Adaptive mesh optimization for simulation of immiscible viscous fingering

Viscous fingering can be a major concern when waterflooding heavy-oil reservoirs. Most commercial reservoir simulators use low-order finite-volume/-difference methods on structured grids to resolve this phenomenon. However, this approach suffers from a significant numerical-dispersion error because of insufficient mesh resolution, which smears out some important features of the flow. We simulate immiscible incompressible two-phase displacements and propose the use of unstructured control-volume finite-element (CVFE) methods for capturing viscous fingering in porous media. Our approach uses anisotropic mesh adaptation where the mesh resolution is optimized on the basis of the evolving features of flow. The adaptive algorithm uses a metric tensor field dependent on solution-interpolation-error estimates to locally control the size and shape of elements in the metric. The mesh optimization generates an unstructured finer mesh in areas of the domain where flow properties change more quickly and a coarser mesh in other regions where properties do not vary so rapidly. We analyze the computational cost of mesh adaptivity on unstructured mesh and compare its results with those obtained by a commercial reservoir simulator on the basis of the finite-volume methods

The Immersed Body Method and Its Use in Modelling Vertical Axis Turbines

The focus of this thesis is on the development of a fluid–solid interaction (FSI) model, based on the idea of the immersed boundary method. The novelty of this approach is the combination of a two–fluid approach to represent the solid phase on a fluid finite–element mesh, with the conservative projection of data between two unrelated meshes. While this is an important feature for two–way coupled FSI models, this thesis analyses the outcome of this method based on one–way coupled FSI problems, in which the solid phase has a prescribed velocity. The presented FSI method is validated on several test cases with static solids as well as solids with a prescribed velocity. For complex computational fluid dynamic (CFD) problems, mesh adaptivity methods are used to reduce the computational effort while obtaining the same accuracy compared to fixed meshes. In this work mesh adaptivity is also used to increase the resolution of the fluid mesh near the solid boundary in order to obtain an accurate representation of the solid’s shape on the fluid mesh. However, spurious peaks in the pressure occur due to the projection of fields after adapting the mesh. This causes peaks in the drag force and results in a potential problem by decreasing the accuracy, especially for two–way coupled FSI problems. Since the FSI method was developed with two–way coupled FSI problems in mind, the occurrence of the spurious peaks was analysed and methods are shown to minimise the peaks in the drag force. Finally, the developed FSI method is applied to rotating vertical axis turbines and the results are compared to experimental results. This again shows the difficulties of applying the method and assesses how it can be used for turbine modelling, and furthermore used for analysing optimised turbine layouts.Open Acces

High fidelity fluid-structure turbulence modeling using an immersed-body method

There is an increasing need for turbulence models with fluid-structure interaction (FSI) in many industrial and environmental high Reynolds number flows. Since the complicated structure boundaries move in turbulent flows, it is quite challenging to numerically apply boundary conditions on these moving fluid-structure interfaces. To achieve accurate and reliable results from numerical FSI simulations in turbulent flows, a high fidelity fluid-structure turbulence model is developed using an immersed-body method in this thesis. It does this by coupling a finite element multiphase fluid model and a combined finite-discrete element solid model via a novel thin shell mesh surrounding solid surfaces. The FSI turbulence model presented has four novelties. Firstly, an unsteady Reynolds-averaged Navier-Stokes (URANS) k−ε turbulence model is coupled with an immersed-body method to model FSI by using this thin shell mesh. Secondly, to reduce the computational cost, a log-law wall function is used within this thin shell to resolve the flow near the boundary layer. Thirdly, in order to improve the accuracy of the wall function, a novel shell mesh external-surface intersection approach is introduced to identify sharp solid-fluid interfaces. Fourthly, the model has been extended to simulate highly compressible gas coupled with fracturing solids. This model has been validated by various test cases and results are in good agreement with both experimental and numerical data in published literature. This model is capable to simulate the aerodynamic and hydrodynamic details of fluids and the stress, vibration, deformation and motion of structures simultaneously. Finally, this model has been applied to several industrial applications including a floating structure being moved around by complex hydrodynamic flows involving wave breaking; a blasting engineering simulation with shock waves, fracture propagation, gas-solid interaction and flying fragments; fluid dynamics, flow-induced vibrations, flow-induced fractures of a full-scale vertical axis turbine. Some useful conclusions, e.g. how to model them, how to make them stable and how to predict when they will break, are obtained by this FSI model when applying it to the above applications.Open Acces