Yongguang Cheng - Academia.edu (original) (raw)

Papers by Yongguang Cheng

Research paper thumbnail of Improving the Stability of the Multiple-Relaxation-Time Lattice Boltzmann Method by a Viscosity Counteracting Approach

Advances in Applied Mathematics and Mechanics, 2015

Research paper thumbnail of Looping dynamic characteristics of a pump-turbine in the S-shaped region during runaway

Journal of Fluids Engineering, 2016

Research paper thumbnail of Three-Dimensional Simulation of Balloon Dynamics by the Immersed Boundary Method Coupled to the Multiple-Relaxation-Time Lattice Boltzmann Method

Communications in Computational Physics, 2015

ABSTRACT

Research paper thumbnail of Numerical analysis of rotating stall instabilities of a pump- turbine in pump mode

IOP Conference Series Earth and Environmental Science

Rotating stall may occur at part load flow of a pump-turbine in pump mode. Unstable flow structur... more Rotating stall may occur at part load flow of a pump-turbine in pump mode. Unstable flow structures developing under stall condition can lead to a sudden drop of efficiency, high dynamic load and even cavitation. CFD simulations on a pump-turbine model in pump mode were carried out to reveal the onset and developed mechanisms of these unstable flow phenomena at part load. The simulation results of energy-discharge and efficiency characteristics are in good agreement with those obtained by experiments. The more deviate from design conditions with decreasing flow rate, the more flow separations within the vanes. Under specific conditions, four stationary separation zones begin to progress on the circumference, rotating at a fraction of the impeller rotation rate. Rotating stalls lead to the flow in the vane diffuser channels alternating between outward jet flow and blockage. Strong jets impact the spiral casing wall causing high pressure pulsations. Severe separations of the stall cells disturb the flow inducing periodical large amplitude pressure fluctuations, of which the intensity at different span wise of the guide vanes is different. The enforced rotating nonuniform pressure distributions on the circumference lead to dynamic uniform forces on the impeller and guide vanes. The results show that the CFD simulations are capable to gain the complicated flow structure information for analysing the unstable characteristics of the pump mode at part load.

Research paper thumbnail of Three-dimensional prediction of reservoir water temperature by the lattice Boltzmann method: Validation

Journal of Hydrodynamics

The water temperature stratification in large reservoirs might have serious ecological and enviro... more The water temperature stratification in large reservoirs might have serious ecological and environmental consequences. The modeling of the temperature distribution and its history is of great importance both for studying the underlying mechanisms and for controlling the adverse effects. To develop an effective and efficient method for simulation of temporal and spatial temperature variations, a lattice Boltzmann method (LBM) model for 3-D thermal buoyancy flows is proposed and validated by the temperature data measured in a model reservoir. This paper discusses important aspects of the LBM and its turbulence model, analyzes the gravity sinking mechanism of cold currents, and demonstrates the complexity of the temperature redistribution process. Good agreement between the simulated and measured results shows that the newly developed method is feasible and powerful, and it will be used for the water temperature prediction in actual reservoirs in a near future.

Research paper thumbnail of Formulae for the intersecting curves of pump-turbine characteristic curves with coordinate planes in three-dimensional parameter space

Proceedings of the Institution of Mechanical Engineers Part A Journal of Power and Energy

Pump-turbine characteristics are important boundary conditions for simulating hydraulic transient... more Pump-turbine characteristics are important boundary conditions for simulating hydraulic transients in pumped-storage hydroelectric power stations. However, the changing laws of the characteristics of different pump turbines are not well understood. Here, two-dimensional characteristic curves are converted into three-dimensional forms, and the intersecting curves, defined by the intersection of the characteristic curves with the coordinate planes in a parameter space defined by the unit rotational speed, unit discharge, and unit torque, are determined to clarify these changing laws. Basic pump-turbine theory of the flow characteristics and idealizations of the “hump” and “S” regions of the characteristic curves are considered to determine formulae for each intersecting curve. Each formula consists of two unknown coefficients that are obtained from fits to measured data. The dependence of the coefficients on specific speed values is clarified to obtain general formulae governed by spe...

Research paper thumbnail of Simulation of the load rejection transient process of a francis turbine by using a 1-D-3-D coupling approach

Journal of Hydrodynamics

This paper presents the simulation and the analysis of the transient process of a Francis turbine... more This paper presents the simulation and the analysis of the transient process of a Francis turbine during the load rejection by employing a one-dimensional and three-dimensional (1-D-3-D) coupling approach. The coupling is realized by partly overlapping the 1-D and 3-D parts, the water hammer wave is modeled by defining the pressure dependent density, and the guide vane closure is treated by a dynamic mesh method. To verify the results of the coupling approach, the transient parameters for both typical models and a real power station are compared with the data obtained by the 1-D approach, and good agreements are found. To investigate the differences between the transient and steady states at the corresponding operating parameters, the flow characteristics inside a turbine of the real power station are simulated by both transient and steady methods, and the results are analyzed in details. Our analysis suggests that there are just a little differences in the turbine outer characteris...

Research paper thumbnail of Dynamic characteristics of a pump-turbine during hydraulic transients of a model pumped-storage system

IOP Conference Series Earth and Environmental Science

The runaway process in a model pumped-storage system was simulated for analyzing the dynamic char... more The runaway process in a model pumped-storage system was simulated for analyzing the dynamic characteristics of a pump-turbine. The simulation was adopted by coupling 1D (One Dimensional) pipeline MOC (Method of Characteristics) equations with a 3D (Three Dimensional) pump-turbine CFD (Computational Fluid Dynamics) model, in which the water hammer wave in the 3D zone was defined by giving a pressure dependent density.

Research paper thumbnail of Accuracy improvement of the immersed boundary–lattice Boltzmann coupling scheme by iterative force correction

Computers & Fluids, 2015

Immersed boundary method Lattice Boltzmann method Fluid-structure interaction Non-slip boundary c... more Immersed boundary method Lattice Boltzmann method Fluid-structure interaction Non-slip boundary condition External forcing term Mechanical heart valves Iterative method a b s t r a c t

Research paper thumbnail of Construction of Pump-Turbine Characteristics at Any Specific Speed by Domain-Partitioned Transformation

Journal of Fluids Engineering, 2014

Pump-turbine characteristics are important for designing pumped-storage plants and indispensable ... more Pump-turbine characteristics are important for designing pumped-storage plants and indispensable for simulating hydraulic transients, but are often not available in the preliminary design stage. Therefore, constructing a set of pump-turbine characteristics is necessary, when no suitable characteristics at the same specific speed can be used for substitution. In this paper, we propose a new method for pump-turbine characterization at any specific speed using a database of 25 available sets of pump-turbine characteristics. The intersecting curves, defined by the intersections of the characteristic curves with a coordinate axis, are formularized to prepare for the characterization primarily. Next is an introduction of a transformation method for characteristic curves base on domain partition, through which the curves are transformed into eight characteristic surface meshes in eight separate domains. Then, we present the construction procedures in each domain, which include merging the transformed surface meshes for all the sets of collected characteristic curves into a cube mesh, constructing a super surface by interpolation to construct the regular characteristic surface meshes for an arbitrary specific speed, and transforming the constructed meshes reversely to get the conventional characteristic curves. This method is verified by comparing them to measured characteristic curves with reasonable accuracies.

Research paper thumbnail of Immersed Boundary-Lattice Boltzmann Coupling Scheme for Fluid-Structure Interaction with Flexible Boundary

Communications in Computational Physics, 2011

Coupling the immersed boundary (IB) method and the lattice Boltzmann (LB) method might be a promi... more Coupling the immersed boundary (IB) method and the lattice Boltzmann (LB) method might be a promising approach to simulate fluid-structure interaction (FSI) problems with flexible structures and complex boundaries, because the former is a general simulation method for FSIs in biological systems, the latter is an efficient scheme for fluid flow simulations, and both of them work on regular Cartesian grids. In this paper an IB-LB coupling scheme is proposed and its feasibility is verified. The scheme is suitable for FSI problems concerning rapid flexible boundary motion and a large pressure gradient across the boundary. We first analyze the respective concepts, formulae and advantages of the IB and LB methods, and then explain the coupling strategy and detailed implementation procedures. To verify the effectiveness and accuracy, FSI problems arising from the relaxation of a distorted balloon immersed in a viscous fluid, an unsteady wake flow caused by an impulsively started circular cylinder at Reynolds number 9500, and an unsteady vortex shedding flow past a suddenly started rotating circular cylinder at Reynolds number 1000 are simulated. The first example is a benchmark case for flexible boundary FSI with a large pressure gradient across the boundary, the second is a fixed complex boundary problem, and the third is a typical moving boundary example. The results are in good agreement with the analytical and existing numerical data. It is shown that the proposed scheme is capable of modeling flexible boundary and complex boundary problems at a second-order spatial convergence; the volume leakage defect of the conventional IB method has been remedied by using a new method of introducing the unsteady and non-uniform external force; and the LB method makes the IB method simulation simpler and more efficient.

Research paper thumbnail of New boundary treatment methods for lattice Boltzmann method

In practical fluid dynamic simulations, the boundary condition should be treated carefully becaus... more In practical fluid dynamic simulations, the boundary condition should be treated carefully because it always has crucial influence on the numerical accuracy, stability and efficiency. Two types of boundary treatment methods for lattice Boltzmann method (LBM) are proposed. One is for the treatment of boundaries situated at lattice nodes, and the other is for the approximation of boundaries that are not located at the regular lattice nodes. The first type of boundary treatment method can deal with various dynamic boundaries on complex geometries by using a general set of formulas, which can maintain second-order accuracy. Based on the fact that the fluid flows simulated by LBM are not far from equilibrium, the unknown distributions at a boundary node are expressed as the analogous forms of their corresponding equilibrium distributions. Therefore, the number of unknowns can be reduced and an alwaysclosed set of equations can be obtained for the solutions to pressure, velocity and special boundary conditions on various geometries. The second type of boundary treatment is a complete interpolation scheme to treat curved boundaries. It comes from careful analysis of the relations between distribution functions at boundary nodes and their neighboring lattice nodes. It is stable for all situations and of second-order accuracy. Basic ideas, implementation procedures and verifications with typical examples for the both treatments are presented. Numerical simulations and analyses show that they are accurate, stable, general and efficient for practical simulations.

Research paper thumbnail of Simulation of hydraulic transients in hydropower systems using the 1-D-3-D coupling approach

Journal of Hydrodynamics, Ser. B, 2012

Although the hydraulic transients in pipe systems are usually simulated by using a one-dimensiona... more Although the hydraulic transients in pipe systems are usually simulated by using a one-dimensional (1-D) approach, local three-dimensional (3-D) simulations are necessary because of obvious 3-D flow features in some local regions of the hydropower systems. This paper combines the 1-D method with a 3-D fluid flow model to simulate the Multi-Dimensional (MD) hydraulic transients in hydropower systems and proposes two methods for modeling the compressible water with the correct wave speed, and two strategies for efficiently coupling the 1-D and 3-D computational domains. The methods are validated by simulating the water hammer waves and the oscillations of the water level in a surge tank, and comparing the results with the 1-D solution data. An MD study is conducted for the transient flows in a realistic water conveying system that consists of a draft tube, a tailrace surge tank and a tailrace tunnel. It is shown that the 1-D-3-D coupling approach is an efficient and promising way to simulate the hydraulic transients in the hydropower systems in which the interactions between 1-D hydraulic fluctuations of the pipeline systems and the local 3-D flow patterns should be considered.

Research paper thumbnail of 3-D simulation of transient flow patterns in a corridor-shaped air-cushion surge chamber based on computational fluid dynamics

Journal of Hydrodynamics, Ser. B, 2013

The 3-D characteristics of the water-air flow patterns in a corridor-shaped air-cushion surge cha... more The 3-D characteristics of the water-air flow patterns in a corridor-shaped air-cushion surge chamber during hydraulic transients need to be considered in the shape optimization. To verify the reliability of the water-air two-phase model, namely, the volume of fluid model, the process of charging water into a closed air chamber is successfully simulated. Using the model, the 3-D flow characteristics under the load rejection and acceptance conditions within the air-cushion surge chamber of a specific hydropower station are studied. The free surface waves, the flow patterns, and the pressure changes during the surge wave process are analyzed in detail. The longitudinal flow of water in the long corridor-shaped surge chamber is similar to the open channel flow with respect to the wave propagation, reflection and superposition characteristics. The lumped parameters of the 3-D numerical simulation agree with the results of a 1-D calculation of hydraulic transients in the whole water conveying system, which validates the 3-D method. The 3-D flow structures obtained can be applied to the shape optimization of the chamber.

Research paper thumbnail of Free surface-pressurized flow in ceiling-sloping tailrace tunnel of hydropower plant: simulation by VOF model

Journal of Hydraulic Research, 2007

Free surface-pressurized flow in ceiling-sloping tailrace tunnel of hydropower plant: simulation ... more Free surface-pressurized flow in ceiling-sloping tailrace tunnel of hydropower plant: simulation by VOF model Écoulement à surface libre pressurisé dans la galerie d'évacuation à plafond-en pente d'une usine hydroélectrique: simulation par un modèle VOF

Research paper thumbnail of Introducing unsteady non-uniform source terms into the lattice Boltzmann model

International Journal for Numerical Methods in Fluids, 2008

Taking body forces into account is not new for the lattice Boltzmann method, yet most of the exis... more Taking body forces into account is not new for the lattice Boltzmann method, yet most of the existing approaches can only treat steady and uniform body forces. To manage situations with time-and spacedependent body forces or source terms, this paper proposes a new approach through theoretical derivation and numerical verification. The method by attaching an extra term to the lattice Boltzmann equation is still used, but the expression of the extra term is modified. It is the modified extra term that achieves the particularity of the new approach. This approach can not only introduce unsteady and non-uniform body forces into momentum equations, but is also able to add an arbitrary source term to the continuity equation. Both the macroscopic equations from multi-scale analysis and the simulated results of typical examples show that the accuracy with second-order convergence can be guaranteed within incompressible limit. 630 Y. CHENG AND J. LI hydraulic problems [1-10]. The rapid increase in the number of published works on LBM in recent years boosts the perspective of practical applications.

Research paper thumbnail of A viscosity counteracting approach in the lattice Boltzmann BGK model for low viscosity flow: Preliminary verification

Computers & Mathematics with Applications, 2011

Due to numerical instability, the lattice Boltzmann model (LBM) with the Bhatnagar-Gross-Krook (B... more Due to numerical instability, the lattice Boltzmann model (LBM) with the Bhatnagar-Gross-Krook (BGK) collision operator has some limitations in the simulation of low viscosity flows. In this paper, we propose a viscosity counteracting approach for simulating a moderate viscosity flow. An extra negative viscosity term is introduced to counteract part of the moderate viscosity by using the lattice Boltzmann equation with a source term. The counteracting viscosity term is treated as a non-uniform unsteady source. The stability is enhanced; thus small viscosity flows can be simulated. Model verification consists of benchmark cases such as those of Poiseuille flow, Couette flow, waterhammer waves, Taylor-Green vortex flow, and lid-driven cavity flow. The flow patterns, error characteristics, and representative parameters are carefully analyzed. It is shown that this approach can simulate flows with lower viscosities than may be simulated using the normal LBGK model; the second-order accuracy of the LBGK model is definitely retained, although a little dissipation is added. These preliminary studies prove the effectiveness and accuracy of the model. Sophisticated analysis and further verification of the stability mechanism will be done in the near future.

Research paper thumbnail of Immersed boundary method and lattice Boltzmann method coupled FSI simulation of mitral leaflet flow

Computers & Fluids, 2010

Coupling the immersed boundary (IB) method and the lattice Boltzmann (LB) method might be a promi... more Coupling the immersed boundary (IB) method and the lattice Boltzmann (LB) method might be a promising approach to simulate fluid-structure interaction (FSI) problems with flexible structures and moving boundaries. To investigate the possibility for future IB-LB coupled simulations of the heart flow dynamics, an IB-LB coupling scheme suitable for rapid boundary motion and large pressure gradient FSI is proposed, and the mitral valve jet flow considering the interaction of leaflets and fluid is simulated. After analyzing the respective concepts, formulae and advantages of the IB and LB methods, we first explain the coupling strategy and detailed implementation procedures, and then verify the effectiveness and second-order accuracy of the scheme by simulating a benchmark case, the relaxation of a stretched membrane immersed in fluid. After that, the diastolic filling jet flow between mitral leaflets in a simplified 2D left heart model is simulated. The model consists of the simplified transmitral passage of the heart and two curvilinear leaflets. In the simulation, the atrial and ventricular pressure histories of normal human are specified as boundary conditions, and the leaflets are treated as fibers that interact with the fluid to define their deformations and movements. The resulting opening and closing movements of the leaflets and the flow patterns of the filling jet are qualitatively reasonable and compare well with existing numerical and measured data. It is shown that this IB-LB coupling method is feasible for treating flexible boundary FSI problems with rapid boundary motion and large pressure gradient, the results of the mitral leaflet flow are valuable for understanding the transmitral FSI dynamics, and it is possible to simulate the more realistic 3D heart flow by the scheme in the future.

Research paper thumbnail of Fluid-Structure Coupled CFD Simulation of the Left Ventricular Flow During Filling Phase

Annals of Biomedical Engineering, 2005

The fluid-structure coupled simulation of the heart, though at its developing stage, has shown gr... more The fluid-structure coupled simulation of the heart, though at its developing stage, has shown great prospect in heart function investigations and clinical applications. The purpose of this paper is to verify a commercial software based fluid-structure interaction scheme for the left ventricular filling. The scheme applies the finite volume method to discretize the arbitrary Lagrangian–Eulerian formulation of the Navier–Stokes equations for the fluid while using the nonlinear finite element method to model the structure. The coupling of the fluid and structure is implemented by combining the fluid and structure equations as a unified system and solving it simultaneously at every time step. The left ventricular filling flow in a three-dimensional ellipsoidal thin-wall model geometry of the human heart is simulated, based on a prescribed time-varying Young’s modulus. The coupling converges smoothly though the deformation is very large. The pressure–volume relation of the model ventricle, the spatial and temporal distributions of pressure, transient velocity vectors as well as vortex patterns are analyzed, and they agree qualitatively and quantitatively well with the existing data. This preliminary study has verified the feasibility of the scheme and shown the possibility to simulate the left ventricular flow in a more realistic way by adding a myocardial constitutive law into the model and using a more realistic heart geometry.

Research paper thumbnail of Improving the Stability of the Multiple-Relaxation-Time Lattice Boltzmann Method by a Viscosity Counteracting Approach

Advances in Applied Mathematics and Mechanics, 2015

Research paper thumbnail of Looping dynamic characteristics of a pump-turbine in the S-shaped region during runaway

Journal of Fluids Engineering, 2016

Research paper thumbnail of Three-Dimensional Simulation of Balloon Dynamics by the Immersed Boundary Method Coupled to the Multiple-Relaxation-Time Lattice Boltzmann Method

Communications in Computational Physics, 2015

ABSTRACT

Research paper thumbnail of Numerical analysis of rotating stall instabilities of a pump- turbine in pump mode

IOP Conference Series Earth and Environmental Science

Rotating stall may occur at part load flow of a pump-turbine in pump mode. Unstable flow structur... more Rotating stall may occur at part load flow of a pump-turbine in pump mode. Unstable flow structures developing under stall condition can lead to a sudden drop of efficiency, high dynamic load and even cavitation. CFD simulations on a pump-turbine model in pump mode were carried out to reveal the onset and developed mechanisms of these unstable flow phenomena at part load. The simulation results of energy-discharge and efficiency characteristics are in good agreement with those obtained by experiments. The more deviate from design conditions with decreasing flow rate, the more flow separations within the vanes. Under specific conditions, four stationary separation zones begin to progress on the circumference, rotating at a fraction of the impeller rotation rate. Rotating stalls lead to the flow in the vane diffuser channels alternating between outward jet flow and blockage. Strong jets impact the spiral casing wall causing high pressure pulsations. Severe separations of the stall cells disturb the flow inducing periodical large amplitude pressure fluctuations, of which the intensity at different span wise of the guide vanes is different. The enforced rotating nonuniform pressure distributions on the circumference lead to dynamic uniform forces on the impeller and guide vanes. The results show that the CFD simulations are capable to gain the complicated flow structure information for analysing the unstable characteristics of the pump mode at part load.

Research paper thumbnail of Three-dimensional prediction of reservoir water temperature by the lattice Boltzmann method: Validation

Journal of Hydrodynamics

The water temperature stratification in large reservoirs might have serious ecological and enviro... more The water temperature stratification in large reservoirs might have serious ecological and environmental consequences. The modeling of the temperature distribution and its history is of great importance both for studying the underlying mechanisms and for controlling the adverse effects. To develop an effective and efficient method for simulation of temporal and spatial temperature variations, a lattice Boltzmann method (LBM) model for 3-D thermal buoyancy flows is proposed and validated by the temperature data measured in a model reservoir. This paper discusses important aspects of the LBM and its turbulence model, analyzes the gravity sinking mechanism of cold currents, and demonstrates the complexity of the temperature redistribution process. Good agreement between the simulated and measured results shows that the newly developed method is feasible and powerful, and it will be used for the water temperature prediction in actual reservoirs in a near future.

Research paper thumbnail of Formulae for the intersecting curves of pump-turbine characteristic curves with coordinate planes in three-dimensional parameter space

Proceedings of the Institution of Mechanical Engineers Part A Journal of Power and Energy

Pump-turbine characteristics are important boundary conditions for simulating hydraulic transient... more Pump-turbine characteristics are important boundary conditions for simulating hydraulic transients in pumped-storage hydroelectric power stations. However, the changing laws of the characteristics of different pump turbines are not well understood. Here, two-dimensional characteristic curves are converted into three-dimensional forms, and the intersecting curves, defined by the intersection of the characteristic curves with the coordinate planes in a parameter space defined by the unit rotational speed, unit discharge, and unit torque, are determined to clarify these changing laws. Basic pump-turbine theory of the flow characteristics and idealizations of the “hump” and “S” regions of the characteristic curves are considered to determine formulae for each intersecting curve. Each formula consists of two unknown coefficients that are obtained from fits to measured data. The dependence of the coefficients on specific speed values is clarified to obtain general formulae governed by spe...

Research paper thumbnail of Simulation of the load rejection transient process of a francis turbine by using a 1-D-3-D coupling approach

Journal of Hydrodynamics

This paper presents the simulation and the analysis of the transient process of a Francis turbine... more This paper presents the simulation and the analysis of the transient process of a Francis turbine during the load rejection by employing a one-dimensional and three-dimensional (1-D-3-D) coupling approach. The coupling is realized by partly overlapping the 1-D and 3-D parts, the water hammer wave is modeled by defining the pressure dependent density, and the guide vane closure is treated by a dynamic mesh method. To verify the results of the coupling approach, the transient parameters for both typical models and a real power station are compared with the data obtained by the 1-D approach, and good agreements are found. To investigate the differences between the transient and steady states at the corresponding operating parameters, the flow characteristics inside a turbine of the real power station are simulated by both transient and steady methods, and the results are analyzed in details. Our analysis suggests that there are just a little differences in the turbine outer characteris...

Research paper thumbnail of Dynamic characteristics of a pump-turbine during hydraulic transients of a model pumped-storage system

IOP Conference Series Earth and Environmental Science

The runaway process in a model pumped-storage system was simulated for analyzing the dynamic char... more The runaway process in a model pumped-storage system was simulated for analyzing the dynamic characteristics of a pump-turbine. The simulation was adopted by coupling 1D (One Dimensional) pipeline MOC (Method of Characteristics) equations with a 3D (Three Dimensional) pump-turbine CFD (Computational Fluid Dynamics) model, in which the water hammer wave in the 3D zone was defined by giving a pressure dependent density.

Research paper thumbnail of Accuracy improvement of the immersed boundary–lattice Boltzmann coupling scheme by iterative force correction

Computers & Fluids, 2015

Immersed boundary method Lattice Boltzmann method Fluid-structure interaction Non-slip boundary c... more Immersed boundary method Lattice Boltzmann method Fluid-structure interaction Non-slip boundary condition External forcing term Mechanical heart valves Iterative method a b s t r a c t

Research paper thumbnail of Construction of Pump-Turbine Characteristics at Any Specific Speed by Domain-Partitioned Transformation

Journal of Fluids Engineering, 2014

Pump-turbine characteristics are important for designing pumped-storage plants and indispensable ... more Pump-turbine characteristics are important for designing pumped-storage plants and indispensable for simulating hydraulic transients, but are often not available in the preliminary design stage. Therefore, constructing a set of pump-turbine characteristics is necessary, when no suitable characteristics at the same specific speed can be used for substitution. In this paper, we propose a new method for pump-turbine characterization at any specific speed using a database of 25 available sets of pump-turbine characteristics. The intersecting curves, defined by the intersections of the characteristic curves with a coordinate axis, are formularized to prepare for the characterization primarily. Next is an introduction of a transformation method for characteristic curves base on domain partition, through which the curves are transformed into eight characteristic surface meshes in eight separate domains. Then, we present the construction procedures in each domain, which include merging the transformed surface meshes for all the sets of collected characteristic curves into a cube mesh, constructing a super surface by interpolation to construct the regular characteristic surface meshes for an arbitrary specific speed, and transforming the constructed meshes reversely to get the conventional characteristic curves. This method is verified by comparing them to measured characteristic curves with reasonable accuracies.

Research paper thumbnail of Immersed Boundary-Lattice Boltzmann Coupling Scheme for Fluid-Structure Interaction with Flexible Boundary

Communications in Computational Physics, 2011

Coupling the immersed boundary (IB) method and the lattice Boltzmann (LB) method might be a promi... more Coupling the immersed boundary (IB) method and the lattice Boltzmann (LB) method might be a promising approach to simulate fluid-structure interaction (FSI) problems with flexible structures and complex boundaries, because the former is a general simulation method for FSIs in biological systems, the latter is an efficient scheme for fluid flow simulations, and both of them work on regular Cartesian grids. In this paper an IB-LB coupling scheme is proposed and its feasibility is verified. The scheme is suitable for FSI problems concerning rapid flexible boundary motion and a large pressure gradient across the boundary. We first analyze the respective concepts, formulae and advantages of the IB and LB methods, and then explain the coupling strategy and detailed implementation procedures. To verify the effectiveness and accuracy, FSI problems arising from the relaxation of a distorted balloon immersed in a viscous fluid, an unsteady wake flow caused by an impulsively started circular cylinder at Reynolds number 9500, and an unsteady vortex shedding flow past a suddenly started rotating circular cylinder at Reynolds number 1000 are simulated. The first example is a benchmark case for flexible boundary FSI with a large pressure gradient across the boundary, the second is a fixed complex boundary problem, and the third is a typical moving boundary example. The results are in good agreement with the analytical and existing numerical data. It is shown that the proposed scheme is capable of modeling flexible boundary and complex boundary problems at a second-order spatial convergence; the volume leakage defect of the conventional IB method has been remedied by using a new method of introducing the unsteady and non-uniform external force; and the LB method makes the IB method simulation simpler and more efficient.

Research paper thumbnail of New boundary treatment methods for lattice Boltzmann method

In practical fluid dynamic simulations, the boundary condition should be treated carefully becaus... more In practical fluid dynamic simulations, the boundary condition should be treated carefully because it always has crucial influence on the numerical accuracy, stability and efficiency. Two types of boundary treatment methods for lattice Boltzmann method (LBM) are proposed. One is for the treatment of boundaries situated at lattice nodes, and the other is for the approximation of boundaries that are not located at the regular lattice nodes. The first type of boundary treatment method can deal with various dynamic boundaries on complex geometries by using a general set of formulas, which can maintain second-order accuracy. Based on the fact that the fluid flows simulated by LBM are not far from equilibrium, the unknown distributions at a boundary node are expressed as the analogous forms of their corresponding equilibrium distributions. Therefore, the number of unknowns can be reduced and an alwaysclosed set of equations can be obtained for the solutions to pressure, velocity and special boundary conditions on various geometries. The second type of boundary treatment is a complete interpolation scheme to treat curved boundaries. It comes from careful analysis of the relations between distribution functions at boundary nodes and their neighboring lattice nodes. It is stable for all situations and of second-order accuracy. Basic ideas, implementation procedures and verifications with typical examples for the both treatments are presented. Numerical simulations and analyses show that they are accurate, stable, general and efficient for practical simulations.

Research paper thumbnail of Simulation of hydraulic transients in hydropower systems using the 1-D-3-D coupling approach

Journal of Hydrodynamics, Ser. B, 2012

Although the hydraulic transients in pipe systems are usually simulated by using a one-dimensiona... more Although the hydraulic transients in pipe systems are usually simulated by using a one-dimensional (1-D) approach, local three-dimensional (3-D) simulations are necessary because of obvious 3-D flow features in some local regions of the hydropower systems. This paper combines the 1-D method with a 3-D fluid flow model to simulate the Multi-Dimensional (MD) hydraulic transients in hydropower systems and proposes two methods for modeling the compressible water with the correct wave speed, and two strategies for efficiently coupling the 1-D and 3-D computational domains. The methods are validated by simulating the water hammer waves and the oscillations of the water level in a surge tank, and comparing the results with the 1-D solution data. An MD study is conducted for the transient flows in a realistic water conveying system that consists of a draft tube, a tailrace surge tank and a tailrace tunnel. It is shown that the 1-D-3-D coupling approach is an efficient and promising way to simulate the hydraulic transients in the hydropower systems in which the interactions between 1-D hydraulic fluctuations of the pipeline systems and the local 3-D flow patterns should be considered.

Research paper thumbnail of 3-D simulation of transient flow patterns in a corridor-shaped air-cushion surge chamber based on computational fluid dynamics

Journal of Hydrodynamics, Ser. B, 2013

The 3-D characteristics of the water-air flow patterns in a corridor-shaped air-cushion surge cha... more The 3-D characteristics of the water-air flow patterns in a corridor-shaped air-cushion surge chamber during hydraulic transients need to be considered in the shape optimization. To verify the reliability of the water-air two-phase model, namely, the volume of fluid model, the process of charging water into a closed air chamber is successfully simulated. Using the model, the 3-D flow characteristics under the load rejection and acceptance conditions within the air-cushion surge chamber of a specific hydropower station are studied. The free surface waves, the flow patterns, and the pressure changes during the surge wave process are analyzed in detail. The longitudinal flow of water in the long corridor-shaped surge chamber is similar to the open channel flow with respect to the wave propagation, reflection and superposition characteristics. The lumped parameters of the 3-D numerical simulation agree with the results of a 1-D calculation of hydraulic transients in the whole water conveying system, which validates the 3-D method. The 3-D flow structures obtained can be applied to the shape optimization of the chamber.

Research paper thumbnail of Free surface-pressurized flow in ceiling-sloping tailrace tunnel of hydropower plant: simulation by VOF model

Journal of Hydraulic Research, 2007

Free surface-pressurized flow in ceiling-sloping tailrace tunnel of hydropower plant: simulation ... more Free surface-pressurized flow in ceiling-sloping tailrace tunnel of hydropower plant: simulation by VOF model Écoulement à surface libre pressurisé dans la galerie d'évacuation à plafond-en pente d'une usine hydroélectrique: simulation par un modèle VOF

Research paper thumbnail of Introducing unsteady non-uniform source terms into the lattice Boltzmann model

International Journal for Numerical Methods in Fluids, 2008

Taking body forces into account is not new for the lattice Boltzmann method, yet most of the exis... more Taking body forces into account is not new for the lattice Boltzmann method, yet most of the existing approaches can only treat steady and uniform body forces. To manage situations with time-and spacedependent body forces or source terms, this paper proposes a new approach through theoretical derivation and numerical verification. The method by attaching an extra term to the lattice Boltzmann equation is still used, but the expression of the extra term is modified. It is the modified extra term that achieves the particularity of the new approach. This approach can not only introduce unsteady and non-uniform body forces into momentum equations, but is also able to add an arbitrary source term to the continuity equation. Both the macroscopic equations from multi-scale analysis and the simulated results of typical examples show that the accuracy with second-order convergence can be guaranteed within incompressible limit. 630 Y. CHENG AND J. LI hydraulic problems [1-10]. The rapid increase in the number of published works on LBM in recent years boosts the perspective of practical applications.

Research paper thumbnail of A viscosity counteracting approach in the lattice Boltzmann BGK model for low viscosity flow: Preliminary verification

Computers & Mathematics with Applications, 2011

Due to numerical instability, the lattice Boltzmann model (LBM) with the Bhatnagar-Gross-Krook (B... more Due to numerical instability, the lattice Boltzmann model (LBM) with the Bhatnagar-Gross-Krook (BGK) collision operator has some limitations in the simulation of low viscosity flows. In this paper, we propose a viscosity counteracting approach for simulating a moderate viscosity flow. An extra negative viscosity term is introduced to counteract part of the moderate viscosity by using the lattice Boltzmann equation with a source term. The counteracting viscosity term is treated as a non-uniform unsteady source. The stability is enhanced; thus small viscosity flows can be simulated. Model verification consists of benchmark cases such as those of Poiseuille flow, Couette flow, waterhammer waves, Taylor-Green vortex flow, and lid-driven cavity flow. The flow patterns, error characteristics, and representative parameters are carefully analyzed. It is shown that this approach can simulate flows with lower viscosities than may be simulated using the normal LBGK model; the second-order accuracy of the LBGK model is definitely retained, although a little dissipation is added. These preliminary studies prove the effectiveness and accuracy of the model. Sophisticated analysis and further verification of the stability mechanism will be done in the near future.

Research paper thumbnail of Immersed boundary method and lattice Boltzmann method coupled FSI simulation of mitral leaflet flow

Computers & Fluids, 2010

Coupling the immersed boundary (IB) method and the lattice Boltzmann (LB) method might be a promi... more Coupling the immersed boundary (IB) method and the lattice Boltzmann (LB) method might be a promising approach to simulate fluid-structure interaction (FSI) problems with flexible structures and moving boundaries. To investigate the possibility for future IB-LB coupled simulations of the heart flow dynamics, an IB-LB coupling scheme suitable for rapid boundary motion and large pressure gradient FSI is proposed, and the mitral valve jet flow considering the interaction of leaflets and fluid is simulated. After analyzing the respective concepts, formulae and advantages of the IB and LB methods, we first explain the coupling strategy and detailed implementation procedures, and then verify the effectiveness and second-order accuracy of the scheme by simulating a benchmark case, the relaxation of a stretched membrane immersed in fluid. After that, the diastolic filling jet flow between mitral leaflets in a simplified 2D left heart model is simulated. The model consists of the simplified transmitral passage of the heart and two curvilinear leaflets. In the simulation, the atrial and ventricular pressure histories of normal human are specified as boundary conditions, and the leaflets are treated as fibers that interact with the fluid to define their deformations and movements. The resulting opening and closing movements of the leaflets and the flow patterns of the filling jet are qualitatively reasonable and compare well with existing numerical and measured data. It is shown that this IB-LB coupling method is feasible for treating flexible boundary FSI problems with rapid boundary motion and large pressure gradient, the results of the mitral leaflet flow are valuable for understanding the transmitral FSI dynamics, and it is possible to simulate the more realistic 3D heart flow by the scheme in the future.

Research paper thumbnail of Fluid-Structure Coupled CFD Simulation of the Left Ventricular Flow During Filling Phase

Annals of Biomedical Engineering, 2005

The fluid-structure coupled simulation of the heart, though at its developing stage, has shown gr... more The fluid-structure coupled simulation of the heart, though at its developing stage, has shown great prospect in heart function investigations and clinical applications. The purpose of this paper is to verify a commercial software based fluid-structure interaction scheme for the left ventricular filling. The scheme applies the finite volume method to discretize the arbitrary Lagrangian–Eulerian formulation of the Navier–Stokes equations for the fluid while using the nonlinear finite element method to model the structure. The coupling of the fluid and structure is implemented by combining the fluid and structure equations as a unified system and solving it simultaneously at every time step. The left ventricular filling flow in a three-dimensional ellipsoidal thin-wall model geometry of the human heart is simulated, based on a prescribed time-varying Young’s modulus. The coupling converges smoothly though the deformation is very large. The pressure–volume relation of the model ventricle, the spatial and temporal distributions of pressure, transient velocity vectors as well as vortex patterns are analyzed, and they agree qualitatively and quantitatively well with the existing data. This preliminary study has verified the feasibility of the scheme and shown the possibility to simulate the left ventricular flow in a more realistic way by adding a myocardial constitutive law into the model and using a more realistic heart geometry.