电动微流体是微全分析系统中样品输运的主要形式，数值模拟是计算和分析电动微流体样品输运特性的重要手段。由于微通道中区带样品在流体内存在大浓度梯度的界面，在计算时很容易引起数值耗散，对计算精度影响严重。本文探讨了电动微流体样品输运计算中数值耗散的产生原因和消除措施，利用有限体积法求解电场分布和流场分布，在此基础上应用自由界面重构技术对数值流向量进行了处理并求解微通道中样品传输，有效消除了界面处数值耗散的影响，并给出微通道中电泳分离和电致混合增强的计算模拟结果，研究结果表明：采用FCT-VOF和基于积分平均型TVD格式的VOF方法对上述算例的样品界面进行重构，能够保持界面平滑的同时，有效抑制了数值耗散，获得较好的计算精度。 Electrokinetic microfluidics is one of the main subcategories for sample transport in Micro-Total- Analysis-System. Numerical simulation is an efficient method for calculating and analyzing trans- port properties of samples in electrokinetic microfluidics. However, since the strong discontinuity exists at the interface of sample zone, the numerical dissipation near the interface becomes a key problem that can seriously influence the computational accuracy. In this paper, the reasons of numerical dissipation are theoretically analyzed and the techniques based on free interface reconstruction are provided to weaken the numerical dissipation. Firstly, the electric field and the flow field are solved by using the finite volume method. Then, in order to constraint the numerical dissipation, several techniques based on the volume of fluid (VOF) are utilized to reconstruct the flow vectors and solve the sample transport. Lastly, with the developed algorithm, capillary electrophoresis and electrokinetically driven enhanced mixing in microchannels are simulated. The results show that both FCT-VOF and TVD-VOF can significantly constraint the numerical dissipation while keeping the interface smoothing and are suitable for the simulation of electrokinetic microfluidics and other applications.

When density variations are sufficiently small the Boussinesq approximation is valid. The approximation is introduced to reduce the degree of the complexity of density variations and implies that density effects are considered only in the buoyancy force term of the momentum equation. Because of its simplicity in practical implementations, the approximation is widely used. Although there are many studies related to the approximation, some important characteristics are still missing. In this article, we compare the Boussinesq approximation and variable density models for the two-dimensional (2D) Rayleigh–Taylor instability with a phase-field method. Numerical experiments indicate that for an initially symmetric perturbation of the interface the symmetry of the heavy and light fronts for the Boussinesq model can be seen for a long time. However, for the variable density model, the symmetry is lost although the flow starts symmetrically.

VOF method which consists in transporting a discontinuous marker variable is widely used to capture the free surface in computational fluid dynamics. There is numerical dissipation in simulations involving the transport of the marker. Numerical dissipation makes the free surface lose its physical nature. A free surface sharpening strategy based on optimization method is presented in the paper. The strategy can keep the location of the free surface and local mass conservation at both time, and can also keep free surface in a constant width. It is independent on the types of solvers and meshes. Two famous cases were chosen for verifying the free surface sharpening strategy performance. Results show that the strategy has a very good performance on keeping local mass conservation. The efficiency of prediction of the free surface is improved by applying the strategy. Accurate modeling of flow details such as drops can also be captured by this method.

Two-phase flows occur regularly in nature and industrial processes and their understanding is of significant interest in engineering research and development. Various numerical methods to predict two-phase phase flows have been developed as a result of extensive research efforts in past decades, however, most methods are limited to Cartesian meshes. A fully-coupled implicit numerical framework for two-phase flows on unstructured meshes is presented, solving the momentum equations and a specifically constructed continuity constraint in a single equation system. The continuity constraint, derived using a momentum interpolation method, satisfies continuity, provides a strong pressure-velocity coupling and ensures a discrete balance between pressure gradient and body forces. The numerical framework is not limited to specific density ratios or a particular interface topology and includes several novelties. A further step towards a more accurate prediction of two-phase flows on unstructured meshes is taken by proposing a new method to evaluate the interface curvature. The curvature estimates obtained with this new method are shown to be as good as or better than methods reported in literature, which are mostly limited to Cartesian meshes, and the accuracy on structured and unstructured meshes is shown to be comparable. Furthermore, lasting contributions are made towards the understanding of convolution methods for twophase flow modelling and the underlying mechanisms of parasitic currents are studied in detailed. The mesh resolution is of particular importance for two-phase flows due to the inherent first-order accuracy of the interface position using interface capturing methods. A mesh adaption algorithm for tetrahedral meshes with application to two-phase flows and its implementation are presented. The algorithm is applied to study mesh resolution requirements at interfaces and force-balancing for surface-tension-dominated two-phase flows on adaptive meshes.

Two principal methods have been used to simulate the evolution of two-phase immiscible flows of liquid and gas separated by an interface. These are the Level-Set (LS) method and the Volume of Fluid (VoF) method. Both methods attempt to represent the very sharp interface between the phases and to deal with the large jumps in physical properties associated with it. Both methods have their own strengths and weaknesses. For example, the VoF method is known to be prone to excessive numerical diffusion, while the basic LS method has some difficulty in conserving mass. Major progress has been made in remedying these deficiencies, and both methods have now reached a high level of physical accuracy. Nevertheless, there remains an issue, in that each of these methods has been developed by different research groups, using different codes and most importantly the implementations have been fine tuned to tackle different applications. Thus, it remains unclear what are the remaining advantages and drawbacks of each method relative to the other, and what might be the optimal way to unify them. In this paper, we address this gap by performing a direct comparison of two current state-of-the-art variations of these methods (LS: RCLSFoam and VoF: interPore) and implemented in the same code (OpenFoam). We subject both methods to a pair of benchmark test cases while using the same numerical meshes to examine a) the accuracy of curvature representation, b) the effect of tuning parameters, c) the ability to minimise spurious velocities and d) the ability to tackle fluids with very different densities. For each method, one of the test cases is chosen to be fairly benign while the other test case is expected to present a greater challenge. The results indicate that both methods can be made to work well on both test cases, while displaying different sensitivity to the relevant parameters.

This work shows that a three-dimensional transient two-phase RANS CFD-VOF model can be used to predict the position of waves and hydraulic jumps within a complex hydraulic flow environment as measured during a series of full-scale experiments. A novel application of LIDAR is used to provide detailed measurements of the position of the water free-surface location during the physical experiments. The test environment is a recreational white-water course that provides a means to vary the flow rates of water and restrict the flow easily as required. Obstructions are added to the channel to create hydraulic jumps and other specific flow features. The influence of these obstructions on the flow has been analysed for size, velocity and position. The results of the study demonstrate that, although computationally intensive, the free-surface CFD approach can reliably predict a range of complex hydraulic flow features in medium/largescale open channel flow conditions. In order to reliably capture the full three-dimensional characteristics of the water free-surface a high resolution mesh (greater than 2.5 million cells) with time-steps in the order of milliseconds is necessary (Simulations presented here represent between 30 and 60 seconds of real-time).

This thesis work deals with fluid structure interaction (FSI), one of the emerging areas of numerical simulation and calculation. FSI occurs when the flow of fluid influences the properties of a structure or vice versa. It is a great challenge to deal with such problems due to its complexity in defining the geometries, nature of interaction between a fluid and solid, multi-physics facts and requirements of computational resources. This kind of interaction occurs in a wide spectrum of engineering problems and as such remains a main attraction of engineering profession. In this thesis, the FSI analysis has been conducted on a typical slender cylindrical structural member used either as a brace or leg of an offshore truss structure. The supporting members of the structure (near to the free surface of the water) are subjected, more dominantly, to wave induced loads. Gradually, this interaction leads to the fatigue of existing structures. In 2009, a numerical study was conducted to predict the wave-in-deck load due to extreme waves on jacket platform near to free surface of the water [1]. The recent developments in numerical abilities provide easier ways to do this analysis. Here, the analysis has been approached using the partitioned method in which two ways of coupling (one-way and two-way) have been applied to simulate this cylindrical member subjected to ocean wave loads. The fluid and structural model have been created with appropriate dimensions. ANSA [3] is used as a pre-processing tool for creating the whole computational domain and volume mesh. In order to model the non-linear high amplitude ocean wave, fifth order Stokes wave theory is used in ANSYS Fluent [4]. For the structural model, ANSYS Mechanical (transient structural) is used to determine the dynamic response of a structure under unsteady wave loads. The free surface of the water phase is tracked by using the volume of fluid (VOF) technique in Fluent. The two solvers (ANSYS Fluent & Mechanical) are coupled (exchange of data) using system coupling in ANSYS Workbench. In order to understand the dynamics of a structural member, modal analysis has been conducted to determine the natural frequencies and its respective mode shapes. The differences in fluid forces acting on a structural member for both the ways of coupling have been analyzed. Also, an investigation has been done on the modes of deformation of a structural member in response to wave loads. Numerical results of wave force have been compared to results using Morison equation and theoretical slamming forces. Comparison work between two methods of coupling on combustion system has been taken as one of the references for the assessment of the two approaches [2].

This paper presents a new volume-of-fluid scheme (M-CICSAM), capable of capturing abrupt interfaces on meshes of arbitrary topology, which is a modification to the compressive interface capturing scheme for arbitrary meshes (CICSAM) proposed in the recent literature. Without resort to any explicit interface reconstruction, M-CICSAM is able to precisely model the complex free surface deformation, such as interface rupture and coalescence. By theoretical analysis, it is shown that the modified CICSAM overcomes three inherent drawbacks of the original CICSAM, concerning the basic differencing schemes, the switching strategy between the compressive downwind and diffusive high-resolution schemes, and the far-upwind reconstruction technique on arbitrary unstructured meshes. To evaluate the performance of the newly proposed scheme, several classic interface capturing methods developed in the past decades are compared with M-CICSAM in four test problems. The numerical results clearly demonstrate that M-CICSAM produces more accurate predictions on arbitrary meshes, especially at high Courant numbers, by reducing the numerical diffusion and preserving the interface shape.

This paper introduces and discusses numerical methods for free-surface flow simulations and applies a large eddy simula tion (LES) based free-surface-resolved CFD method to a couple of flows of hydraulic engineering interest. The advantages, dis-advantages and limitations of the various methods are discussed. The review prioritises interface capturing methods over interface tracking methods, as these have shown themselves to be more generally applicable to viscous flows of practical engineering interest, particularly when complex and rapidly changing surface topologies are encountered. Then, a LES solver that employs the level set method to capture free-surface deformation in 3-D flows is presented, as are results from two example calculations that concern complex low submergence turbulent flows over idealised roughness elements and bluff bodies. The results show that the method is capable of predicting very complex flows that are characterised by strong interactions between the bulk flow and the free-surface, and permits the identification of turbulent events and structures that would be very difficult to measure experimentally.

This paper discusses the features and applications of interface tracking techniques for modeling droplet-based microfluidics. The paper reviews the state of the art of methods for tracking and capturing the interface. These methods are categorized as the implicit and the explicit methods. The explicit methods need to reconstruct the interface by recon- necting the markers on the interface. The implicit techniques implicitly describe the interface as a simple function. Thus, complicated topological changes of interface can be handled naturally and automatically. The implicit methods reviewed in this paper are the volume-of-fluid method, the phase-field method, the Lattice-Boltzmann method and the level-set method. The explicit methods mainly include the boundary-integral methods and the tracking methods.

This paper discusses the features and applications of interface tracking techniques for modeling deroplet-based microfluidics. The paper reviews the state of the art of methods for tracking and capturing the interface. These methods are categorized as the implicit and the explicit methods. The implicit methods need to reconstruct the interface by reconnecting the markers on the interface. The implicit techniques implicitly describe the interface as a simple function. Thus, complicated topological changes of interface can be handled naturally and automatically. The implicit methods reviewed in this paper are the volume-offluid method, the phase-field method, the Lattice-Boltzmann method and the level-set method. The explicit methods mainly include the boundary-integral methods and the tracking methods.

The two-phase and multiphase incompressible solvers of the OpenFOAM R CFD package were used to simulate the shape, gentle deposition and low-speed impact of water drops on solid surfaces characterized by means of the contact angle. Smooth flat and curved surfaces and microfinned surfaces were used to benchmark such solvers by comparison with numerically-integrated analytical results, models and experimental results. The simulation were performed on single personal computers to verify what can be obtained with a conventional hardware, without using a cluster. The observed performances are satisfactory even if some problems were encountered.

The situation of two immiscible fluids through a deformable granular material is widely encountered in nature and in many areas of engineering and science. To understand the physical evolution of the multiphase system is of great importance for the applications. It requires the knowledge of all component phases, their distribution and interactions. A pore-scale coupled hydromechanical model is presented in this thesis based on previous work, aiming at simulating the quasi-static drainage of a deformable granular materials. The model combines a pore network approach and the discrete element method (DEM) for the fluids and grains, respectively. A local criterion for determining the local movements of the fluids interfaces established to approximate the role of the local pore geometry on capillarity and namely on the forces exerted on the solid grains inside each pore. Special attentions have been paid to the entrapment events of the receding fluid and to the preferential invasion along the boundaries. The model is validated through comparisons with experimental results (water retention curves). We apply the model for examining two issues: (1) finite size effects and the concept of representative elementary volume (REV); (2) Bishop's effective stress parameter and to the relationship between macro-scale effective stress and micro-scale contact stress. Finally, an extension to the pendular regimes is proposed and first results are presented and analyzed.

The simulation of the flow of fresh grout allows predicting the filling behavior at defined conditions. Hereby, the production process can be optimized. In Mol (Belgium), a field testing site for storage of nuclear waste material has been developed in the past decades. At this location, the Economic Interest Grouping EURIDICE investigates the possibility to store containers with nuclear waste material in horizontal underground tunnels. The space between the tunnel lining and the waste containers could be filled with a cement-based grout. The aim is to create a solid body without voids that can withstand ground settlements. Since the filling process cannot be visually observed a study was carried out in order to predict the filling by simulating the fresh grout flow with computer software. The objective of the study was to determine a proper filling strategy for this project. The experiments were carried out at the Delft University of Technology in The Netherlands. The Nuclear Research & Consultancy Group in Petten, The Netherlands, simulated the filling process with OpenFOAM, a software package that is based on Computational Fluid Dynamics (CFD). The study consisted of three parts: firstly, the rheological characteristics (thixotropy, yield value and plastic viscosity) of the fresh grout were determined, secondly, a parameter-study was carried out on the filling behavior of the grout with a scaled Plexiglas version (maximum length: 5.76 m) of the underground tunnel and finally, simulations were performed and compared to experimental results. This paper describes the case-study and experimental results, and compares the filling behavior determined from experiments and computer simulations using CFD.

The present thesis considers numerical computations of fully nonlinear fluid-structure interaction. The aim of the thesis is to establish a well validated fully nonlinear numerical wave tank for the simulations of complex fluid-structure interaction of moored floating offshore structures. The numerical computations are carried out using the fully nonlinear numerical fluid-structure interaction solvers, interFoam and interDyMFoam, from the open-source CFD libraryOpenFOAM®. These solvers make use of a fully nonlinearNavier-Stokes/VOF solver for the computations of the two-phase flow field, where the interDyMFoam solver also utilises a 6-DOF motion solver to compute the motions of the floating structure. These solvers were extended with waves2Foam, a wave generation and absorption toolbox, developed by Jacobsen et al. (2012). The extended interFoam and interDyMFoam solvers, hereafter referred to as the waveFoam and interDyMFoam solver, were utilised for the computations of fluid-structure with fixed and moving structures respectively. Furthermore, an implementation of catenary mooring lines is provided by Niels Jacobsen (Researcher/advisor at Deltares), for the simulations of the mooring system of the floating structure. Finally, the fully nonlinear potential flow solver, OceanWave3D, was utilised in a fully nonlinear and fully parallelised domain decomposed solver, developed by Paulsen (2013) and Paulsen et al. (2014b), for the efficient computations of realistic sea states. Here, the outer wave field is described by the potential flow solver, whereas the inner wave field, in the vicinity of a given structure, is described by the interDyMFoam solver. The waveFoam and waveDyMFoam solvers are carefully validated either in terms of convergence by grid refinement or by comparisons to experimental measurements. Special attention is paid to the waveDyMFoam solver with respect to the computation of the flow induced motions of a moored floating wind turbine. The ability of the numerical model to accurately reproduce experiments, performed with the generic OC5 floating wind turbine model in the MARIN Concept Basin, is also investigated. The Navier-Stokes/VOF basedwaveFoam andwaveDyMFoam solvers were evaluated with respect to wave propagation and wave structure interaction in a two-dimensional set-up. A grid convergence study was performed on the propagation of a fully nonlinear stream function wave. The convergence rate of the two-phase numerical solution to the single-phase stream function solution was verified. Successful computation of wave loading on a partially submerged fixed horizontal cylinder was provided. The accuracy of the numerical model, with respect to fluid-structure interaction for free and forced motion of a structure, was verified with the generation of surface waves by the forced oscillation and the free heave decay of a horizontal cylinder. These two-dimensional cases were also used to verify the efficiency of two meshing tools, provided by the OpenFOAM® toolbox. These meshing tools were shown to provide excellent results, and more importantly, provided a less complicated method for generating surface boundary meshes of complex three-dimensional structures. The waveDyMFoam solver is used for the computation of free and moored decay test of the three-dimensional floating wind turbine model. The accuracy of the numerical solution was verified against numerical computations from the work of Dunbar et al. (2015) and physical experiments performed at MARIN. Finally, a proof-of-concept case is performed, involving the modelling of a three-dimensional moored floating wind turbine subjected to irregular uni-directional waves. In these numerical computations, the potential of the waveDyMFoam and the domain decomposition strategy are evaluated.

The present paper focuses on the simulation of the expansion and aspherical collapse of a laser-generated bubble subjected to an acceleration field and comparison of the results with instances from high-speed videos. The interaction of the liquid and gas is handled with the volume of fluid method. Compressibility effects have been included for each phase to predict the propagation of pressure waves. Initial conditions were estimated through the Rayleigh Plesset equation, based on the maximum bubble size and collapse time. The simulation predictions indicate that during the expansion the bubble shape is very close to spherical. On the other hand, during the collapse the bubble point closest to the bottom of the container develops a slightly higher collapse velocity than the rest of the bubble surface. Over time, this causes momentum focusing and leads to a positive feedback mechanism that amplifies the collapse locally. At the latest collapse stages, a jet is formed at the axis of symmetry, with opposite direction to the acceleration vector, reaching velocities of even 300 m/s. The simulation results agree with the observed bubble evolution and pattern from the experiments, obtained using high speed imaging, showing the collapse mechanism in great detail and clarity.

The flows in stage-wise liquid-liquid extraction devices include both phase segregated and dispersed flow regimes. As a additional layer of complexity, for extraction equipment such as the annular centrifugal contactor, free-surface flows also play a critical role in both the mixing and separation regions of the device and cannot be neglected. Traditionally, computional fluid dynamics (CFD) of multiphase systems is regime dependent—different methods are used for segregated and dispersed flows. A hybrid multiphase method based on the combination of an Eulerian multifluid solution framework (per-phase momentum equations) and sharp interface capturing using Volume of Fluid (VOF) on selected phase pairs has been developed using the open-source CFD toolkit OpenFOAM. Demonstration of the solver capability is presented through various examples relevant to liquid-liquid extraction device flows including three-phase, liquid-liquid-air simulations in which a sharp interface is maintained between each liquid and air, but dispersed phase modeling is used for the liquid-liquid interactions.

The flow after the rupture of a dam on an inclined plane of arbitrary slope, and the induced transport of non-cohesive sediment, is analysed using the shallow-water approximation. An asymptotic (analytical) solution is presented for the flow hydrodynamics, and compared with the numerical simulation of the dam-break flood. Differences arise due to the appearance of hydrodynamic instabilities (hereafter called roll-waves) in the numerical solution. These roll-waves point out the unstable behaviour of the dam-break wave. It is found that roll-waves enhance the transport of suspended sediment. The limitations of this simple model to predict the transport of sediment in dam-break floods on steep inclines are discussed. Then, a numerical experiment is designed to analyse the unstable character of the dam-break wave, that constitutes a non-parallel and unsteady base flow. By analysing the linear and non-linear numerical evolution of small perturbations, it is possible to reveal how the nature of the ensuing flow depends not only on the Froude number (as it happens in the classical problem of roll-waves over a uniform and steady flow) but also on the non-parallel and time-varying characteristics of the background flow. Consequently, it is also shown that these effects stabilise turbulent roll-waves and raise the critical Froude number required to achieve an unstable flow. This stability result differs with that obtained with a non-parallel spatial stability analysis, pointing out the strong influence of the base-flow time-dependence in the stability criteria. A novel Continuum Mechanics model is presented to study the transport of sediment in a laminar/turbulent free-surface flow. The mixture equations for non-cohesive sediment transport in turbulent free-surface flow are derived from the ensemble averaged Navier-Stokes equations of the three-phases (water, sediment and air). This model avoids the limitations of traditional shallow-water models, and is suitable to study, for instance, the transport of sediment in non-hydrostatic shallow-water flows over bed of arbitrary bottom slopes. The model developed in this work reveals a mathematical equivalence between the propagation of the volumetric concentration of the sediment and the phase function used to capture the free surface. We take advantage of this fact to formulate an explicit Finite Volume Method (FVM) with the exact conservation property, that is implemented in the open source software OpenFOAM. Finally, this model is applied to solve the problem of local scour around a pipeline and the transport of sediment after the rupture of a horizontal dam. It is demonstrated that one-dimensional models based on depth-averaged variables (e.g. generalisations of the one-dimensional Saint-Venant equations to predict mor- phological changes) are superseded by more sophisticated and accurate procedures valid for hyperconcentrated shallow-water flows over bed of arbitrary bottom slopes (e.g. the model described herein).

The dynamics of liquid water in the gas channels with rectangular sections (REC), trapezoidal sections with open angles of 60 degrees (T60), and trapezoidal sections with open angles of 72 degrees (T72) are numerically investigated via the volume of fluid method. The effects of the contact angle of the top and side walls (CATS), the water inlet configuration, and the air inlet velocity are studied based on the temporal evolution of gas-liquid interface, the water volume fraction (WVF), the water coverage ratio of the gas diffusion layer (GDL) surface (GWCR), and the pressure drop between the air inlet and the outlet. For the hydrophobic GDL surface and the hydrophilic top and side walls, the T72 provides the lowest WVF and GWCR of around 7 percent due to periodic pressure spikes. The REC and T60 show a higher WVF and a lower GWCR as most of liquid water moves along the channel while attached to the top wall. As the CATS increases from 60 to 120 degrees, the behaviors of liquid water become similar for the three cross-sectional shapes. The T72 shows especially similar results irrespective of the CATS. When the liquid water emergence is concentrated along the side wall, the T72 shows the best water removal characteristics. For all the three channel cross-sectional shapes, water slugs move faster and have smaller sizes as the air inlet velocity increases.

The current study investigates the dynamics of a two phase flow in a circular cylinder under zero gravity conditions. Zero gravity fluid mechanics is applicable in both spaceflight and terrestrial applications. Plug formation obstruct lung passages and is the cause of asthma and pulmonary disease .In spaceflight applications, the helium bubbles occlude fuel lines of hydrazine arc-jet thrusters on satellites which cause operational problems. The process of plug formation is not fully understood. Previous studies have been limited to static solutions in cylinders. The goal of the current study is to investigate the dynamics of two phase flow under zero gravity conditions and low bond numbers. The specific objectives of this study are to model a two-phase flow into a circular cylinder via a side tube in zero gravity and confirm the existence of droplet, annulus, and plug topologies through experiment and simulation. Interim stages between topologies and steps of transition are analyzed in terms of flow quantities such as flow rate and kinetic energy. The effect of varying inlet pressure on topology formation is also determined and investigated. An experiment was conducted to determine the dynamics of silicone oil-air interface in a cylinder under low bond number conditions using a high speed camera to capture the fluid. Numerical simulations were carried out for both static and dynamic conditions. Results confirmed the existence of droplet, annulus and plug for the dynamic system. A characteristic time where plug is formed is identified and has a value of approximately 13. An instability regime between the annulus and plug was identified which gets narrower for higher pressure differences. Stable plug solutions are delayed for lower pressure differences. Stable plug solutions in dynamic systems were found to be higher than those in static systems under the same conditions.