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Träfflista för sökning "WFRF:(Jansson Johan) ;srt2:(2015-2019)"

Sökning: WFRF:(Jansson Johan) > (2015-2019)

  • Resultat 11-20 av 198
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11.
  • Degirmenci, Niyazi Cem, 1982- (författare)
  • Adaptive Finite Element Methods for Fluid Structure Interaction Problems with Applications to Human Phonation
  • 2018
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • This work presents a unified framework for numerical solution of Fluid Structure Interaction (FSI) and acoustics problems with focus on human phonation. The Finite Element Method is employed for numerical investigation of partial differential equations that model conservation of momentum and mass. Since the resulting system of equations is very large, an efficient open source high performance implementation is constructed and provided. In order to gain accuracy for the numerical solutions, an adaptive mesh refinement strategy is employed which reduces the computational cost in comparison to a uniform refinement. Adaptive refinement of the mesh relies on computable error indicators which appear as a combination of a computable residual and the solution of a so-called dual problem acting as weights on computed residuals. The first main achievement of this thesis is to apply this strategy to numerical simulations of a benchmark problem for FSI. This FSI model is further extended for contact handling and applied to a realistic vocal folds geometry where the glottic wave formation was captured in the numerical simulations. This is the second achievement in the presented work. The FSI model is further coupled to an acoustics model through an acoustic analogy, for vocal folds with flow induced oscillations for a domain constructed to create the vowel /i/. The comparisons of the obtained pressure signal at specified points with respect to results from literature for the same vowel is reported, which is the final main result presented.
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12.
  • Hoffman, Johan, et al. (författare)
  • New Theory of Flight
  • 2016
  • Ingår i: Journal of Mathematical Fluid Mechanics. - : Springer. - 1422-6928 .- 1422-6952. ; 18:2, s. 219-241
  • Tidskriftsartikel (refereegranskat)abstract
    • We present a new mathematical theory explaining the fluid mechanics of sub-sonic flight, which is fundamentally different from the existing boundary layer-circulation theory by Prandtl-Kutta-Zhukovsky formed 100 year ago. The new the-ory is based on our new resolution of d’Alembert’s paradox showing that slightlyviscous bluff body flow can be viewed as zero-drag/lift potential flow modified by3d rotational slip separation arising from a specific separation instability of po-tential flow, into turbulent flow with nonzero drag/lift. For a wing this separationmechanism maintains the large lift of potential flow generated at the leading edgeat the price of small drag, resulting in a lift to drag quotient of size15
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13.
  • Jansson, Johan, et al. (författare)
  • Adaptive simulation of unsteady flow past the submerged part of a floating wind turbine platform
  • 2015
  • Ingår i: MARINE 2015 - Computational Methods in Marine Engineering VI. - : International Center for Numerical Methods in Engineering (CIMNE). - 9788494392863 ; , s. 35-46
  • Konferensbidrag (refereegranskat)abstract
    • Offshore floating platforms for wind turbines represent challenging concepts for designers trying to combine an optimal compromise between cost effectiveness and performance. Modelling of the hydrodynamic behaviour of the structure is still the subject of wide debate in the technical communities. The assessment of the hydrodynamics of the support structure is not an easy task as the floaters consist of an assembly of columns, braces and pontoons, commonly also with heave plates: Each of these components corresponds to a different hydrodynamic model and it further interacts with the other elements. This results in very complex non-linear modeling, which makes it necessary to resort to computational fluid dynamics (CFD) methods for the evaluation of the combined hydrodynamics. In the framework of the collaboration between the Basque Centre for Applied Mathematics (BCAM) and Tecnalia R&I, the interaction of the sea flow with a semisubmersible floating offshore wind platform have been calculated by using the open source solver Unicorn in the FEniCS-HPC framework when subject to a steady inflow. The prototype of the platform consists in a semi-submersible 4-columns column stabilized platform - NAUTILUS Floating Solutions concept-; columns are connected by a rigid ring pontoon provided with heave damping plates at the bottom. The novelty of the approach in FEniCS-HPC hinges upon an implicit formulation for the turbulence, a cheap free slip model of the boundary layer and goal-oriented mesh adaptivity [8, 6, 9, 20, 1]. We find that the results are consistent with experimental results for cylinders at high Reynolds number.
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14.
  • Jansson, Johan, et al. (författare)
  • Adaptive unified continuum FEM modeling of a 3D FSI benchmark problem
  • 2017
  • Ingår i: International Journal for Numerical Methods in Biomedical Engineering. - : Wiley-Blackwell. - 2040-7939 .- 2040-7947. ; 33:9
  • Tidskriftsartikel (refereegranskat)abstract
    • In this paper, we address a 3D fluid-structure interaction benchmark problem that represents important characteristics of biomedical modeling. We present a goal-oriented adaptive finite element methodology for incompressible fluid-structure interaction based on a streamline diffusion–type stabilization of the balance equations for mass and momentum for the entire continuum in the domain, which is implemented in the Unicorn/FEniCS software framework. A phase marker function and its corresponding transport equation are introduced to select the constitutive law, where the mesh tracks the discontinuous fluid-structure interface. This results in a unified simulation method for fluids and structures. We present detailed results for the benchmark problem compared with experiments, together with a mesh convergence study.
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15.
  • Kádas, Krisztina, et al. (författare)
  • AlM2B2 (M = Cr, Mn, Fe, Co, Ni) : a group of nanolaminated materials
  • 2017
  • Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 29:15
  • Tidskriftsartikel (refereegranskat)abstract
    • Combining theory with experiments, we study the phase stability, elastic properties, electronic structure and hardness of layered ternary borides AlCr2B2, AlMn2B2, AlFe2B2, AlCo2B2, and AlNi2B2. We find that the first three borides of this series are stable phases, while AlCo2B2 and AlNi2B2 are metastable. We show that the elasticity increases in the boride series, and predict that AlCr2B2, AlMn2B2, and AlFe2B2 are more brittle, while AlCo2B2 and AlNi2B2 are more ductile. We propose that the elasticity of AlFe2B2 can be improved by alloying it with cobalt or nickel, or a combination of them. We present evidence that these ternary borides represent nanolaminated systems. Based on SEM measurements, we demonstrate that they exhibit the delamination phenomena, which leads to a reduced hardness compared to transition metal mono-and diborides. We discuss the background of delamination by analyzing chemical bonding and theoretical work of separation in these borides.
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16.
  • Krishnasamy, E., et al. (författare)
  • Direct FEM large scale computation of turbulent multiphase flow in urban water systems and marine energy
  • 2016
  • Ingår i: ECCOMAS Congress 2016 - Proceedings of the 7th European Congress on Computational Methods in Applied Sciences and Engineering. - Athens : National Technical University of Athens. - 9786188284401 ; , s. 1339-1351
  • Konferensbidrag (refereegranskat)abstract
    • High-Reynolds number turbulent incompressible multiphase flow represents a large class of engineering problems of key relevance to society. Here we describe our work on modeling two such problems: 1. The Consorcio de Aguas Bilbao Bizkaia is constructing a new storm tank system with an automatic cleaning system, based on periodically pushing tank water out in a tunnel 2. In the framework of the collaboration between BCAM - Basque Center for Applied Mathematics and Tecnalia R & I, the interaction of the sea flow with a semi submersible floating offshore wind platform is computationally investigated. We study the MARIA' benchmark modeling breaking waves over objects in marine environments. Both of these problems are modeled in the the Direct FEM/General Galerkin methodology for turbulent incompressible variable-densitv flow 1,2 in the FEniCS software framework.
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17.
  • Lyth, Johan, 1980-, et al. (författare)
  • Trends in cutaneous malignant melanoma in Sweden 1997-2011: Thinner tumours and improved survival among men
  • 2015
  • Ingår i: British Journal of Dermatology. - : Wiley-Blackwell. - 0007-0963 .- 1365-2133. ; 172:3, s. 700-706
  • Tidskriftsartikel (refereegranskat)abstract
    • Background: Both patient survival and the proportion of patients diagnosed with thin cutaneous malignant melanoma (CMM) have been steadily rising in Sweden as in most western countries, though the rate of improvement in survival appears to have declined in Sweden at the end of last millennium.Objectives: To analyse the most recent trends in the distribution of tumour thickness (T-category) as well as CMM-specific survival in Swedish patients diagnosed 1997-2011.Methods: This nationwide population-based study included 30 590 patients registered in the Swedish Melanoma Register (SMR) and diagnosed with a first primary invasive CMM 1997-2011. The patients were followed through 2012 in the national Cause-of-Death Register.Results: Logistic and Cox regression analyses adjusting for age at diagnosis, tumour site, and health care region were carried out. The odds ratio for being diagnosed with thicker tumours was significantly reduced (P = 0·0008) and the CMM-specific survival significantly improved in men diagnosed 2007-2011 compared to men diagnosed 1997-2001 (hazard ratio=0·81; 95% CI 0·72-0·91, P = 0·0009) while the corresponding differences for women were not significant. Women were diagnosed with significantly thicker tumours during 2002-2006 and a tendency towards decreased survival was observed compared to those diagnosed earlier 1997-2001 and later 2007-2011.Conclusion: In Sweden, the CMMs of men are detected earlier over time and this seems to be followed by an improved CMM-specific survival for men. Women are still diagnosed with considerably thinner tumours and they experience a better survival than men.
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18.
  • Moragues Ginard, M, et al. (författare)
  • Simulation of floating platforms for marine energy generation
  • 2018
  • Ingår i: 10th International Conference on Computational Fluid Dynamics, ICCFD 2018. - : International Conference on Computational Fluid Dynamics 2018.
  • Konferensbidrag (refereegranskat)abstract
    • The goal of this work is to study the dynamics of floating platforms that are designed for marine energy generation. This work is done in collaboration with Tecnalia R&I, a company settled in the Basque Country which designs this kind of platforms. To our purpose we present a method for the simulation of two-phase flow with the presence of floating bodies. We consider the variable density incompressible Navier-Stokes equations and discretize them by the finite element method with a variational multiscale stabilization. A level-set type method is adopted to model the interphase between the two fluids. The mixing or smearing in the interphase is prevented with a compression technique. Turbulence is implicitly modeled by the numerical stabilization. The floating device simulation is done by a rigid body motion scheme where a deforming mesh approach is used. The mesh deforms elastically following the movement of the body. Simulation of a decay test on a cube is performed and the results are presented in this paper. All the simulations are done with the open source finite elements parallel software FEniCS-HPC. 
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19.
  • Nguyen, Van Dang, 1985-, et al. (författare)
  • A fluid-structure interaction model with weak slip velocity boundary conditions on conforming internal interfaces
  • 2018
  • Konferensbidrag (populärvet., debatt m.m.)abstract
    • We develop a PUFEM–Partition of Unity Finite Element Method to impose slip velocity boundary conditions on conforming internal interfaces for a fluid-structure interaction model. The method facilitates a straightforward implementation on the FEniCS/FEniCS-HPC platform. We show two results for 2D model problems with the implementation on FEniCS: (1) optimal convergence rate is shown for a stationary Navier-Stokes flow problem, and (2) the slip velocity conditions give qualitatively the correct result for the Euler flow. 
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20.
  • Nguyen, Van Dang, 1985-, et al. (författare)
  • A partition of unity finite element method for computational diffusion MRI
  • 2018
  • Ingår i: Journal of Computational Physics. - : Elsevier. - 0021-9991 .- 1090-2716. ; 375, s. 271-290
  • Tidskriftsartikel (refereegranskat)abstract
    • The Bloch–Torrey equation describes the evolution of the spin (usually water proton) magnetization under the influence of applied magnetic field gradients and is commonly used in numerical simulations for diffusion MRI and NMR. Microscopic heterogeneity inside the imaging voxel is modeled by interfaces inside the simulation domain, where a discontinuity in the magnetization across the interfaces is produced via a permeability coefficient on the interfaces. To avoid having to simulate on a computational domain that is the size of an entire imaging voxel, which is often much larger than the scale of the microscopic heterogeneity as well as the mean spin diffusion displacement, smaller representative volumes of the imaging medium can be used as the simulation domain. In this case, the exterior boundaries of a representative volume either must be far away from the initial positions of the spins or suitable boundary conditions must be found to allow the movement of spins across these exterior boundaries.Many approaches have been taken to solve the Bloch–Torrey equation but an efficient high-performance computing framework is still missing. In this paper, we present formulations of the interface as well as the exterior boundary conditions that are computationally efficient and suitable for arbitrary order finite elements and parallelization. In particular, the formulations are based on the partition of unity concept which allows for a discontinuous solution across interfaces conforming with the mesh with weak enforcement of real (in the case of interior interfaces) and artificial (in the case of exterior boundaries) permeability conditions as well as an operator splitting for the exterior boundary conditions. The method is straightforward to implement and it is available in FEniCS for moderate-scale simulations and in FEniCS-HPC for large-scale simulations. The order of accuracy of the resulting method is validated in numerical tests and a good scalability is shown for the parallel implementation. We show that the simulated dMRI signals offer good approximations to reference signals in cases where the latter are available and we performed simulations for a realistic model of a neuron to show that the method can be used for complex geometries.
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