how this can be done using quadrature by expansion (QBX), a relatively new method based on local expansions of the layer potential. We present results for the Laplace single layer potential and the Stokes double layer potential. QBX can be used to evaluate the potentials to high accuracy arbitrarily close to the wall and on the wall. We also discuss how some quantities can be precomputed and how geometric symmetries can be used to reduce precomputation and storage.
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| 22. |
- Bagge, Joar, 1991-, et al.
(författare)
-
Accurate quadrature via line extrapolation and rational approximation with application to boundary integral methods for Stokes flow
- 2023
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- In boundary integral methods, special quadrature methods are needed to approximate layer potentials, integrals where the integrand is singular or sharply peaked for evaluation points on or close to the boundaries. In this paper, we study a method based on extrapolation or interpolation along a line, sometimes called the Hedgehog method. In this method, the layer potential is evaluated with a regular quadrature method for evaluation points along a line, and an approximant is constructed and evaluated in an area of interest where the original layer potential is difficult to evaluate due to it being singular or sharply peaked.We analyze the errors in the Hedgehog method with polynomial approximation, and use this to construct optimal distributions of sample points. Furthermore, rational approximation is introduced in the Hedgehog method, and compared with polynomial approximation. It is found that rational approximation can typically achieve a lower error than polynomial approximation, and does not increase the computational cost of the method significantly. Strategies for avoiding and dealing with spurious poles in rational approximation are discussed.We compare extrapolation (no sample point on the boundary) with interpolation (sample point present) in the Hedgehog method, and find that the error in our example is lower in the interpolation case by around one order of magnitude, compared to the extrapolation case.We consider a specific test case, consisting of two rigid rodlike particles in Stokes flow. Parameter selection and error estimation for the Hedgehog method is discussed for this test case. The accuracy and computational cost of the Hedgehog method is examined, and compared with another special quadrature method, namely quadrature by expansion (QBX). We find that the Hedgehog method should be able to compete with QBX in this context, but further investigation is needed for strict tolerances.
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| 23. |
- Bagge, Joar, 1991-, et al.
(författare)
-
Fast Ewald summation for Stokes flow with arbitrary periodicity
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- A fast and spectrally accurate Ewald summation method for the evaluation of stokeslet, stresslet and rotlet potentials of three-dimensional Stokes flow is presented. This work extends the previously developed Spectral Ewald method for Stokes flow to periodic boundary conditions in any number (three, two, one, or none) of the spatial directions, in a unified framework. The periodic potential is split into a short-range and a long-range part, where the latter is treated in Fourier space using the fast Fourier transform. A crucial component of the method is the modified kernels used to treat singular integration. We derive new modified kernels, and new improved truncation error estimates for the stokeslet and stresslet. An automated procedure for selecting parameters based on a given error tolerance is designed and tested. Analytical formulas for validation in the doubly and singly periodic cases are presented. We show that the computational time of the method scales like O(N log N) for N sources and targets, and investigate how the time depends on the error tolerance and window function, i.e. the function used to smoothly spread irregular point data to a uniform grid. The method is fastest in the fully periodic case, while the run time in the free-space case is around three times as large. Furthermore, the highest efficiency is reached when applying the method to a uniform source distribution in a primary cell with low aspect ratio. The work presented in this paper enables efficient and accurate simulations of three-dimensional Stokes flow with arbitrary periodicity using e.g. boundary integral and potential methods.
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| 24. |
- Bagge, Joar, 1991-, et al.
(författare)
-
Fast Ewald summation for Stokes flow with arbitrary periodicity
- 2023
-
Ingår i: Journal of Computational Physics. - : Elsevier BV. - 0021-9991 .- 1090-2716. ; 493, s. 112473-
-
Tidskriftsartikel (refereegranskat)abstract
- A fast and spectrally accurate Ewald summation method for the evaluation of stokeslet, stresslet and rotlet potentials of three-dimensional Stokes flow is presented. This work extends the previously developed Spectral Ewald method for Stokes flow to periodic boundary conditions in any number (three, two, one, or none) of the spatial directions, in a unified framework. The periodic potential is split into a short-range and a long-range part, where the latter is treated in Fourier space using the fast Fourier transform. A crucial component of the method is the modified kernels used to treat singular integration. We derive new modified kernels, and new improved truncation error estimates for the stokeslet and stresslet. An automated procedure for selecting parameters based on a given error tolerance is designed and tested. Analytical formulas for validation in the doubly and singly periodic cases are presented. We show that the computational time of the method scales like O(NlogN) for N sources and targets, and investigate how the time depends on the error tolerance and window function, i.e. the function used to smoothly spread irregular point data to a uniform grid. The method is fastest in the fully periodic case, while the run time in the free-space case is around three times as large. Furthermore, the highest efficiency is reached when applying the method to a uniform source distribution in a primary cell with low aspect ratio. The work presented in this paper enables efficient and accurate simulations of three-dimensional Stokes flow with arbitrary periodicity using e.g. boundary integral and potential methods.
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| 25. |
- Bagge, Joar, 1991-, et al.
(författare)
-
Highly accurate special quadrature methods for Stokesian particle suspensions in confined geometries
- 2021
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Ingår i: International Journal for Numerical Methods in Fluids. - : Wiley. - 0271-2091 .- 1097-0363. ; 93:7, s. 2175-2224
-
Tidskriftsartikel (refereegranskat)abstract
- Boundary integral methods are highly suited for problems with complicated geometries, but require special quadrature methods to accurately compute the singular and nearly singular layer potentials that appear in them. This article presents a boundary integral method that can be used to study the motion of rigid particles in three-dimensional periodic Stokes flow with confining walls. A centerpiece of our method is the highly accurate special quadrature method, which is based on a combination of upsampled quadrature and quadrature by expansion, accelerated using a precomputation scheme. The method is demonstrated for rodlike and spheroidal particles, with the confining geometry given by a pipe or a pair of flat walls. A parameter selection strategy for the special quadrature method is presented and tested. Periodic interactions are computed using the spectral Ewald fast summation method, which allows our method to run in O(n log n) time for n grid points in the primary cell, assuming the number of geometrical objects grows while the grid point concentration is kept fixed.
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| 26. |
- Bagge, Joar, 1991-, et al.
(författare)
-
Parabolic velocity profile causes shape-selective drift of inertial ellipsoids
- 2021
-
Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 926
-
Tidskriftsartikel (refereegranskat)abstract
- Understanding particle drift in suspension flows is of the highest importance in numerous engineering applications where particles need to be separated and filtered out from the suspending fluid. Commonly known drift mechanisms such as the Magnus force, Saffman force and Segre-Silberberg effect all arise only due to inertia of the fluid, with similar effects on all non-spherical particle shapes. In this work, we present a new shape-selective lateral drift mechanism, arising from particle inertia rather than fluid inertia, for ellipsoidal particles in a parabolic velocity profile. We show that the new drift is caused by an intermittent tumbling rotational motion in the local shear flow together with translational inertia of the particle, while rotational inertia is negligible. We find that the drift is maximal when particle inertial forces are of approximately the same order of magnitude as viscous forces, and that both extremely light and extremely heavy particles have negligible drift. Furthermore, since tumbling motion is not a stable rotational state for inertial oblate spheroids (nor for spheres), this new drift only applies to prolate spheroids or tri-axial ellipsoids. Finally, the drift is compared with the effect of gravity acting in the directions parallel and normal to the flow. The new drift mechanism is stronger than gravitational effects as long as gravity is less than a critical value. The critical gravity is highest (i.e. the new drift mechanism dominates over gravitationally induced drift mechanisms) when gravity acts parallel to the flow and the particles are small.
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| 27. |
- Bergerbrant, Sophie, 1968, et al.
(författare)
-
Identifying commoners in the Bronze Age: burials outside barrows
- 2017
-
Ingår i: New Perspectives on the Bronze Age: Proceedings from the 13th Nordic Bronze Age Symposium, held in Gothenburg 9th June to 13th June 2015. - Oxford : Archaeopress. - 9781784915988 ; , s. 37-64
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- This article discusses the possibility of social division and the presence of commoners in south Scandinavia during the Early Bronze Age. The discussion is based on new scientific and archaeological data generated in the project Travels, transmissions and transformations in temperate northern Europe during the 3rd and 2nd millennium BC: The rise of Bronze Age societies. Based on a comprehensive radiocarbon dating program, we were able to re-assign many skeletons, previously assumed to be Late Neolithic, to the Bronze Age. This accounted for a significant proportion of non-elite burials (including those of children) that had previously been ‘mysteriously’ missing in the archaeological Bronze Age record. Moreover, strontium isotope analyses reveal that individuals seem to be mobile regardless of their wealth status and burial rituals. It suggests a society where workers and perhaps even nonfree labourers were mobile, not only the elite segment.
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| 28. |
- Blank, Malou, 1975, et al.
(författare)
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New Perspectives on the Late Neolithic of South-Western Sweden. An Interdisciplinary Investigation of the Gallery Grave Falköping Stad 5
- 2018
-
Ingår i: Open Archaeology. - : Walter de Gruyter GmbH. - 2300-6560. ; 4:1, s. 1-35
-
Tidskriftsartikel (refereegranskat)abstract
- This article presents the results of an interdisciplinary study combining archaeology, osteology, and stable isotope analyses. The geological conditions and richness of megalithic graves in Falbygden is suitable for studies of Neolithic human remains. Nevertheless, the Late Neolithic period (2350-1700 BC) is poorly investigated. This paper explores new knowledge of the Late Neolithic megalithic population in Falbygden. In-depth osteological and archaeological studies focusing on a single gallery grave (Falkoping stad 5) were conducted. Radiocarbon dating and carbon, nitrogen, and strontium isotope analyses of teeth from twenty-one individuals revealed the time of the grave's use, as well as the subsistence and mobility practices of the buried individuals. The grave was already in use during the first part of the Late Neolithic and used into the second part of the period by individuals of different origin. Furthermore, the results indicated changing population dynamics in the Late Neolithic Falbygden, with increased human mobility, variability in subsistence strategies, and growing population density.
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| 29. |
- Bystricky, Lukas, et al.
(författare)
-
An accurate integral equation method for Stokes flow with piecewise smooth boundaries
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Two-dimensional Stokes flow through a periodic channel is considered. The channel walls need only be Lipschitz continuous, in other words they are allowed to have corners. Boundary integral methods are an attractive numerical method to solve the Stokes equations, as the problem can be reformulated into a problem that must be solved only over the boundary of the domain. When the boundary is at least C1 smooth, the boundary integral kernel is a compact operator, and traditional Nyström methods can be used to obtain highly accurate solutions. In the case of Lipschitz continuous boundaries however, obtaining accurate solutions using the standard Nyström method can require high resolution. We adapt a technique known as recursively compressed inverse preconditioning to accurately solve the Stokes equations without requiring any more resolution than is needed to resolve the boundary. Combined with a periodic fast summation method we construct a method that is O(N log N ) where N is the number of quadrature points on the boundary. We demonstrate the robustness of this method by extending an existing boundary integral method for viscous drops to handle the movement of drops near corners.
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| 30. |
- Bystricky, Lukas, et al.
(författare)
-
An accurate integral equation method for Stokes flow with piecewise smooth boundaries
- 2020
-
Ingår i: BIT Numerical Mathematics. - : Springer. - 0006-3835 .- 1572-9125.
-
Tidskriftsartikel (refereegranskat)abstract
- Two-dimensional Stokes flow through a periodic channel is considered. The channel walls need only be Lipschitz continuous, in other words they are allowed to have corners. Boundary integral methods are an attractive tool for numerically solving the Stokes equations, as the partial differential equation can be reformulated into an integral equation that must be solved only over the boundary of the domain. When the boundary is at least C1 smooth, the boundary integral kernel is a compact operator, and traditional Nyström methods can be used to obtain highly accurate solutions. In the case of Lipschitz continuous boundaries, however, obtaining accurate solutions using the standard Nyström method can require high resolution. We adapt a technique known as recursively compressed inverse preconditioning to accurately solve the Stokes equations without requiring any more resolution than is needed to resolve the boundary. Combined with a periodic fast summation method we construct a method that is O(Nlog N) where N is the number of quadrature points on the boundary. We demonstrate the robustness of this method by extending an existing boundary integral method for viscous drops to handle the movement of drops near corners.
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| 31. |
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| 32. |
- Engblom, Stefan, et al.
(författare)
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On diffuse interface modeling and simulation of surfactants in two-phase fluid flow
- 2013
-
Ingår i: Communications in Computational Physics. - : Global Science Press. - 1815-2406 .- 1991-7120. ; 14:4, s. 879-915
-
Tidskriftsartikel (refereegranskat)abstract
- An existing phase-fieldmodel of two immiscible fluids with a single soluble surfactant present is discussed in detail. We analyze the well-posedness of the model and provide strong evidence that it is mathematically ill-posed for a large set of physically relevant parameters. As a consequence, critical modifications to the model are suggested that substantially increase the domain of validity. Carefully designed numerical simulations offer informative demonstrations as to the sharpness of our theoretical results and the qualities of the physical model. A fully coupled hydrodynamic test-case demonstrates the potential to capture also non-trivial effects on the overall flow.
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| 33. |
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| 34. |
- Engquist, Björn, et al.
(författare)
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Discretization of Dirac delta functions in level set methods
- 2005
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Ingår i: Journal of Computational Physics. - : Elsevier BV. - 0021-9991 .- 1090-2716. ; 207:1, s. 28-51
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Tidskriftsartikel (refereegranskat)abstract
- Discretization of singular functions is an important component in many problems to which level set methods have been applied. We present two methods for constructing consistent approximations to Dirac delta measures concentrated on piecewise smooth curves or surfaces. Both methods are designed to be convenient for level set simulations and are introduced to replace the commonly used but inconsistent regularization technique that is solely based on a regularization parameter proportional to the mesh size. The first algorithm is based on a tensor product of regularized one-dimensional delta functions. It is independent of the irregularity relative to the grid. In the second method, the regularization is constructed from a one-dimensional regularization that is extended to multi-dimensions with a variable support depending on the orientation of the singularity relative to the computational grid. Convergence analysis and numerical results are given. © 2005 Published by Elsevier Inc.
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| 35. |
- Engquist, Björn, et al.
(författare)
-
High-frequency wave propagation by the segment projection method
- 2002
-
Ingår i: Journal of Computational Physics. - : Elsevier BV. - 0021-9991 .- 1090-2716. ; 178:2, s. 373-390
-
Tidskriftsartikel (refereegranskat)abstract
- Geometrical optics is a standard technique used for the approximation of high-frequency wave propagation. Computational methods based on partial differential equations instead of the traditional ray tracing have recently been applied to geometrical optics. These new methods have a number of advantages but typically exhibit difficulties with linear superposition of waves. In this paper we introduce a new partial differential technique based on the segment projection method in phase space. The superposition problem is perfectly resolved and so is the problem of computing amplitudes in the neighborhood of caustics. The computational complexity is of the same order as that of ray tracing. The new algorithm is described and a number of computational examples are given. including a simulation of waveguides.
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| 36. |
- Engquist, Björn, et al.
(författare)
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On Consistent Boundary Conditions for the Yee Scheme in 3D
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The standard staircase approximation of curved boundaries in the Yee scheme is inconsistent. Consistency can however be achieved by modifying the algorithm close to the boundary. We consider a technique to consistently model curved boundaries where the coefficients of the update stencil is modified, thus preserving the Yee structure. The method has previously been successfully applied to acoustics in two and three dimension, as well as electromagnetics in two dimensions. In this paper we generalize to electromagnetics in three dimensions. Unlike in previous cases there is a non-zero divergence growth along the boundary that needs to be projected away. We study the convergence and provide numerical examples that demonstrates the improved accuracy.
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| 37. |
- Fahrenholtz, I. L., et al.
(författare)
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Within-day energy deficiency and reproductive function in female endurance athletes
- 2018
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Ingår i: Scandinavian Journal of Medicine and Science in Sports. - : Wiley-Blackwell. - 0905-7188 .- 1600-0838. ; 28:3, s. 1139-1146
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Tidskriftsartikel (refereegranskat)abstract
- We aimed to estimate and compare within-day energy balance (WDEB) in athletes with eumenorrhea and menstrual dysfunction (MD) with similar 24-hour energy availability/energy balance (EA/EB). Furthermore, to investigate whether within-day energy deficiency is associated with resting metabolic rate (RMR), body composition, S-cortisol, estradiol, T-3, and fasting blood glucose. We reanalyzed 7-day dietary intake and energy expenditure data in 25 elite endurance athletes with eumenorrhea (n=10) and MD (n=15) from a group of 45 subjects where those with disordered eating behaviors (n=11), MD not related to low EA (n=5), and low dietary record validity (n=4) had been excluded. Besides gynecological examination and disordered eating evaluation, the protocol included RMR measurement; assessment of body composition by dual-energy X-ray absorptiometry, blood plasma analysis, and calculation of WDEB in 1-hour intervals. Subjects with MD spent more hours in a catabolic state compared to eumenorrheic athletes; WDEB<0kcal: 23.0hour (20.8-23.4) vs 21.1hour (4.7-22.3), P=.048; WDEB<-300kcal: 21.8hour (17.8-22.4) vs 17.6hour (3.9-20.9), P=.043, although similar 24-hour EA: 35.6 (11.6) vs 41.3 (12.7) kcal/kg FFM/d, (P=.269), and EB: -659 (551) vs -313 (596) kcal/d, (P=.160). Hours with WDEB <0kcal and <-300kcal were inversely associated with RMRratio (r=-.487, P=.013, r=-.472, P=.018), and estradiol (r=-.433, P=.034, r=-.516, P=.009), and positively associated with cortisol (r=.442, P=.027, r=.463, P=.019). In conclusion, although similar 24-hour EA/EB, the reanalysis revealed that MD athletes spent more time in a catabolic state compared to eumenorrheic athletes. Within-day energy deficiency was associated with clinical markers of metabolic disturbances.
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| 38. |
- Forsberg, Anna, et al.
(författare)
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Once-only colonoscopy or two rounds of faecal immunochemical testing 2 years apart for colorectal cancer screening (SCREESCO): preliminary report of a randomised controlled trial
- 2022
-
Ingår i: The Lancet Gastroenterology & Hepatology. - : ELSEVIER INC. - 2468-1253. ; 7:6, s. 513-521
-
Tidskriftsartikel (refereegranskat)abstract
- Background Screening for colorectal cancer is done with lower gastrointestinal endoscopy or stool-based tests. There is little evidence from randomised trials to show primary colonoscopy reduces mortality in colorectal cancer We aimed to investigate the effect of screening with once-only colonoscopy or two rounds of faecal immunochemical test screening on colorectal cancer mortality and incidence. Methods We did a randomised controlled trial in Sweden (SCREESCO). Residents in 18 of 21 regions who were age 60 years in the year of randomisation were identified from a population register maintained by the Swedish Tax Agency. A statistician with no further involvement in the trial used a randomised block method to assign individuals to once-only colonoscopy, two rounds of faecal immunochemical testing (OC-Sensor; 2 years apart), or a control group (no intervention; standard diagnostic pathways), in a ratio of 1:6 for colonoscopy versus control and 1:2 for faecal immunochemical testing versus control. Masking was not possible due to the nature of the trial. The primary endpoints of the trial are colorectal cancer mortality and colorectal cancer incidence. Here, we report preliminary participation rates, baseline findings, and adverse events from March, 2014, to December, 2020, in the two intervention groups after completion of recruitment and screening, up to the completion of the second faecal immunochemical testing round. Analyses were done in the intention-to-screen population, defined as all individuals who were randomly assigned to the respective study group. This study is registered with Clinical Trials.gov, NCT02078804. Findings Between March 1, 2014, and Dec 31, 2020, 278 280 people were induded in the study; 31 140 were assigned to the colonoscopy group, 60 300 to the faecal immunochemical test group, and 186 840 to the control group. 10 679 (35.1%) of 30 400 people who received an invitation for colonoscopy participated. 33 383 (55.5%) of 60 137 people who received a postal faecal immunochemical test participated. In the intention-to-screen analysis, colorectal cancer was detected in 49 (0.16%) of 31140 people in the colonoscopy group versus 121 (0. 20%) of 60 300 in the faecal immunochemical test group (relative risk [RR] 0.78, 95% CI 0.56-1.09). Advanced adenomas were detected in 637 (2.05%) people in the colonoscopy group and 968 (1.61%) in the faecal immunochemical test group (RR 1.27, 95% CI 1.15-1.41). Colonoscopy detected more right-sided advanced adenomas than faecal immunochemical testing. There were two perforations and 15 major bleeds in 16 555 colonoscopies. No intervention-related deaths occurred. Interpretation The diagnostic yield and the low number of adverse events indicate that the design from this trial, both for once-only colonoscopy and faecal immunochemical test screening, could be transferred to a population-based screening service if a benefit in disease-specific mortality is subsequently shown. Copyright (C) 2022 Elsevier Ltd. All rights reserved.
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| 39. |
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| 40. |
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| 41. |
- Fryklund, Fredrik, 1988-, et al.
(författare)
-
An adaptive kernel-split quadrature method for parameter-dependent layer potentials
- 2022
-
Ingår i: Advances in Computational Mathematics. - : Springer Nature. - 1019-7168 .- 1572-9044. ; 48:2
-
Tidskriftsartikel (refereegranskat)abstract
- Panel-based, kernel-split quadrature is currently one of the most efficient methods available for accurate evaluation of singular and nearly singular layer potentials in two dimensions. However, it can fail completely for the layer potentials belonging to the modified Helmholtz, modified biharmonic, and modified Stokes equations. These equations depend on a parameter, denoted alpha, and kernel-split quadrature loses its accuracy rapidly when this parameter grows beyond a certain threshold. This paper describes an algorithm that remedies this problem, using per-target adaptive sampling of the source geometry. The refinement is carried out through recursive bisection, with a carefully selected rule set. This maintains accuracy for a wide range of the parameter alpha, at an increased cost that scales as log alpha. Using this algorithm allows kernel-split quadrature to be both accurate and efficient for a much wider range of problems than previously possible.
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| 42. |
- Fryklund, Fredrik, et al.
(författare)
-
An integral equation-based numerical method for the forced heat equation on complex domains
- 2020
-
Ingår i: Advances in Computational Mathematics. - : Springer Nature. - 1019-7168 .- 1572-9044. ; 46:5
-
Tidskriftsartikel (refereegranskat)abstract
- Integral equation-based numerical methods are directly applicable to homogeneous elliptic PDEs and offer the ability to solve these with high accuracy and speed on complex domains. In this paper, such a method is extended to the heat equation with inhomogeneous source terms. First, the heat equation is discretised in time, then in each time step we solve a sequence of so-called modified Helmholtz equations with a parameter depending on the time step size. The modified Helmholtz equation is then split into two: a homogeneous part solved with a boundary integral method and a particular part, where the solution is obtained by evaluating a volume potential over the inhomogeneous source term over a simple domain. In this work, we introduce two components which are critical for the success of this approach: a method to efficiently compute a high-regularity extension of a function outside the domain where it is defined, and a special quadrature method to accurately evaluate singular and nearly singular integrals in the integral formulation of the modified Helmholtz equation for all time step sizes.
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| 43. |
- Fryklund, Fredrik, 1988-, et al.
(författare)
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An integral equation method for the advection-diffusion equation on time-dependent domains in the plane
- 2023
-
Ingår i: Journal of Computational Physics. - : Elsevier BV. - 0021-9991 .- 1090-2716. ; 475
-
Tidskriftsartikel (refereegranskat)abstract
- Boundary integral methods are attractive for solving homogeneous elliptic partial differen-tial equations on complex geometries, since they can offer accurate solutions with a com-putational cost that is linear or close to linear in the number of discretization points on the boundary of the domain. However, these numerical methods are not straightforward to ap-ply to time-dependent equations, which often arise in science and engineering. We address this problem with an integral equation-based solver for the advection-diffusion equation on moving and deforming geometries in two space dimensions. In this method, an adap-tive high-order accurate time-stepping scheme based on semi-implicit spectral deferred correction is applied. One time-step then involves solving a sequence of non-homogeneous modified Helmholtz equations, a method known as elliptic marching. Our solution method-ology utilizes several recently developed methods, including special purpose quadrature, a function extension technique and a spectral Ewald method for the modified Helmholtz kernel. Special care is also taken to handle the time-dependent geometries. The numerical method is tested through several numerical examples to demonstrate robustness, flexibility and accuracy.
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| 44. |
- Fryklund, Fredrik, 1988-
(författare)
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Integral equations and function extension techniques for numerical solution of PDEs
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Today many phenomena from science and engineering can be simulated accurately thanks to computational methods. Still, many challenges remain, one of them being close interface interactions when simulating e.g. the dynamics of a substance concentration in multiphase flows at the micro level. The challenge is to maintain high accuracy and efficiency as drops, vesicles, etc. are very close to each other, which many numerical methods struggle with. Also, the drops' geometries undergo changes over time. Thus far there is no standardized method for solving the equation modeling a concentration on time-dependent geometries efficiently and accurately. Boundary integral methods are powerful in handling moving and complex geometries, and maintaining high accuracy throughout the domain, even for close interactions. However, they are only efficient for a limited class of problems, and thus do not apply to our problem at hand.The focus of this dissertation is to expand the class of problems boundary integral methods are applicable to, without sacrificing their most attractive properties, plus presenting how the resulting equations time-dependent geometries can be solved on time-dependent geometries. This is achieved by the development of the algorithm partition of unity extension (PUX). It smoothly extends data from its domain of definition, with compact support, which allows for application of established fast methods.With our PUX method and state-of-the-art computational algorithms new problems could be studied, and to new levels of accuracy. In the process new underlying dynamics that were previously obscured by large errors appeared. These findings spurred a new set of questions, leading to the design of accurate algorithms for a class of problems that appear when applying boundary integral methods in conjunction with discretizing the governing equations first in time.In sum, we are now closer to a complete solver for the evolution of the substance concentration on time-dependent geometries. In this endeavor, methods have been development and studied that have applications outside our scope, and have already been applied successfully in other researchers' work. The work with PUX has thus been fruitful and will be developed and investigated further in the future, with adaptivity as the goal.
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| 45. |
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| 46. |
- Fryklund, Fredrik, et al.
(författare)
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Partition of unity extension of functions on complex domains
- 2018
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Ingår i: Journal of Computational Physics. - : Academic Press Inc.. - 0021-9991 .- 1090-2716. ; 375, s. 57-79
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Tidskriftsartikel (refereegranskat)abstract
- We introduce an efficient algorithm, called partition of unity extension or PUX, to construct an extension of desired regularity of a function given on a complex multiply connected domain in 2D. Function extension plays a fundamental role in extending the applicability of boundary integral methods to inhomogeneous partial differential equations with embedded domain techniques. Overlapping partitions are placed along the boundaries, and a local extension of the function is computed on each patch using smooth radial basis functions; a trivially parallel process. A partition of unity method blends the local extrapolations into a global one, where weight functions impose compact support. The regularity of the extended function can be controlled by the construction of the partition of unity function. We evaluate the performance of the PUX method in the context of solving the Poisson equation on multiply connected domains using a boundary integral method and a spectral solver. With a suitable choice of parameters the error converges as a tenth order method down to 10−14.
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| 47. |
- Gustavsson, Katarina, et al.
(författare)
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Gravity induced sedimentation of slender fibers
- 2009
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 21:12
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Tidskriftsartikel (refereegranskat)abstract
- Gravity induced sedimentation of slender, rigid fibers in a highly viscous fluid is investigated by large scale numerical simulations. The fiber suspension is considered on a microscopic level and the flow is described by the Stokes equations in a three dimensional periodic domain. Numerical simulations are performed to study in great detail the complex dynamics of a cluster of fibers. A repeating cycle is identified. It consists of two main phases: a densification phase, where the cluster densifies and grows, and a coarsening phase, during which the cluster becomes smaller and less dense. The dynamics of these phases and their relation to fluctuations in the sedimentation velocity are analyzed. Data from the simulations are also used to investigate how average fiber orientations and sedimentation velocities are influenced by the microstructure in the suspension. The dynamical behavior of the fiber suspension is very sensitive to small random differences in the initial configuration and a number of realizations of each numerical experiment are performed. Ensemble averages of the sedimentation velocity and fiber orientation are presented for different values of the effective concentration of fibers and the results are compared to experimental data. The numerical code is parallelized using the Message Passing Instructions (MPI) library and numerical simulations with up 800 fibers can be run for very long times which is crucial to reach steady levels of the averaged quantities. The influence of the periodic boundary conditions on the process is also carefully investigated.
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| 48. |
- Gustavsson, Katarina, et al.
(författare)
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Numerical Simulations of Rigid Fiber Suspensions
- 2008
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Konferensbidrag (refereegranskat)abstract
- In this paper, we present a numerical method designed to simulate the challenging problem of the dynamics of slender fibers immersed in an incompressible fluid. Specifically, we consider microscopic, rigid fibers, that sediment due to gravity. Such fibers make up the micro-structure of many suspensions for which the macroscopic dynamics are not well understood. Our numerical algorithm is based on a non-local slender body approximation that yields a system of coupled integral equations, relating the forces exerted on the fibers to their velocities, which takes into account the hydrodynamic interactions of the fluid and the fibers. The system is closed by imposing the constraints of rigid body motions. The fact that the fibers are straight have been further exploited in the design of the numerical method, expanding the force on Legendre polynomials to take advantage of the specific mathematical structure of a finite-part integral operator, as well as introducing analytical quadrature in a manner possible only for straight fibers. We have carefully treated issues of accuracy, and present convergence results for all numerical parameters before we finally discuss the results from simulations including a larger number of fibers.
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| 49. |
- Hagson, Anders, 1950, et al.
(författare)
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Redefining Urban Development Strategies for Effective and Efficient Future Mobility Solutions
- 2018
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The main question of this pre-study is if there is a coherence between future sustainable urban development and urban transportation models. The current rapid development of new technologies will result in new innovative transportation models. To what extent are they compatible with urban models? The population and urban growth change urban mobility in space and stress the word infrastructure. Urban transport is one of the major challenges on a local, regional and national level, where decision and planning processes in cities are extremely complex. A polycentric model of growth to avoid pressures on the urban centres, has been accompanied by developing transport networks along existing corridors. The problems however remain: shortage of land, growing traffic congestion, pollution, etc. This aims to define new innovative urban transport solutions with a larger flexibility in response to the growing dynamic urban development. Is there a paradox between urban development along existing corridors and the idea of flexibility of mobility? New technologies and new transportation modes are already accessible. However, alternative city structures for new transportation system models are not widely discussed in the city development strategies. This pre-study is based in Gothenburg as a case city. Research outputs are presented as following: I. Historical perspective. Description of how urban forms and land use changes as a consequence of new transportation technologies; II. How will different new innovative transportation systems (i.e. private, mixed, and public) affect a) accessibility to destinations for different socio-economic groups b) distribution of goods?
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| 50. |
- Hagson, Anders, 1950, et al.
(författare)
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Vad hindrar respektive möjliggör tät och blandad stadsutveckling?
- 2015
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Diskussionen om framtidens hållbara och mer resursvänliga samhälle tar ofta sikte på den täta och funktionsblandade staden. Men om många kan vara överens om principerna, så visar sig den så kallade blandstaden svår att definiera mer precist. Vad innebär en tät och blandad stad, och hur ser förutsättningarna ut för att genomföra den?
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