Gradient-augmented hybrid interface capturing method for incompressible two-phase flow

Motivated by inconveniences of present hybrid methods,a gradient-augmented hybrid interface capturing method（GAHM） is presented for incompressible two-phase flow.A front tracking method（FTM） is used as the skeleton of the GAHM for low mass loss and resources.Smooth eulerian level set values are calculated from the FTM interface,and are used for a local interface reconstruction.The reconstruction avoids marker particle redistribution and enables an automatic treatment of interfacial topology change.The cubic Hermit interpolation is employed in all steps of the GAHM to capture subgrid structures within a single spacial cell.The performance of the GAHM is carefully evaluated in a benchmark test.Results show significant improvements of mass loss,clear subgrid structures,highly accurate derivatives（normals and curvatures） and low cost.The GAHM is further coupled with an incompressible multiphase flow solver,Super CE/SE,for more complex and practical applications.The updated solver is evaluated through comparison with an early droplet research.

Accurate Determination for the Energy & Temperature Dependence of Electron Capture CrossSection of Si-SiO_2 Interface States Using a New Method

A new technique for accurate determination of the electron and hole capture cross-sections of interface states at the insulator-semiconductor interface has been developed through measuring the initial time variation in the carrier filling capacitance transient, and full consideration is given to the charge-potential feedback effect on carrier capture process. A simplified calculation of the effect is also given. The interface states have been investigated with this technique at the Si-SiO_2 interface in an n-type Si MOS diode. The results show that the electron capture cross-section strongly depends on both temperature and energy.

A High Order Sharp-Interface Method with Local Time Stepping for Compressible Multiphase Flows

In this paper,a new sharp-interface approach to simulate compressible multiphase flows is proposed.The new scheme consists of a high order WENO finite volume scheme for solving the Euler equations coupled with a high order pathconservative discontinuous Galerkin finite element scheme to evolve an indicator function that tracks the material interface.At the interface our method applies ghost cells to compute the numerical flux,as the ghost fluid method.However,unlike the original ghost fluid scheme of Fedkiw et al.[15],the state of the ghost fluid is derived from an approximate-state Riemann solver,similar to the approach proposed in[25],but based on a much simpler formulation.Our formulation leads only to one single scalar nonlinear algebraic equation that has to be solved at the interface,instead of the system used in[25].Away from the interface,we use the new general Osher-type flux recently proposed by Dumbser and Toro[13],which is a simple but complete Riemann solver,applicable to general hyperbolic conservation laws.The time integration is performed using a fully-discrete one-step scheme,based on the approaches recently proposed in[5,7].This allows us to evolve the system also with time-accurate local time stepping.Due to the sub-cell resolution and the subsequent more restrictive time-step constraint of the DG scheme,a local evolution for the indicator function is applied,which is matched with the finite volume scheme for the solution of the Euler equations that runs with a larger time step.The use of a locally optimal time step avoids the introduction of excessive numerical diffusion in the finite volume scheme.Two different fluids have been used,namely an ideal gas and a weakly compressible fluid modeled by the Tait equation.Several tests have been computed to assess the accuracy and the performance of the new high order scheme.A verification of our algorithm has been carefully carried out using exact solutions as well as a comparison with other numerical reference solutions.The material interface is resolved sharply and accurately without spurious oscillations in the pressure field.

Capturing Moving Interfaces by LevelSet Method with Ghost Technique

SMAC method is adopted to solve Navier-Stokes equations, by using fifth-order WENO scheme and TVD R-K scheme, LevelSet methods are used to capture moving interfaces with improved Ghost techniques. Some examples, e.g. the incompressible inviscid flame interfaces of one and two dimensions and the rising-merging of oil bubbles in water, are computed and the comparison with the result of commercial CFD software Fluent has been done. The rationality of LevelSet methods with Ghost technique in capturing moving interfaces with jump conditions is affirmed. It is shown that Level Set method can capture interfaces sharply without complicated reconstruction and can be easily programmed.

Phase-Field Models for Multi-Component Fluid Flows

In this paper,we review the recent development of phase-field models and their numerical methods for multi-component fluid flows with interfacial phenomena.The models consist of a Navier-Stokes system coupled with a multi-component Cahn-Hilliard system through a phase-field dependent surface tension force,variable density and viscosity,and the advection term.The classical infinitely thin boundary of separation between two immiscible fluids is replaced by a transition region of a small but finite width,across which the composition of the mixture changes continuously.A constant level set of the phase-field is used to capture the interface between two immiscible fluids.Phase-field methods are capable of computing topological changes such as splitting andmerging,and thus have been applied successfully to multi-component fluid flows involving large interface deformations.Practical applications are provided to illustrate the usefulness of using a phase-field method.Computational results of various experiments show the accuracy and effectiveness of phase-field models.

Numerical simulation of 3-D water collapse with an obstacle by FEM-level set method

An interface capturing approach based on a level set function for simulating transient two-phase viscous incompressible flows is applied in this paper. A narrow-band signed distance function is adopted to indicate the phase fields and the interface. The multiphase flow is numerically solved by three stages with finite element method （FEM）： （1） solving a two-fluid Navier-Stokes （N-S） equations over the whole domain, （2） transporting the level set function with the obtained velocity field, （3） the level set function correction through a renormalization with continuous penalization which preserves the thickness of the interface. In this paper, the 3-D water colunm collapse with an obstacle is simulated, which yielded good agreement with the experimental data.

ASSESSMENT OF A CENTRAL DIFFERENCE FINITE VOLUME SCHEME FOR MODELING OF CAVITATING FLOWS USING PRECONDITIONED MULTIPHASE EULER EQUATIONS

A numerical treatment for the prediction of cavitating flows is presented and assessed. The algorithm uses the preconditioned multiphase Euler equations with appropriate mass transfer terms. A central difference finite volume scheme with suitable dissipation terms to account for density jumps across the cavity interface is shown to yield an effective method for solving the multiphase Euler equations. The Euler equations are utilized herein for the cavitation modeling, because some certain characteristics of cavitating flows can be obtained using the solution of this system of equations with relative low computational effort. In addition, the Euler equations are appropriate for the assessment of the numerical method used, because of the sensitivity of the solution to the numerical instabilities. For this reason, a sensitivity study is conducted to evaluate the effects of various parameters, such as numerical dissipation coefficients and grid size, on the accuracy and performance of the solution. The computations are performed for steady cavitating flows around the NACA 0012 and NACA 66 （MOD） hydrofoils and also an axisymmetric hemispherical fore-body under different conditions and the results are compared with the available numerical and experimental data. The solution procedure presented is shown to be accurate and efficient for predicting steady sheet- and super-cavitation for 2D/axisymmetric geometries.

An Improved Gas-Kinetic Scheme for Multimaterial Flows

The efficiency of recently developed gas-kinetic scheme for multimaterial flows is increased through the adoption of a new iteration method in the kinetic non-mixing Riemann solver and an interface sharpening reconstruction method at a cell interface.The iteration method is used to determine the velocity of fluid interface,based on the force balance between both sides due to the incidence and bounce back of particles at the interface.An improved Aitken method is proposed with a simple hybrid of the modified Aitken method(Aitken-Chen)and the Steffensen method.Numerical tests validate its efficiency with significantly less calls to the function not only for the average number but also for the maximum.The new reconstruction is based on the tangent of hyperbola for interface capturing(THINC)but applied only to the volume fraction,which is very simple to be implemented under the stratified frame-work and capable of resolving fluid interface in mixture.Furthermore,the directional splitting is adopted rather than the previous quasi-one-dimensional method.Typical numerical tests,including several watergas shock tube flows,and the shock-water cylinder interaction flow show that the improved gas-kinetic scheme can capture fluid interfaces much sharper,while preserving the advantages of the original one.

Development of a hybrid photo-bioreactor and nanoparticle adsorbent system for the removal of CO2,and selected organic and metal co-pollutants

Fossil fuel combustion and many industrial processes generate gaseous emissions that contain a number of toxic organic pollutants and carbon dioxide（CO_2） which contribute to climate change and atmospheric pollution.There is a need for green and sustainable solutions to remove air pollutants,as opposed to conventional techniques which can be expensive,consume additional energy and generate further waste.We developed a novel integrated bioreactor combined with recyclable iron oxide nano/micro-particle adsorption interfaces,to remove CO_2,and undesired organic air pollutants using natural particles,while generating oxygen.This semi-continuous bench-scale photo-bioreactor was shown to successfully clean up simulated emission streams of up to 45% CO_2 with a conversion rate of approximately 4%CO_2 per hour,generating a steady supply of oxygen（6 mmol/hr）,while nanoparticles effectively remove several undesired organic by-products.We also showed algal waste of the bioreactor can be used for mercury remediation.We estimated the potential CO_2 emissions that could be captured from our new method for three industrial cases in which,coal,oil and natural gas were used.With a 30% carbon capture system,the reduction of CO_2 was estimated to decrease by about 420,000,320,000 and 240,000 metric tonnes,respectively for a typical 500 MW power plant.The cost analysis we conducted showed potential to scale-up,and the entire system is recyclable and sustainable.We further discuss the implications of usage of this complete system,or as individual units,that could provide a hybrid option to existing industrial setups.