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- Jonsson, Patrick
(författare)
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Smoothed Particle Hydrodynamic of Hydraulic Jumps in Spillways
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focus on the complex natural phenomena of hydraulic jumps using the numerical method Smoothed Particle Hydrodynamics (SPH). A hydraulic jump is highly turbulent and associated with turbulent energy dissipation, air entrainment, surface waves and spray and strong dissipative processes. It can be found not only in natural streams and in engineered open channels, but also in your kitchen sink at home. The dissipative features are utilized in hydropower spillways and stilling basins to reduce high velocity flows. Potentially, such flow can cause erosion and reduce the lifetime and increase maintenance costs of spillways and related structures which must be avoided. Usually, spillways are engaged to safely pass extreme flooding events and redirect the flow during maintenance shutdown of the production units, i.e. turbines and generators. It is hence vital to understand and be able to predict the involved processes in a hydraulic jump. The Lagrangian, meshless particle based numerical method SPH has been considered as the main computational method throughout this thesis. The ability of the SPH method to capture complex free-surfaces with large deformation and fragmentation, found in hydraulic jumps, makes it a strong modelling tool. However, the SPH method is less developed compared to the established Finite Volume- (FVM) and Finite Element (FEM) methods. Initially, focus was on reproducing the results of previous studies where the geometrical aspect of hydraulic jumps was the main consideration (Paper A). Several modelling parameters were re-evaluated using a dam-break test case in Paper B and later applied in Paper C. Paper C, focused not only on the geometrical aspect of the hydraulic jump but also on the internal flow field and its relation to the free-surface. Later in Paper D, a new strategy on how to perform SPH hydraulic jump simulations based on periodic open boundaries was developed. Finally, the method developed was applied in two separate studies. In Paper E, the SPH method was compared with experiments performed at Vattenfall Research & Development in ¨Alvkarleby, Sweden. The SPH model, comprised of a channel and a scaled spillway outlet chute, not only captured the jump position but also large scale flow features. The final Paper F, was a continuation of Paper C where the internal flow field and its dynamical relationship with the free surface was reinvestigated using the more sophisticated SPH model.
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| 2. |
- Sjökvist, Linnea
(författare)
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Wave Loads and Peak Forces on Moored Wave Energy Devices in Tsunamis and Extreme Waves
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Surface gravity waves carry enormous amounts of energy over our oceans, and if their energy could be harvested to generate electricity, it could make a significant contribution to the worlds power demand. But the survivability of wave energy devices in harsh operating conditions has proven challenging, and for wave energy to be a possibility, peak forces during storms and extreme waves must be studied and the devices behaviour understood. Although the wave power industry has benefited from research and development in traditional offshore industries, there are important differences. Traditional offshore structures are designed to minimize power absorption and to have small motion response, while wave power devices are designed to maximize power absorption and to have a high motion response. This increase the difficulty of the already challenging survivability issue. Further, nonlinear effects such as turbulence and overtopping can not be neglected in harsh operating conditions. In contrast to traditional offshore structures, it is also important to correctly account for the power take off system in a wave energy converter (WEC), as it is strongly coupled to the devices behaviour.The focus in this thesis is the wave loads and the peak forces that occur when a WEC with a limited stroke length is operated in waves higher than the maximum stroke length. The studied WEC is developed at Uppsala University, Sweden, and consists of a linear generator at the seabed that is directly driven by a surface buoy. A fully nonlinear CFD model is developed in the finite volume software OpenFOAM, and validated with physical wave tank experiments. It is then used to study the motion and the forces on the WEC in extreme waves; high regular waves and during tsunami events, and how the WECs behaviour is influenced by different generator parameters, such as generator damping, friction and the length of the connection line. Further, physical experiments are performed on full scale linear generators, measuring the total speed dependent damping force that can be expected for different loads. The OpenFOAM model is used to study how the measured generator behaviour affects the force in the connection line.
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