Current emissions targets have created a strong need for introducing more renewable energy sources into the energy mixture. The oscillating-foil turbine (OFT) has gained interest in recent years for renewable energy extraction. Experimental and numerical studies on the OFT experience different levels of wall confinement than what may be experienced at a natural site. Walls in close proximity will direct the flow at the turbine, causing a greater perceived velocity by the turbine, and thus a higher theoretical performance. This work aims to increase understanding of flow confinement on the fully-passive OFT. This is motivated by (1) enabling comparison between turbine performance operating at different confinement levels, and (2) potentially providing a means to enhance performance by designing a turbine which uses confinement to its advantage. The experiments were performed using a NACA0015 foil with an aspect ratio of 7.5 in a water tunnel equipped with adjustable lateral walls. The foil was undergoing passive oscillations in pitch and heave degrees of freedom. The kinematic parameters of the foil oscillations and its energy harvesting performance were measured at eight blockage ratios, ranging from 22% to 60%, for two structural configurations of the turbine. Quantitative flow imaging was performed using particle image velocimetry (PIV), at three confinement levels, to observe the timing of the leading-edge vortex (LEV) formation and shedding throughout the foil oscillation cycle. Loading on the foil was related to the flow structure by calculating the moments of vorticity with respect to the pitching axis of the foil. The results showed that the efficiency and the power coefficient increased with increasing confinement. This was expected due to the higher incident velocity on the foil in the presence of the confining walls. At the highest level of confinement, the close proximity of the foil to the walls during parts of the oscillation cycle resulted in a change in the phase lag between the pitching and the heaving components of the foil motion. In turn, this shift in the kinematic parameters led to a sharp decrease in the energy-extraction performance of the turbine. / Graduate

Solving the fluid-structure interaction (FSI) problems is particularly challenging. This is because the coupling of the fluid and structure may require different solvers in different points of the solution domain, and with different mesh requirements. In this thesis, a partitioned approach is considered. Two solvers are employed to deal with each part of the problem (fluid and structure), where the interaction process is realized via exchanging information from the fluid-structure interface in a staggered fashion. One of the advantages of this approach is that we can take advantage of the existing algorithms that have been used for solving fluid or structural problems, which leads a reduction in the code development time, Hou (2012). However, it requires careful implementation so that spurious results in terms of stability and accuracy can be avoided. We found that most fluid-structure interaction computations through a staggered approach are based on at most second order time integration methods. In this thesis we studied the performance of some high order fluid and structure dynamic methods, when applied in a staggered approach to an FSI problem in a structure prediction way by combining predictors with time integration schemes to obtain stable schemes. Nonlinear Euler equations for gas dynamics were investigated and the analysis was realized through the piston problem. An adapted one-dimensional high order finite volume WENO₃ scheme for nonlinear hyperbolic conservation laws-Dumbser (2007a), Dumbser et al. 2007b)-was considered and a numerical flux was proposed. The numerical results of the proposed method show the non-oscillatory property when compared with traditional numerical methods such as the Local Lax-Friedrichs. So far to our knowledge, the WENO₃₋ as proposed in this work- has not been applied to FSI problems. Thus, it was proposed to discretize the fluid domain in space, and in order to adapt it to a moving mesh was reformulated to couple with an Arbitrary Lagrangian Eulerian (ALE) approach. To integrate in time the structure we started by using Newmark schemes as well as the trapezoidal-rule backward differentiation formulae of order 2 (TR − BDF2). Two study cases were carried out by taking into account the transient effects on the fluid behaviour. In the first case, we only consider the structural mass in the dynamic coupled system and in the second case, a quasi-steady fluid was considered. In order to test the performance of the structural solvers, simulations were carried out, firstly, without the contribution of fluid mass, and then a comparative study of the performance of various structure solvers in a staggered approach framework were realized in order to study the temporal accuracy for the partitioned fluid-structure interaction coupling. For a quasi-steady fluid case, the oscillation frequency of the coupled system was successfully estimated using the TR-BDF2 scheme, and the coupled system was solved for various Courant numbers in a structural predictor fashion. The results showed better performance of the TR-BDF2 scheme. Newmark’s schemes as well as the TR-BDF2 are only second order accuracy. However, the Newmark (average acceleration) is traditionally preferred by researchers as a structure solver in a staggered approach for FSI problems, although higher order schemes do exist. van Zuijlen (2004), in his partitioned algorithm proposed the explicit singly diagonal implicit Runge-Kutta (ESDIRK) family of schemes of order 3 to 5 to integrate both fluid and structure. Therefore in this work, these schemes were considered and applied as structural solvers. Their performance was studied through numerical experiments, and comparisons were realized with the performance of the traditional Newmark’s schemes. The results show that although their computational cost is high, they present a high order of accuracy. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2013.

Harbours play a vital role in the economies of most countries since a significant amount of international trade is conducted through them. Ships rely on harbours for the safe loading and unloading of cargo and the harbour infrastructure relies on breakwaters for protection. As a result, the design and analysis of breakwaters receives keen interest from the engineering community. Coastal engineers need an easy-to-use tool that can model the way in which waves interact with large numbers of interlocking armour units. Although the study of fluid–structure interaction generates a lot of research activity, none of the reviewed literature describes a suitable method of analysis. The goal of the research was to develop a simulation algorithm that meets all the criteria by allowing CFD software and physics middleware to work in unison. The proposed simulation algorithm used Linux “shell scripts” to coordinate the actions of commercial CFD software (Star-CCM+) and freely available physics middleware (PhysX). The CFD software modelled the two-phase fluid and provided force and moment data to the physics middleware so that the movement of the armour units could be determined. The simulation algorithm was verified numerically and experimentally. The numerical verification exercise was of limited value due to unresolved issues with the CFD software chosen for the analysis, but it was shown that PhysX responds appropriately given the correct force data as input. Experiments were conducted in a hydraulics laboratory to study the interaction of a solitary wave and cubes stacked on a platform. Fiducial markers were used to track the movement of the cubes. The phenomenon of interest was the transfer of momentum from the wave to the rigid bodies, and the results confirmed that the effect was captured adequately. The study concludes with suggestions for further study. / MIng (Mechanical Engineering), North-West University, Potchefstroom Campus, 2014

Harbours play a vital role in the economies of most countries since a significant amount of international trade is conducted through them. Ships rely on harbours for the safe loading and unloading of cargo and the harbour infrastructure relies on breakwaters for protection. As a result, the design and analysis of breakwaters receives keen interest from the engineering community. Coastal engineers need an easy-to-use tool that can model the way in which waves interact with large numbers of interlocking armour units. Although the study of fluid–structure interaction generates a lot of research activity, none of the reviewed literature describes a suitable method of analysis. The goal of the research was to develop a simulation algorithm that meets all the criteria by allowing CFD software and physics middleware to work in unison. The proposed simulation algorithm used Linux “shell scripts” to coordinate the actions of commercial CFD software (Star-CCM+) and freely available physics middleware (PhysX). The CFD software modelled the two-phase fluid and provided force and moment data to the physics middleware so that the movement of the armour units could be determined. The simulation algorithm was verified numerically and experimentally. The numerical verification exercise was of limited value due to unresolved issues with the CFD software chosen for the analysis, but it was shown that PhysX responds appropriately given the correct force data as input. Experiments were conducted in a hydraulics laboratory to study the interaction of a solitary wave and cubes stacked on a platform. Fiducial markers were used to track the movement of the cubes. The phenomenon of interest was the transfer of momentum from the wave to the rigid bodies, and the results confirmed that the effect was captured adequately. The study concludes with suggestions for further study. / MIng (Mechanical Engineering), North-West University, Potchefstroom Campus, 2014

In the recent decades the Fluid-Structure Interaction (FSI) problem has been of great interest to many researchers and a variety of methods have been proposed for its numerical simulation. As FSI simulation is a multi-discipline and a multi-physics problem, its full simulation consists of many details and sub-procedures. On the other hand, reliable FSI simulations are required in various applications ranging from hemo-dynamics and structural engineering to aero-elasticity. In hemo-dynamics an incompressible fluid is coupled with a flexible structure with similar density (e.g. blood in arteries). In aero-elasticity a compressible fluid interacts with a stiff structure (e.g. aircraft wing) or an incompressible flow is coupled with a very light structure (e.g. Parachute or sail), whereas in some other engineering applications an incompressible flow interacts with a flexible structure with large displacement (e.g. oil risers in offshore industries). Therefore, various FSI models are employed to simulate a variety of different applications. An initial vital step to conduct an accurate FSI simulation is to perform a study of the physics of the problem which would be the main criterion on which the full FSI simulation procedure will then be based. In this thesis, interaction of an incompressible fluid flow at low Reynolds number with a flexible circular cylinder in two dimensions has been studied in detail using some of the latest published methods in the literature. The elements of procedures have been chosen in a way to allow further development to simulate the interaction of an incompressible fluid flow with a flexible oil riser with large displacement in three dimensions in future. To achieve this goal, a partitioned approach has been adopted to enable the use of existing structural codes together with an Immersed Boundary (IB) method which would allow the modelling of large displacements. A direct forcing approach, interpolation / reconstruction, type of IB is used to enforce the moving boundary condition and to create sharp interfaces with the possibility of modelling in three dimensions. This provides an advantage over the IB continuous forcing approach which creates a diffused boundary. And also is considered as a preferred method over the cut cell approach which is very complex in three dimensions with moving boundaries. Different reconstruction methods from the literature have been compared with the newly proposed method. The fluid governing equation is solved only in the fluid domain using a Cartesian grid and an Eulerian approach while the structural analysis was performed using Lagrangian methods. This method avoids the creation of secondary fluid domains inside the solid boundary which occurs in some of the IB methods. In the IB methods forces from the Eulerian flow field are transferred onto the Lagrangian marker points on the solid boundary and the displacement and velocities of the moving boundary are interpolated in the flow domain to enforce no-slip boundary conditions. Various coupling methods from the literature were selected and improved to allow modelling the interface and to transfer the data between fluid and structure. In addition, as an alternative method to simulate FSI for a single object in the fluid flow as suggested in the literature, the moving frame of reference method has been applied for the first time in this thesis to simulate Fluid-Structure interaction using an IB reconstruction approach. The flow around a cylinder in two dimensions was selected as a benchmark to validate the simulation results as there are many experimental and analytical results presented in the literature for this specific case.

No description available.

This work details the development and implementation of a numerical model capable of solving strongly-coupled fluid-structure interaction problems involving long thin structures, which are common multi-physics problems encountered in many applications. In most fluid-structure interaction problems the deformation of the slender elastic bodies is significant and cannot be described by a purely linear analysis. We present a new formulation to model these larger displacements. By extending the standard modal decomposition technique for linear structural analysis, the governing equations and boundary conditions are updated to account for the leading-order non-linear terms and a new modal formulation with quadratic modes is derived. The quadratic modal approach is tested on standard benchmark problems of increasing complexity and compared with analytical and full non-linear numerical solutions. Two computational fluid-structure interaction approaches are then implemented in a partitioned manner: a finite volume method for discretisation of both the fluid and solid domains and the quadratic modal formulation for the structure coupled with a finite volume fluid solver. Strong-coupling is achieved by means of a fixed-point solver with dynamic relaxation. The fluid-structure interaction approaches are validated and compared on benchmark problems of increasing complexity and strength of coupling between the fluid and solid domains. Fluid-structure interaction systems may become unstable due to the interaction between the fluid-induced pressure and structural rigidity. A thorough stability analysis of finite elastic plates in uniform flow is conducted by varying the structural length and flow velocity showing that these are critical parameters. Validation of the results with those from analytical methods is done. An analysis of the dynamic interactions between multiple finite plates in various configurations is also conducted.

A study of the interaction of one-sided flow over a compliant surface is presented. When fluid passes over a flexible surface the simultaneous interaction between the flow and structure gives rise to vibrations and instabilities on the surface as well as in the fluid. The fluid-structure interaction (FSI) has potential to be used in the control of boundary layer dynamics to achieve drag reduction through transition delay. The modelling and control of FSI systems apply to many fields of engineering beyond drag reduction, for example: the modelling and analysis of biomechanical systems; natural environmental systems; aero-elastics; and other areas where flow interacts moving or compliant boundaries. The investigation is performed through numerical simulation. This returns more detail than could be resolved through experiments, while also permitting the study of finite compliant surfaces that are prohibitively difficult, or impossible, to study with analytical techniques. In the present work, novel numerical modelling methods are developed from linear system analysis through to nonlinear disturbances and viscous effects. / Two numerical modelling techniques are adopted to approach the analysis of the FSI system. A potential-flow method is used for the modelling of flows in the limit of infinite Reynolds numbers, while a grid-free Discrete Vortex Method (DVM) is used for the modelling of the rotational boundary-layer flow at moderate Reynolds numbers. In both inviscid and viscous studies, significant contributions are made to the numerical modelling techniques. The application of these methods to the study of flow over compliant panels gives new insight to the nature of the FSI system. In the linear inviscid model, a novel hybrid computational/theoretical method is developed that evaluates the eigenvalues and eigenmodes from a discretised FSI system. The results from the non-linear inviscid model revealed that the steady-state of the non-linear wall motion is independent of initial excitation. For the viscous case, the first application of a DVM to model the interaction of a viscous, rotational flow with a compliant surface is developed. This DVM is successfully applied to model boundary-layer flow over a finite compliant surface.

The development of a numerical model of a two-stroke engine is undertaken to study the scavenging characteristics of the engine. The engine design is unique in its use of 16 passive intake valves in the cylinder head which, along with the exhaust ports located at bottom centre (BC), give the engine a top-down uniflow-scavenged configuration. Each valve constitutes a small stainless steel platelet within a cavity in the cylinder head which reacts to the pressure difference across the cylinder head. The principle focus of this study is the transient simulation of the scavenging flow using dynamic meshing to model the piston motion and the response of the passive intake valves to the scavenging flow for varied engine speed and peak pressure. A flowbench study of the steady flow through the cylinder head into a duct is incorporated as a step in the development of the transient numerical model. Validation of the numerical predictions is undertaken by comparing results from an experimental flowbench for the steady case and using a cold-flow scavenging rig for the transient simulations. Both the steady flow through the cylinder head and the unsteady flow within the cylinder indicate the presence of a recirculation region on the cylinder axis. As a result, short-circuiting of scavenging gas becomes considerable and leads to scavenging characteristics comparable to Hopkinson’s perfect mixing one-dimensional scavenging model. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2008-09-26 18:38:53.375

Water hammer represents a complex hydraulic phenomenon with significant consequences on the proper functioning and safety of operation for pipe and conduit systems. The complexity and intricate physics of water hammer translated into significant difficulties associated firstly, with finding a proper solution for understanding the mechanism of its occurrence and, secondly, relating to proposing technically and economically viable design methods and devices that would help reduce and mitigate water hammer effects. In this context, the present thesis deals with the numerical modeling of the transient behaviour of water pipe segments. Following an extensive literature review of the state-of-the-art on the water hammer mechanisms and past work on experimental, analytical and numerical analysis of this phenomenon, a three dimensional numerical model of the water hammer in a pipe which considers the fluid-structure interaction (FSI) is developed using a Finite Element Method – Finite Volume Method (FEM-FVM) technique. Structural and fluid computational results based on rapid and slow gate closure scenarios are compared with existing closed-form solutions of the water hammer. A parametric study is also performed on a simply supported pipe segment to determine the influence of various design parameter. A systematic sensitivity analysis was conducted and a ranking mechanism was established for the importance of each parameter on the fluid fields and structural response. A first comparative analysis is conducted on horizontally and vertically bent elevated pipe segments to quantify the influence of the bend angle on the results. A second comparative analysis is performed on a horizontally bent segment buried in soil to determine the influence of the pipe interaction with the soil on the response. It is observed that the thickness, span, initial velocity and bend angle had a significant impact on the pressure and structural response. The presence of soil was observed to have a significant benefit in decreasing the von-Mises stresses.