| 1. |
- Nguyen, Van-Dang, 1985-
(författare)
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High-Performance Finite Element Methods : with Application to Simulation of Diffusion MRI and Vertical Axis Wind Turbines
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The finite element methods (FEM) have been developed over decades, and together with the growth of computer engineering, they become more and more important in solving large-scale problems in science and industry. The objective of this thesis is to develop high-performance finite element methods (HP-FEM), with two main applications in mind: computational diffusion magnetic resonance imaging (MRI), and simulation of the turbulent flow past a vertical axis wind turbine (VAWT). In the first application, we develop an efficient high-performance finite element framework HP-PUFEM based on a partition of unity finite element method to solve the Bloch-Torrey equation in heterogeneous domains. The proposed framework overcomes the difficulties that the standard approaches have when imposing the microscopic heterogeneity of the biological tissues. We also propose artificial jump conditions at the external boundaries to approximate the pseudo-periodic boundary conditions which allows for the water exchange at the external boundaries for non-periodic meshes. The framework is of a high level simplicity and efficiency that well facilitates parallelization. It can be straightforwardly implemented in different FEM software packages and it is implemented in FEniCS for moderate-scale simulations and in FEniCS-HPC for the large-scale simulations. The framework is validated against reference solutions, and implementation shows a strong parallel scalability. Since such a high-performance simulation framework is still missing in the field, it can become a powerful tool to uncover diffusion in complex biological tissues. In the second application, we develop an ALE-DFS method which combines advanced techniques developed in recent years to simulate turbulence. We apply a General Galerkin (G2) method which is continuous piecewise linear in both time and space, to solve the Navier-Stokes equations for a rotating turbine in an Arbitrary Lagrangian-Eulerian (ALE) framework. This method is enhanced with dual-based a posterior error control and automated mesh adaptation. Turbulent boundary layers are modeled by a slip boundary condition to avoid a full resolution which is impossible even with the most powerful computers available today. The method is validated against experimental data of parked turbines with good agreements. The thesis presents contributions in the form of both numerical methods for high-performance computing frameworks and efficient, tested software, published open source as part of the FEniCS-HPC platform.
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| 2. |
- Fredén Jansson, Karl-Johan, 1988
(författare)
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A New Audiometric Bone Vibrator, Radioear B81, and the Bone Conduction Implant with Emphasis on Magnetic Resonance Imaging
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Hearing by air conduction (AC) and bone conduction (BC) are attributed to be the natural ways of stimulating the cochlea. With AC hearing, the cochlea is stimulated by air pressure variations via the ear canal, whereas with BC hearing, sound vibrations are transmitted thru the skull bone to the cochlea. Sensorineural hearing losses are commonly rehabilitated with conventional AC hearing aids in the ear canal, but patients who are suffering from conductive or mixed hearing losses, and who are unable to use AC hearing aids, may instead use bone conduction devices (BCDs). In order to determine the type and degree of hearing loss, the BC hearing thresholds are measured using a bone vibrator, and then analyzed together with the AC hearing thresholds to suggest an appropriate rehabilitation alternative.This thesis deals with two BC hearing related topics. The first topic is evaluating a new audiometric bone vibrator, Radioear B81, which is assumed to offer more accurate BC hearing threshold measurements. The second topic is related to a new type of active transcutaneous BCD, called the Bone Conduction Implant (BCI), which leaves the skin intact by using a wireless solution that does not require a permanent skin penetration. Even though the applications are different, both devices use the same Balanced Electromagnetic Separation Transducer (BEST) principle as motor unit in their design.The audiometric bone vibrator Radioear B81 was found to have an improved low frequency performance and can produce higher output levels with less harmonic distortion than was possible before. In a clinical study of the first six patients, it was found that the BCI is a realistic alternative to already commercially available BCDs. In technical evaluations, the BCI was shown to be insensitive to skin thickness variations and to have robust output, and that it possibly tolerates magnetic resonance imaging at 1.5 Tesla.
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| 3. |
- Jansson, Johan
(författare)
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Process-Induced Local Material Variations in Finite Element Simulations of Cast and Fibre Reinforced Injection Moulded Components
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The purpose of this thesis is to provide an overview of the methods used in the appended papers, in order to consider heterogeneous material properties in finite element simulations by using process simulations as input. The work deals with both injection moulded and cast components, and focuses on process-induced local material variations and their effect on component performance.The influence of heterogeneous properties originating from the casting process as well as some other common simplifications, which are made in finite element analyses, are evaluated for a cast iron component. It is found that commonly neglected properties such as compressive strength, residual stresses, temperature dependency and heterogeneous properties have a non-trivial and potentially large influence on the simulation results.Lastly, a computational method for fibre reinforced plastics is presented. The methodology enables designers to consider the non-linear anisotropic properties of fibre-reinforced polymers, due to the flow-induced fibre orientation predicted by injection moulding simulations. The method allows material data assignment in each integration-point of the structural mesh. The method is demonstrated to capture the behaviour of the full range of fibre orientations simultaneously with good accuracy.
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