| 1. |
- Berggren, Martin, et al.
(författare)
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Well-posed variational formulations of Friedrichs-type systems
- 2021
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Ingår i: Journal of Differential Equations. - : Elsevier. - 0022-0396 .- 1090-2732. ; 292, s. 90-131
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Tidskriftsartikel (refereegranskat)abstract
- All finite element methods, as well as much of the Hilbert-space theory for partial differential equations, rely on variational formulations, that is, problems of the type: find u∈V such that a(v,u)=l(v) for each v∈L, where V,L are Sobolev spaces. However, for systems of Friedrichs type, there is a sharp disparity between established well-posedness theories, which are not variational, and the very successful discontinuous Galerkin methods that have been developed for such systems, which are variational. In an attempt to override this dichotomy, we present, through three specific examples of increasing complexity, well-posed variational formulations of boundary and initial–boundary-value problems of Friedrichs type. The variational forms we introduce are generalizations of those used for discontinuous Galerkin methods, in the sense that inhomogeneous boundary and initial conditions are enforced weakly through integrals in the variational forms. In the variational forms we introduce, the solution space is defined as a subspace V of the graph space associated with the differential operator in question, whereas the test function space L is a tuple of L2 spaces that separately enforce the equation, boundary conditions of characteristic type, and initial conditions.
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| 2. |
- Hassan, Emadeldeen, et al.
(författare)
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Topology optimization of compact wideband coaxial-to-waveguide transitions with minimum-size control
- 2018
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Ingår i: Structural and multidisciplinary optimization (Print). - New York : Springer. - 1615-147X .- 1615-1488. ; 57:4, s. 1765-1777
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a density-based topology optimization approach to design compact wideband coaxial-to-waveguide transitions. The underlying optimization problem shows a strong self penalization towards binary solutions, which entails mesh-dependent designs that generally exhibit poor performance. To address the self penalization issue, we develop a filtering approach that consists of two phases. The first phase aims to relax the self penalization by using a sequence of linear filters. The second phase relies on nonlinear filters and aims to obtain binary solutions and to impose minimum-size control on the final design. We present results for optimizing compact transitions between a 50-Ohm coaxial cable and a standard WR90 waveguide operating in the X-band (8-12 GHz).
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| 3. |
- Hägg, Linus, 1988-, et al.
(författare)
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1D-model of the interaction between a stack of wood and an imposed electromagnetic wave
- 2015
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We have developed and investigated a 1D-model for the interaction between a stack of wood and an impinging electromagnetic field. Maxwell's equations are used to model the electromagnetic interaction and each layer in a stack of boards has been modeled as a homogenous lossy dielectric slab. The main reason for developing this model has been to investigate the possibility of measuring the moisture content of wood inside a drying kiln using electromagnetic waves. Our investigations show that it is in principle possible to measure the moisture content, since the electromagnetic field is sensitive to changes in the moisture content of the wood. We also show that it might be possible to measure the average moisture content, without detailed knowledge of the distribution of moisture content between different boards.
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| 4. |
- Hägg, Linus, et al.
(författare)
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Estimating the neutron yield in a deuterium plasma with the JET neutron camera
- 2023
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Ingår i: Review of Scientific Instruments. - : American Institute of Physics (AIP). - 0034-6748 .- 1089-7623. ; 94:7
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Tidskriftsartikel (refereegranskat)abstract
- The JET neutron camera is a well-established detector system at JET, which has 19 sightlines each equipped with a liquid scintillator. The system measures a 2D profile of the neutron emission from the plasma. A first principle physics method is used to estimate the DD neutron yield that is based on JET neutron camera measurements and is independent of other neutron measurements. This paper details the data reduction techniques, models of the neutron camera, simulations of neutron transport, and detector responses used to this end. The estimate uses a simple parameterized model of the neutron emission profile. The method makes use of the JET neutron camera’s upgraded data acquisition system. It also accounts for neutron scattering near the detectors and transmission through the collimator. These components together contribute to 9% of the detected neutron rate above a 0.5 MeVee energy threshold. Despite the simplicity of the neutron emission profile model, the DD neutron yield estimate falls on average within 10% agreement with a corresponding estimate from the JET fission chambers. The method can be improved by considering more advanced neutron emission profiles. It can also be expanded to estimate the DT neutron yield with the same methodology.
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| 5. |
- Hägg, Linus, et al.
(författare)
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Finita elementberäkningar av KL-skivor i Matlab : Slutrapport
- 2021
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Den platt- och skivlösare som utvecklats inom projektet Träinnovation i Norr (TiiN) har vidareutvecklats för beräkning av ortotropa skivor och plattor. Den vidareutvecklade lösaren är att betrakta som ett steg på vägen mot det långsiktiga målet att utveckla en enkel men riktig platt- och skivlösare för KL-trä. Plattlösaren har med goda resultat verifierats mot såväl en konstruerad lösning som mot beräkningar i Abaqus. Det har tyvärr inte varit möjligt att inom ramen för projektet verifiera skivlösaren.
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| 6. |
- Hägg, Linus, et al.
(författare)
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Influence of Fan Speed on Airflow distribution in a Scandinavian Drying Kiln
- 2012
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Konferensbidrag (refereegranskat)abstract
- From the beginning of the 90’s the development of drying kilns in Scandinavia has been focusing on increased drying capacity and also making the kilns more flexible in terms of handling differences in dimensions and initial moisture content. In order to facilitate the demand of higher airflows there has been an increase in circulation fan capacity. In a sawmill with modern kilns the fan capacity in a single batch kiln can be over 90 kW, making the drying kiln fans the single biggest consumer of electric power.Today, more and more sawmills are reviewing their consumption of electric power due to increasing prices. One way of reducing the consumption is to reduce the fan speed when the moisture content is so low that the drying rate is mainly limited by the diffusion properties of the wood and not the airflow. Since modern kilns are designed for high capacity fans there is a lack of knowledge of how a reduced fan speed affects the airflow distribution. This poses a risk of getting reduced quality of the final product due to increased moisture content variation in a batch.In this study the airflow has been measured inside an industrial drying kiln. For this study two experiments were done with 20 airflow gauges placed inside a kiln. During both experiments the kiln was fully loaded with pre dried Scotts pine (Pinus sylvestris) boards with a thickness of 50 mm. The reason for using pre-dried boards was simply that the gauge was not able to withstand the climate produced during a real process. In order to cover as much of the kiln as possible the gauges were distributed differently for the two experiments.The results show that about 30 % of the total airflow passes through the bolster spaces which are only about 20 % of the total flow area. This means that a notable volume of air might not be participating in drying of the boards.The results show also that the relative airflow distribution between the middle and the side of board stacks becomes more heterogeneous at a low fan speed. This trend is also seen for the relative distribution of airflow between bolster and sticker spaces.
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| 7. |
- Hägg, Linus
(författare)
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Matchning av stockar till rätt träd
- 2020
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- I projektet har möjligheten att på individnivå sammankoppla timmersorteringsdata med skördardata undersökts. Syftet med en sådan undersökning är att åstadkomma digitalspårbarhet från skog till såg. En kopplad datamängd har insamlats och använts för att utforma algoritmer för automatisk koppling av timmersorteringsdata och skördardata påindividnivå. Förutsatt att stockar kan sättas samman till stammar med hjälp av timmersorteringsdata gäller att omkring 16% av alla stammar med minst 3 sågstockar medhög säkerhet kan kopplas samman i de två datamängderna. En algoritm för sammansättningav stockar till stammar med hjälp av timmersorteringsdata har också utvecklats men denna bedöms i nuvarande form ej nå tillräcklig prestanda. Potentialen att höja prestandan är dock till stor del ännu outforskad.
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| 8. |
- Hägg, Linus, 1988-, et al.
(författare)
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Nonlinear filters in topology optimization : existence of solutions and efficient implementation for minimum compliance problems
- 2017
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Ingår i: Structural and multidisciplinary optimization (Print). - : Springer. - 1615-147X .- 1615-1488. ; 55:3, s. 1017-1028
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Tidskriftsartikel (refereegranskat)abstract
- Material distribution topology optimization problems are generally ill-posed if no restriction or regularization method is used. To deal with these issues, filtering procedures are routinely applied. In a recent paper, we presented a framework that encompasses the vast majority of currently available density filters. In this paper, we show that these nonlinear filters ensure existence of solutions to a continuous version of the minimum compliance problem. In addition, we provide a detailed description on how to efficiently compute sensitivities for the case when multiple of these nonlinear filters are applied in sequence. Finally, we present large-scale numerical experiments illustrating some characteristics of these cascaded nonlinear filters.
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| 9. |
- Hägg, Linus, 1988-, et al.
(författare)
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Nonlinear filters in topology optimization : existence of solutions and efficient implementation for minimal compliance problems
- 2016
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- It is well known that material distribution topology optimization problems often are ill-posed if no restriction or regularization method is used. A drawback with the standard linear density filter is that the resulting designs have large areas of intermediate densities, so-called gray areas, especially when large filter radii are used. To produce final designs with less gray areas, several different methods have been proposed; for example, projecting the densities after the filtering or using a nonlinear filtering procedure. In a recent paper, we presented a framework that encompasses a vast majority of currently available density filters. In this paper, we show that all these nonlinear filters ensure existence of solutions to a continuous version of the minimal compliance problem. In addition, we provide a detailed description on how to efficiently compute sensitivities for the case when multiple of these nonlinear filters are applied in sequence. Finally, we present a numerical experiment that illustrates that these cascaded nonlinear filters can be used to obtain independent size control of both void and material regions in a large-scale setting.
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| 10. |
- Hägg, Linus, 1988-, et al.
(författare)
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On minimum length scale control in density based topology optimization
- 2018
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Ingår i: Structural and multidisciplinary optimization (Print). - : Springer Science and Business Media LLC. - 1615-147X .- 1615-1488. ; 58:3, s. 1015-1032
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Tidskriftsartikel (refereegranskat)abstract
- The archetypical topology optimization problem concerns designing the layout of material within a given region of space so that some performance measure is extremized. To improve manufacturability and reduce manufacturing costs, restrictions on the possible layouts may be imposed. Among such restrictions, constraining the minimum length scales of different regions of the design has a significant place. Within the density filter based topology optimization framework the most commonly used definition is that a region has a minimum length scale not less than D if any point within that region lies within a sphere with diameter D > 0 that is completely contained in the region. In this paper, we propose a variant of this minimum length scale definition for subsets of a convex (possibly bounded) domain We show that sets with positive minimum length scale are characterized as being morphologically open. As a corollary, we find that sets where both the interior and the exterior have positive minimum length scales are characterized as being simultaneously morphologically open and (essentially) morphologically closed. For binary designs in the discretized setting, the latter translates to that the opening of the design should equal the closing of the design. To demonstrate the capability of the developed theory, we devise a method that heuristically promotes designs that are binary and have positive minimum length scales (possibly measured in different norms) on both phases for minimum compliance problems. The obtained designs are almost binary and possess minimum length scales on both phases.
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