| 1. |
- Björkenstam, Staffan C, 1981, et al.
(författare)
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A framework for motion planning of digital humans using discrete mechanics and optimal control
- 2017
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Ingår i: Proceedings of the 5th International Digital Human Modeling Symposium. - : Federal Institute for Occupational Safety and Health. ; , s. 64-71
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Konferensbidrag (refereegranskat)abstract
- In this paper we present a framework for digital human modelling using discrete mechanics and optimal control. Discrete mechanics is particularly well suited for modelling the dynamics of constrained mechanical systems, which is almost always the case when considering complex human models interacting with the environment. We demonstrate that, by using recently developed recursive dynamics algorithms, we are able to efficiently use discrete mechanics in direct optimal control methods to plan for complex motions. Besides a proper mechanical model, an appropriate objective function is paramount to achieve realistic motions as a solution to an optimal control problem. Hence, several different objective functions, such as for example minimum time or minimum applied torque over the joints, are compared, and the resulting motions are analyzed and evaluated. To further improve the model, we include basic muscular models for the muscles of the shoulder, arm and wrist, and examine how this affects the motions.
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| 2. |
- Johnson, Tomas, 1979, et al.
(författare)
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A Multi-Scale Simulation Method for the Prediction of Edge Wicking in Multi-Ply Paperboard
- 2015
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Ingår i: Nordic Pulp and Paper Research Journal. - 2000-0669 .- 0283-2631. ; 30:4, s. 640-650
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Tidskriftsartikel (refereegranskat)abstract
- When liquid packaging board is made aseptic in the filling machine the unsealed edges of the board are exposed to a mixture of water and hydrogen peroxide. A high level of liquid penetration may lead to aesthetic as well as functional defects. To be able to make a priori predictions of the edge wicking properties of a certain paperboard material is therefore of great interest to the paper industry as well as to packaging manufacturers. In this paper an extended multi-scale model of edge wicking in multi-ply paperboard is presented. The geometric and physical properties of the paperboard are modeled on the micro-scale, and include fillers and fines. The absolute air permeabilities and pore size distributions are validated with experimental and tomographic values. On the macro-scale random porosity and sizing distributions, time and sizing dependent contact angles, and inter-ply dependence are modeled. Arbitrary shapes of the paperboard are handled through an unstructured 3D surface mesh. Stationary and transient edge wicking simulations are validated against experiments with excellent agreement. The simulations show that the diffusive menisci between the liquid and air phases together with the two-ply model is necessary to achieve good agreement with the transient edge wicking experiments.
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| 3. |
- Olsson, Caroline, 1970, et al.
(författare)
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Initial experience with introducing national guidelines for CT- and MRI-based delineation of organs at risk in radiotherapy
- 2019
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Ingår i: Physics and Imaging in Radiation Oncology. - : Elsevier. - 2405-6316. ; 11, s. 88-91
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Tidskriftsartikel (refereegranskat)abstract
- A fundamental problem in radiotherapy is the variation of organ at risk (OAR) volumes. Here we present our initial experience in engaging a large Radiation Oncology (RO) community to agree on national guidelines for OAR delineations. Our project builds on associated standardization initiatives and invites professionals from all radiotherapy departments nationwide. Presently, one guideline (rectum) has successfully been agreed on by a majority vote. Reaching out to all relevant parties in a timely manner and motivating funding agencies to support the work represented early challenges. Population-based data and a scalable methodological approach are major strengths of the proposed strategy.
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| 4. |
- Eriksson, Anders, 1975, et al.
(författare)
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Bottom-up derivation of an effective thermostat for united atoms simulations of water
- 2009
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Ingår i: JOURNAL OF CHEMICAL PHYSICS. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 130:16
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Tidskriftsartikel (refereegranskat)abstract
- In this article we derive the effective pairwise interactions in a Langevin-type united atoms model of water. The interactions are determined from the trajectories of a detailed molecular dynamics simulation of simple point charge water. A standard method is used for estimating the conservative interaction, whereas a new "bottom-up" method is used to determine the effective dissipative and stochastic interactions. We demonstrate that when compared to the standard united atoms model, the transport properties of the coarse-grained model is significantly improved by the introduction of the derived dissipative and stochastic interactions. The results are compared to a previous study, where a "top-down" approach was used to obtain transport properties consistent with those of the simple point charge water model. © 2009 American Institute of Physics.
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| 5. |
- Eriksson, Anders, 1975, et al.
(författare)
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Effective thermostat induced by coarse graining of simple point charge water
- 2008
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 129:2
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Tidskriftsartikel (refereegranskat)abstract
- We investigate how the transport properties of a united atom fluid with a dissipative particle dynamics thermostat depend on the functional form and magnitude of both the conservative and the stochastic interactions. We demonstrate how the thermostat strongly affects the hydrodynamics, especially diffusion, viscosity, and local escape times. As model system we use simple point charge (SPC) water, from which projected trajectories are used to determine the effective interactions in the united atom model. The simulation results support our argument that the thermostat should be viewed as an integral part of the coarse-grained dynamics rather than a tool for approaching thermal equilibrium. As our main result we show that the united atom model with the adjusted effective interactions approximately reproduces the diffusion constant and the viscosity of the underlying detailed SPC water model.
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| 6. |
- Eriksson, Anders, 1975, et al.
(författare)
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On the microscopic foundation of dissipative particle dynamics
- 2009
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Ingår i: Europhysics Letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 86:4
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Tidskriftsartikel (refereegranskat)abstract
- Mesoscopic-particle-based fluid models, such as dissipative particle dynamics, are usually assumed to be coarse-grained representations of an underlying microscopic fluid. A fundamental question is whether there exists a map from microscopic particles in these systems to the corresponding coarse-grained particles, such that the coarse-grained system has the same bulk and transport properties as the underlying system. In this letter, we investigate the coarse-graining of microscopic fluids using a Voronoi-type projection that has been suggested in several studies. The simulations show that the projection fails in defining coarse-grained particles that have a physically meaningful connection to the microscopic fluid. In particular, the Voronoi projection produces identical coarse-grained equilibrium properties when applied to systems with different microscopic interactions and different bulk properties. © EPLA, 2009
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| 7. |
- Eriksson, Anders, 1975, et al.
(författare)
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Using force covariance to derive effective stochastic interactions in dissipative particle dynamics.
- 2008
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Ingår i: Physical Review E. - 2470-0045 .- 2470-0053. ; 77:1
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Tidskriftsartikel (refereegranskat)abstract
- There exist methods for determining effective conservative interactions in coarse-grained particle-based mesoscopic simulations. The resulting models can be used to capture thermal equilibrium behavior, but the model system we study does not correctly represent transport properties. We suggest the use of force covariance to determine the full functional form of dissipative and stochastic interactions. We show that a combination of the RDF and a force covariance function can be used to determine all interactions in dissipative particle dynamics (DPD). Furthermore, we use the method to test whether the effective interactions in DPD can be adjusted to produce a force covariance consistent with the projection of a microscopic Lennard-Jones simulation. The results indicate that the DPD ansatz may not be consistent with the underlying microscopic dynamics. We discuss how this result relates to theoretical studies reported in the literature.
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| 8. |
- Frasca, Serena, 1992-
(författare)
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Lignocellulosic Biomass Components for Materials Applications
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents a comprehensive study of the use of lignocellulosic biomass components in materials, targeting two distinct applications: conductive materials and additive manufacturing. The lignocellulosic biomass components -lignin, cellulose, and hemicellulose- have been identified as promising renewable feedstocks to replace fossil resources and contribute to the green transition. The first work presented focuses on the synthesis of conductive polymers, specifically polypyrroles. The synthesis of the monomers, N-functionalized pyrroles, was achieved by a modified Clauson-Kaas protocol, both performed in continuous flow and in batch. The substrates used, 2,5-dimethoxytetrahydrofuran and anilines, are promising candidates for biomass-based chemical production. The produced N-functionalised pyrroles were then deposited onto electrodes via electropolymerization to obtain thin films and their electrical properties were characterized. Next, the thesis delves into the isolation and valorisation of lignin, specifically into softwood lignin, modified and isolated via a phenol-assisted fractionation. This approach supresses the formation of condensed lignin while simultaneously introducing new functional groups that could be beneficial for a number of applications. Phenolated lignin was obtained with a high degree of functionalization, a well-defined structure and relatively low molecular weight. Detailed analysis of the fractionation conditions and of the corresponding lignin structures gave insights on how to tailor lignin on demand. The potential of the one-step phenolated lignin was investigated for materials applications.Filaments of lignin and polylactic acid (PLA) were produced to be used in additive manufacturing. The study focused on high lignin incorporation to PLA at three different concentrations (30, 50, and 70 wt%). The lignin-PLA filaments were used for 3D printing of dog bone shaped specimens to examine their mechanical properties. Additionally, detailed thermal and chemical analysis were performed to get an in-depth understanding of the materials. The results were compared to the performance of technical lignins that were also included in the study. Importantly, recycling studies of the filaments indicated good printing performance up to three recycling cycles.Another application explored was the production of conductive carbon materials, starting from the modified lignin. The carbonization was performed using a CO2 laser engraver and lignin was the main component in the wet film formulation. The optimized carbonization parameters afforded carbonized films with low sheet resistance (< 7 Ω sq-1). The structural analysis of the carbonized materials revealed the formation of few-layers graphene-like carbon structures. Further applications of these materials are under investigation.These innovative applications showcase the potential for sustainable, biomass-based materials. The lignin fractionation method reported herein can contribute to further advancements in lignin research. A phenol modified lignin with defined structure offers more opportunities in comparison with bulk lignins, with the advantage of tailoring lignin properties to its end use in the same number steps. Biomass-based organic electronics will help paving the way for environmentally friendly advancements in the energy sector.
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| 9. |
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| 10. |
- Nyström, Johan, 1981
(författare)
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Deriving coarse grained descriptions of particle based simulations
- 2008
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the field of molecular simulations, the long standing aim has been to study mechanisms that cannot easily be observed in experiments or understood in terms of more abstract models. Fully detailed molecular dynamics simulations can be extremely successful in many areas, but when applied to mesoscopic systems with a large number of molecules, the time and length scales on which important phenomena take place are often far beyond what is feasible to simulate with modern computers. To overcome this issue, several different coarse graining schemes have been developed, resulting in effective dynamics evolving on larger length and time scales than the fully detailed microscopic systems. A simulation technique that has received increasing interest in the last decade is Dissipative Particle Dynamics (DPD), but despite its popularity, the physical interpretation of DPD is not fully satisfying. In particular, no clear explanation has been given concerning the connection between the mesoscopic DPD technique and the assumed microscopic systems from which the mesoscopic dynamics originates. In paper I a bottom-up method is presented for deriving effective interaction parameters for coarse grained representations of microscopic simulations. The method relies on the DPD ansatz for the coarse grained dynamics. We apply the method to a projection where several particles are clustered together into larger, mesoscopic, particles and derive effective coarse grained equations of motion for the mesoscopic particles. Conservative interactions are recreated using the well known inverse Monte Carlo (IMC) technique, whereas dissipative and stochastic interactions are derived using a newly introduced quantity termed the force covariance.Paper II concerns the use of DPD as a thermostat for coarse grained simulations of liquid water. We systematically test how the functional form of the DPD interactions affect transport properties. The main result is that with a hard core potential derived using the IMC technique, it is possible to resolve the so called Schmidt number problem of DPD, i.e. to find a functional form of the stochastic and dissipative interactions such that the diffusion and the viscosity of the underlying system is simultaneously retained on the coarse grained level of description, something that is not possible with classical united atoms simulations.
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| 11. |
- Nyström, Johan, 1981
(författare)
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Deriving effective forces for mesoscopic particle simulations
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the field of molecular simulations, the long-standing aim has been to study mechanisms that cannot easily be observed in experiments or understood in terms of more abstract models. This is hard for systems of mesoscopic size (proteins, membranes etc.) as the time and length scales on which important phenomena take place are often far beyond what is feasible to simulate using an atomistic level of detail. To overcome this issue, several different coarse graining schemes have been developed, resulting in effective dynamics evolving on larger length and time scales than the fully detailed microscopic systems. A simulation technique that has received increasing interest in the last decade is Dissipative Particle Dynamics (DPD), but despite its popularity, the physical interpretation of DPD is not complete. In particular, no strict explanation has been given concerning the connection between the mesoscopic DPD technique and the microscopic systems from which the mesoscopic dynamics originates. This thesis establishes the link between the DPD-representation of a system, and the same system on a fine-grained (microscopic) level. This is achieved by adapting the non-equilibrium statistical mechanics technique of Mori-Zwanzig projections to many-particle systems (Papers I, III), giving a formal footing on which the DPD technique can stand. The theory is also explicitly tested using computer simulations in Paper IV. In that paper, a molecular dynamics simulation of water (using water molecules with one oxygen and two hydrogen atoms) and a DPD representation of the same system with mass-center approximation of the water molecules are compared. The results are compared and contrasted to a DPD-model with parameters manually tuned to give correct transport properties (presented in Paper II).For particle-based coarse-graining of fluids, the particles are often identified with clusters of fluid molecules. In Paper V, the bearings of this view on equilibrium and transport properties of the coarse-grained fluid are investigated. It is shown for the case of coarse-graining through Voronoi tessellation that this representation cannot capture the properties of the microscopic system.Papers VI and VII concern the use of methods from coarse-grained molecular simulations for estimating interactions between flocking animals. In particular, paper VI shows how a force-matching method can be used to faithfully recreate all interactions in a simulated bird flock, and paper VII contains a comment stating that interaction rules used by flocking animals can most likely not be deduced from static photographs.
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| 12. |
- Nyström, Johan, 1981, et al.
(författare)
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Design-Build, innovation, and competition: The role of smaller contractors
- 2017
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Ingår i: Proceedings of the International Conference on Construction and Real Estate Management 2017. - Reston, VA : American Society of Civil Engineers. - 9780784481080 ; , s. 11-18
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Konferensbidrag (refereegranskat)abstract
- Design-build (DB) contracting can be used to promote innovation in the infrastructure sector. Alongside innovation, public clients often wish to encourage competition. At a first glance, these two ambitions could be conflicting. More risk on the contractors stemming from DB might not appeal to small and medium-sized enterprises (SMEs), as they, ceteris paribus, have a risk disadvantage compared to larger firms. The research question is how more DB contracting will affect competition from a SME perspective. A simple model is provided to show how DBB stimulates competition and DB encourages innovation. Supported by semi-structured SME interviews from Sweden, it is discussed how the hybrid version of a controlled DB contract is inferior to alternative contract designs. The controlled DB contract does not stimulate competition nor provide incentives for innovation
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