A P - σ regional climate model using a parameterization scheme to account for the thermal effects of the sub-grid scale orography was used to simulate the three heavy rainfall events that occurred within the Yangtze ...A P - σ regional climate model using a parameterization scheme to account for the thermal effects of the sub-grid scale orography was used to simulate the three heavy rainfall events that occurred within the Yangtze River Valley during the mei-yu period of 1991. The simulation results showed that by considering the sub-grid scale topography scheme, one can significantly improve the performance of the model for simulating the rainfall distribution and intensity during these three heavy rainfall events, most especially the second and third. It was also discovered that the rainfall was mainly due to convective precipitation. The comparison between experiments, either with and without the sub-grid scale topography scheme, showed that the model using the scheme reproduced the convergence intensity and distribution at the 850 hPa level and the ascending motion and moisture convergence center located at 500 hPa over the Yangtze River valley. However, some deviations still exist in the simulation of the atmospheric moisture content, the convergence distribution and the moisture transportation route, which mainly result in lower simulated precipitation levels. Further analysis of the simulation results demonstrated that the sub-grid topography scheme modified the distribution of the surface energy budget components, especially at the south and southwest edges of the Tibetan Plateau, leading to the development and eastward propagation of the negative geopotential height difference and positive temperature-lapse rate difference at 700 hPa, which possibly led to an improved precipitation simulation over eastern China.展开更多
In this paper, a modified sub-gridding scheme that hybridizes the conventional finite-difference time-domain(FDTD)method and the unconditionally stable locally one-dimensional(LOD) FDTD is developed for analyzing ...In this paper, a modified sub-gridding scheme that hybridizes the conventional finite-difference time-domain(FDTD)method and the unconditionally stable locally one-dimensional(LOD) FDTD is developed for analyzing the periodic metallic nanoparticle arrays. The dispersion of the metal, caused by the evanescent wave propagating along the metal-dielectric interface, is expressed by the Drude model and solved with a generalized auxiliary differential equation(ADE) technique.In the sub-gridding scheme, the ADE–FDTD is applied to the global coarse grids while the ADE–LOD–FDTD is applied to the local fine grids. The time step sizes in the fine-grid region and coarse-grid region can be synchronized, and thus obviating the temporal interpolation of the fields in the time-marching process. Numerical examples about extraordinary optical transmission through the periodic metallic nanoparticle array are provided to show the accuracy and efficiency of the proposed method.展开更多
The finite-difference time-domain(FDTD)method is used effectively to solve electromagnetic(EM)scattering and radiation problems using a 3D sub-gridding algorithm.For accuracy and stability of the FDTD method,the compu...The finite-difference time-domain(FDTD)method is used effectively to solve electromagnetic(EM)scattering and radiation problems using a 3D sub-gridding algorithm.For accuracy and stability of the FDTD method,the computational domain of EM problems with locally fine structures or electrically small objects is discretized with finer grids.This sub-gridding algorithm for different regions of the computational domain was implemented to increase the accuracy and reduce the computational time and memory requirements compared to those of the traditional FDTD method.In the sub-gridding algorithm,the FDTD computational domain is divided into separate regions:coarse grid and fine grid regions.Since the cell sizes and time steps are different in the coarse and fine grid regions,interpolations in both time and space are used to evaluate the electric and magnetic fields on the boundaries between different regions.The accuracy of the developed 3D sub-gridding algorithm has been verified for radiation and scattering problems,including multiple fine grid regions.Excellent performance is obtained even for higher and different contrast ratios in fine grid regions.展开更多
In this paper, two sub-grid scale (SGS) models are introduced into the Lattice Boltzmann Method (LBM), i.e., the dynamics SGS model and the dynamical system SGS model, and applied to numerically solving three-dimensio...In this paper, two sub-grid scale (SGS) models are introduced into the Lattice Boltzmann Method (LBM), i.e., the dynamics SGS model and the dynamical system SGS model, and applied to numerically solving three-dimensional high Re turbulent cavity flows. Results are compared with those obtained from the Smagorinsky model and direct numerical simulation for the same cases. It is shown that the method with LBM dynamics SGS model has advantages of fast computation speed, suitable to simulate high Re turbulent flows. In addition, it can capture detailed fine structures of turbulent flow fields. The method with LBM dynamical system SGS model dose not contain any adjustable parameters, and can be used in simulations of various complicated turbulent flows to obtain correct information of sub-grid flow field, such as the backscatter of energy transportation between large and small scales. A new average method of eliminating the inherent unphysical oscillation of LBM is also given in the paper.展开更多
An LES/FDF model was developed by the authors to investigate the SGS effect on the particle motion in the gas-particle two-phase plane wake flow.The simulation results of dispersion rate for different particles were c...An LES/FDF model was developed by the authors to investigate the SGS effect on the particle motion in the gas-particle two-phase plane wake flow.The simulation results of dispersion rate for different particles were compared with the results without using the FDF model.It was shown that the large eddy structure is the dominant factor influencing the particle diffu-sion in space for small particles(small Stokes-number particles),but for intermediate or large diameter particles,the influence of the sub-grid scale eddies on the dispersion rate is in the same order as that of the large eddies.The sub-grid scale eddies increase the particle dispersion rate in most time,but sometimes they decrease the dispersion rate.The sub-grid scale particle dispersion rate is decided not only by the intensity of sub-grid scale eddies and the Stokes number of the particles,but also by the large eddy structure of the flow field.For the particles in isotropic turbulence,the dispersion rate decreases as the particle diameter increases.展开更多
The accuracy of large eddy simulation(LES)is highly dependent on the performance of sub-grid scale(SGS)model.In the present paper,a dynamic cubic nonlinear sub-grid scale model(DCNM)proposed by Huang et al.is implemen...The accuracy of large eddy simulation(LES)is highly dependent on the performance of sub-grid scale(SGS)model.In the present paper,a dynamic cubic nonlinear sub-grid scale model(DCNM)proposed by Huang et al.is implemented for the simulation of unsteady cavitating flow around a 3-D Clark-Y hydrofoil in OpenFOAM.Its performance in predicting the evolution of cloud cavitation is discussed in detail.The simulation with a linear model,the dynamic Smagorinsky model(DSM),is also conducted as a comparison.The results with DCNM show a better agreement with the available experimental observation.The comparison between DCNM and DSM further suggests that the DCNM is able to predict the backscatter more precisely,which is an important feature in LES.The characteristics of DCNM is analyzed to account for its advantages in the prediction of unsteady cloud cavitation as well.The results reveal that it is the nonlinear terms of DCNM that makes DCNM capture sub-grid scale vortices better and more suitable for studying the transient behaviors of cloud cavitation than DSM.展开更多
In this study,we developed a novel optical-flow algorithm for determining the wall shear-stress on the surface of objects.The algorithm solves the thin-oil-film equation using a numerical scheme that recovers local fe...In this study,we developed a novel optical-flow algorithm for determining the wall shear-stress on the surface of objects.The algorithm solves the thin-oil-film equation using a numerical scheme that recovers local features neglected by smoothing filters.A variational formulation with a smoothness constraint was applied to extract the global shear-stress fields.The algorithm was then applied to scalar images generated using direct numerical simulation(DNS)method,which revealed that the errors were smaller than those of conventional methods.The application of the proposed algorithm to recover the wall shear-stress on a low-aspect-ratio wing and on an axisymmetric boattail model taken as examples in this study showed a strong potential for analysing shear-stress fields.Compared to the methods used in previous studies,proposed method reveals more local features of separation line and singular points on object surface.展开更多
In this paper,we present an efficient computational methodology for diffusion and convection-diffusion problems in highly heterogeneous media as well as convection-dominated diffusion problem.It is well known that the...In this paper,we present an efficient computational methodology for diffusion and convection-diffusion problems in highly heterogeneous media as well as convection-dominated diffusion problem.It is well known that the numerical computation for these problems requires a significant amount of computermemory and time.Nevertheless,the solutions to these problems typically contain a coarse component,which is usually the quantity of interest and can be represented with a small number of degrees of freedom.There are many methods that aim at the computation of the coarse component without resolving the full details of the solution.Our proposed method falls into the framework of interior penalty discontinuous Galerkin method,which is proved to be an effective and accurate class of methods for numerical solutions of partial differential equations.A distinctive feature of our method is that the solution space contains two components,namely a coarse space that gives a polynomial approximation to the coarse component in the traditional way and a multiscale space which contains sub-grid structures of the solution and is essential to the computation of the coarse component.In addition,stability of the method is proved.The numerical results indicate that the method can accurately capture the coarse behavior of the solution for problems in highly heterogeneous media as well as boundary and internal layers for convection-dominated problems.展开更多
文摘A P - σ regional climate model using a parameterization scheme to account for the thermal effects of the sub-grid scale orography was used to simulate the three heavy rainfall events that occurred within the Yangtze River Valley during the mei-yu period of 1991. The simulation results showed that by considering the sub-grid scale topography scheme, one can significantly improve the performance of the model for simulating the rainfall distribution and intensity during these three heavy rainfall events, most especially the second and third. It was also discovered that the rainfall was mainly due to convective precipitation. The comparison between experiments, either with and without the sub-grid scale topography scheme, showed that the model using the scheme reproduced the convergence intensity and distribution at the 850 hPa level and the ascending motion and moisture convergence center located at 500 hPa over the Yangtze River valley. However, some deviations still exist in the simulation of the atmospheric moisture content, the convergence distribution and the moisture transportation route, which mainly result in lower simulated precipitation levels. Further analysis of the simulation results demonstrated that the sub-grid topography scheme modified the distribution of the surface energy budget components, especially at the south and southwest edges of the Tibetan Plateau, leading to the development and eastward propagation of the negative geopotential height difference and positive temperature-lapse rate difference at 700 hPa, which possibly led to an improved precipitation simulation over eastern China.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.61471105 and 61331007)
文摘In this paper, a modified sub-gridding scheme that hybridizes the conventional finite-difference time-domain(FDTD)method and the unconditionally stable locally one-dimensional(LOD) FDTD is developed for analyzing the periodic metallic nanoparticle arrays. The dispersion of the metal, caused by the evanescent wave propagating along the metal-dielectric interface, is expressed by the Drude model and solved with a generalized auxiliary differential equation(ADE) technique.In the sub-gridding scheme, the ADE–FDTD is applied to the global coarse grids while the ADE–LOD–FDTD is applied to the local fine grids. The time step sizes in the fine-grid region and coarse-grid region can be synchronized, and thus obviating the temporal interpolation of the fields in the time-marching process. Numerical examples about extraordinary optical transmission through the periodic metallic nanoparticle array are provided to show the accuracy and efficiency of the proposed method.
文摘The finite-difference time-domain(FDTD)method is used effectively to solve electromagnetic(EM)scattering and radiation problems using a 3D sub-gridding algorithm.For accuracy and stability of the FDTD method,the computational domain of EM problems with locally fine structures or electrically small objects is discretized with finer grids.This sub-gridding algorithm for different regions of the computational domain was implemented to increase the accuracy and reduce the computational time and memory requirements compared to those of the traditional FDTD method.In the sub-gridding algorithm,the FDTD computational domain is divided into separate regions:coarse grid and fine grid regions.Since the cell sizes and time steps are different in the coarse and fine grid regions,interpolations in both time and space are used to evaluate the electric and magnetic fields on the boundaries between different regions.The accuracy of the developed 3D sub-gridding algorithm has been verified for radiation and scattering problems,including multiple fine grid regions.Excellent performance is obtained even for higher and different contrast ratios in fine grid regions.
基金Supported by the Key Project of National Natural Science Foundation of China (Grant No. 10532030)
文摘In this paper, two sub-grid scale (SGS) models are introduced into the Lattice Boltzmann Method (LBM), i.e., the dynamics SGS model and the dynamical system SGS model, and applied to numerically solving three-dimensional high Re turbulent cavity flows. Results are compared with those obtained from the Smagorinsky model and direct numerical simulation for the same cases. It is shown that the method with LBM dynamics SGS model has advantages of fast computation speed, suitable to simulate high Re turbulent flows. In addition, it can capture detailed fine structures of turbulent flow fields. The method with LBM dynamical system SGS model dose not contain any adjustable parameters, and can be used in simulations of various complicated turbulent flows to obtain correct information of sub-grid flow field, such as the backscatter of energy transportation between large and small scales. A new average method of eliminating the inherent unphysical oscillation of LBM is also given in the paper.
基金supported by the National Natural Science Foundation of China (Grant No.10502044,10772162)the Defense-based research project(Grant No.A1420080144)the Major projects on control and rectification of water body pollution (Grant No.2009ZX07424-001)
文摘An LES/FDF model was developed by the authors to investigate the SGS effect on the particle motion in the gas-particle two-phase plane wake flow.The simulation results of dispersion rate for different particles were compared with the results without using the FDF model.It was shown that the large eddy structure is the dominant factor influencing the particle diffu-sion in space for small particles(small Stokes-number particles),but for intermediate or large diameter particles,the influence of the sub-grid scale eddies on the dispersion rate is in the same order as that of the large eddies.The sub-grid scale eddies increase the particle dispersion rate in most time,but sometimes they decrease the dispersion rate.The sub-grid scale particle dispersion rate is decided not only by the intensity of sub-grid scale eddies and the Stokes number of the particles,but also by the large eddy structure of the flow field.For the particles in isotropic turbulence,the dispersion rate decreases as the particle diameter increases.
基金Supported by the National Natural Science Foundation of China(Grant Nos.51822903,11772239).
文摘The accuracy of large eddy simulation(LES)is highly dependent on the performance of sub-grid scale(SGS)model.In the present paper,a dynamic cubic nonlinear sub-grid scale model(DCNM)proposed by Huang et al.is implemented for the simulation of unsteady cavitating flow around a 3-D Clark-Y hydrofoil in OpenFOAM.Its performance in predicting the evolution of cloud cavitation is discussed in detail.The simulation with a linear model,the dynamic Smagorinsky model(DSM),is also conducted as a comparison.The results with DCNM show a better agreement with the available experimental observation.The comparison between DCNM and DSM further suggests that the DCNM is able to predict the backscatter more precisely,which is an important feature in LES.The characteristics of DCNM is analyzed to account for its advantages in the prediction of unsteady cloud cavitation as well.The results reveal that it is the nonlinear terms of DCNM that makes DCNM capture sub-grid scale vortices better and more suitable for studying the transient behaviors of cloud cavitation than DSM.
文摘In this study,we developed a novel optical-flow algorithm for determining the wall shear-stress on the surface of objects.The algorithm solves the thin-oil-film equation using a numerical scheme that recovers local features neglected by smoothing filters.A variational formulation with a smoothness constraint was applied to extract the global shear-stress fields.The algorithm was then applied to scalar images generated using direct numerical simulation(DNS)method,which revealed that the errors were smaller than those of conventional methods.The application of the proposed algorithm to recover the wall shear-stress on a low-aspect-ratio wing and on an axisymmetric boattail model taken as examples in this study showed a strong potential for analysing shear-stress fields.Compared to the methods used in previous studies,proposed method reveals more local features of separation line and singular points on object surface.
基金supported by a grant from the Research Grant Council of the Hong Kong SAR(Project No.CUHK401010).
文摘In this paper,we present an efficient computational methodology for diffusion and convection-diffusion problems in highly heterogeneous media as well as convection-dominated diffusion problem.It is well known that the numerical computation for these problems requires a significant amount of computermemory and time.Nevertheless,the solutions to these problems typically contain a coarse component,which is usually the quantity of interest and can be represented with a small number of degrees of freedom.There are many methods that aim at the computation of the coarse component without resolving the full details of the solution.Our proposed method falls into the framework of interior penalty discontinuous Galerkin method,which is proved to be an effective and accurate class of methods for numerical solutions of partial differential equations.A distinctive feature of our method is that the solution space contains two components,namely a coarse space that gives a polynomial approximation to the coarse component in the traditional way and a multiscale space which contains sub-grid structures of the solution and is essential to the computation of the coarse component.In addition,stability of the method is proved.The numerical results indicate that the method can accurately capture the coarse behavior of the solution for problems in highly heterogeneous media as well as boundary and internal layers for convection-dominated problems.