中观交通流建模与系统仿真研究综述*

在简要描述了宏微观交通流仿真的建模思路和仿真软件开发情况的基础上,重点对近四十年来中观交通流仿真建模的研究成果进行了总结和归类,并对不同中观仿真模型的优缺点进行了比较;最后指出了中观交通流建模与仿真的研究方向。     

城市中观交通仿真的交通流车簇模型

为了解决城市交通仿真模型仿真速度和仿真精确度的平衡问题,针对城市交通流运行特征,同时考虑影响交通流运行的若干因素,如车辆数目、红绿灯控制等,提出一种中观交通仿真车辆群体模法.该模型结合了宏观模型和微观模型的优点,同时对应的交通流配流算法解决了传统模型的不足,城市交通仿真实验结果表明了车辆群体模型和分配算法的合理性和有效性.     

Concentration effects in mesoscopic simulation of coarsening

The self-organization of particles in a two-phase system in the coexistence region through a diffusive mechanism is known as Ostwald ripening. A mesoscopic model derived from the spin exchange Ising model is used here in a computational study of Ostwald ripening. The typical cluster size is observed to grow like Lifshitz–Slyozov growth law over an early range of times with faster growth at later times. The effect of changing the concentration of particles on the morphological evolution of the system is also discussed.     

Supporting real-world network-oriented mesoscopic traffic simulation on GPU

Mesoscopic traffic simulation is a promising technology for handling large-scale dynamic traffic assignment problems. Meanwhile, GPU has been a research hotspot in the last decade because of its massive parallel performance compared with CPU. Although several works deal with GPU-based parallel performance, few have addressed the issue of real-world network applications. Thus, the performance of GPU on realistic road networks is still not clear. In this paper, an ideal grid network and a realistic Singapore expressway network are implemented and compared. The former gets a speedup of 10.00 while the latter only get a speedup of 2.37. The two main reasons for this lower efficiency of a real network are the more complex data structure caused by network topology and fewer numbers of threads due to the scale of the expressway network. Future works need more effort to develop a system with better data structure and better designed concurrent threads.     

Computational model of mesoscopic structure of concrete for simulation of fracture processes

This paper describes mathematical modelling and computational tool for simulation of fracture processes of cementitious composites at the mesoscopic level. The tool relies on highly realistic 3D- and 2D-representations of the heterogeneous internal structure of concrete for understanding the micromechanics of aggregate–matrix interactions. The generation mechanism allows control of aggregate volume content, shape and size distribution. The allocation procedure proved capable to produce numerical concrete with aggregate distributions comparable to real concrete. The continuum was discretised into lattices of linear elements, for structural analyses. Compression, direct tension and wedge-splitting tests were simulated. Parametrical study was carried out to investigate effects of different material properties and proportions in concrete admixtures.     

A mesoscopic simulation study of distributions of droplets in a bifurcating channel

A transport of a suspension of slightly deformed immiscible droplets in a bifurcating channel is studied by using a mesoscale simulation technique. The distribution of the droplets is represented by the fractional droplet flux into two daughters as a function of the volumetric flow ratio between the daughters. The data obtained in our simulations is compared with theoretical predictions obtained by assuming an exponential function for the distribution for positions of the droplets in the mother channel. The theoretical predictions show good agreement with the simulation results. Further, we compare our results with an experimental study of transports of spherical and disk-shaped particles in a bifurcating channel, and we confirm that our results show the same tendency towards non-linearity as the experimental result do. A non-uniform distribution of the droplets in the mother channel affects the non-linear separation of the droplet flux at the bifurcation.     

Multi-objective traffic signal optimization using 3D mesoscopic simulation and evolutionary algorithms

Modern cities are currently facing rapid urban growth and struggle to maintain a sustainable development. In this context, “eco-neighbourhoods” became the perfect place for testing new innovative ideas that would reduce congestion and optimize traffic flow. The main motivation of this work is a true and stated need of the Department of Transport in Nancy, France, to improve the traffic flow in a central eco-neighbourhood currently under reconfiguration, reduce travel times and test various traffic control scenarios for a better interconnectivity between urban intersections. Therefore, this paper addresses a multi-objective simulation-based signal control problem through the case study of “Nancy Grand Cœur” (NGC) eco-neighbourhood with the purpose of finding the optimal traffic control plan to reduce congestion during peak hours. Firstly, we build the 3D mesoscopic simulation model of the most circulated intersection (C129) based on specifications from the traffic management centre. The simulation outputs from various scenario testing will be then used as inputs for the optimisation and comparative analysis modules. Secondly, we propose a multi-objective optimization method by using evolutionary algorithms and find the optimal traffic control plan to be used in C129 during morning and evening rush hours. Lastly, we take a more global view and extend the 3D simulation model to three other interconnected intersections, in order to analyse the impact of local optimisation on the surrounding traffic conditions in the eco-neighbourhood. The current proposed simulation-optimisation framework aims at supporting the traffic engineering decision-making process and the smart city dynamic by favouring a sustainable mobility.     

用于低速渗流实验的低渗流传感器设计

针对岩土体低速渗流实验中μL/min级别流量难于测量的问题,提出了一种利用电解极化示踪的低渗流流量测量方法。介绍了传感器的测量原理、结构及测控电路。制作了量程为10～100μL/min的渗流传感器,并运用毛细管整体柱电渗泵进行了流量测定实验。结果表明：传感器结构简单、体积小,在10～60μL/min的流量范围内,传感器的线性相关性大于0.998,能够满足岩土体低速渗流实验的要求,具有良好的实用价值。     

基于CUDA的泥石流模拟计算研究

针对泥石流仿真过程中海量数据计算问题,采用CUDA技术即结合CPU与GPU的优点研究了一种协同计算方法以提高数据计算效率和仿真性能。同时,搭建了基于GPU的泥石流仿真计算平台,对优化的CUDA并行计算方法进行验证。实验结果表明,该方法对海量数据的计算具有快速准确、低成本、低功耗的特点,能为灾害预测提供及时准确的决策支持,满足了高密集型计算的需求。     

连续交通流可视化仿真

在大型城市的交通态势模拟中,宏观模型相比于微观模型,具有效率高,物理意义明确的优点,但缺乏车辆在各个时刻的细节描述。为了使宏观模型的交通模拟的显示更加直观,结合微观模型的更新方式,提出了基于宏观模型的可视化方法。通过道路上的车辆初始密度得到所有车辆的初始位置和速度;根据宏观路段速度更新车辆信息,使得宏观路段密度值与微观车辆所构成的车辆流密度相符。该方法能够给出宏观模型中车辆在各个时刻的运动情况,并添加了微观上的细节。以实例证明了该方法的可行性。     

Simulation of slow flow of a fluid past a solid sphere

The problem of slow flow past a solid sphere is considered when the outer fluid is of infinite extent. Inertial effect is included in the analysis. A numerical method is employed to obtain solutions of the equation of motion. Solutions are obtained for the spherical coordinates components of velocity. The results obtained are in complete agreement with those obtained by other workers for the flow of the Newtonian fluid past a solid sphere.     

Numerical simulation of separating boundary-layer flow

A two-dimensional boundary-layer flow over a bump has been studied computationally by solving the incompressible Navier–Stokes equations. The numerical solution procedure uses a mapping transforming the physical coordinate system into a Cartesian one. The resulting system is solved using a mixed finite differences––Chebyshev collocation discretization. We compare the influence matrix technique with a fractional time-step procedure used to enforce the divergence-free condition. Numerical experiments are performed for bumps with different aspect ratios. The separation structure is investigated for increasing Reynolds number and self-induced vortex shedding is reproduced numerically.     

Cell cycle simulation for flow cytometry

A program has been implemented on a VAX computer that simulates the progression of cells through the cell cycle and generates data similar to those obtained in flow cytometry with different techniques. Features of the program are general applicability and flexibility, options including consideration of (a) mean duration of cell cycle phases, (b) their inter-cell distribution, (c) a first order commitment from G0 into G1 phase, and (d) a total or partial block of the output from any phase. Examples are given of simulated flow cytometric experiments of drug-induced cell cycle perturbation and of bromodeoxyuridine pulse labeling. This program should help to acquire a correct understanding of the relationship between kinetic features and flow cytometric data.     

Computer simulation of wind flow around buildings

This paper looks at the relationship between wind flow round a building and heat loss from it. The relative merits of numerical and wind tunnel models are discussed and various numerical techniques examined.     

Supercompact multiwavelets for flow field simulation

A supercompact multiwavelet scheme for computational fluid dynamics is presented. Beam and Warming's supercompact wavelet method is an appropriate wavelet for fluid simulation data in the sense that it can provide compact support. The compactness of the wavelets avoids unnecessary interaction with remotely located data (e.g. across a shock discontinuity or vortex) and significantly reduces computational data processing time. Thresholding for data compression with the supercompact wavelets is applied based on a covariance vector structure of multiwavelets. The extension of this scheme to three dimensions is analyzed. Numerical tests demonstrate that the analytic advantages actually result in large data compression ratios.     

Large eddy simulation of complex flow fields

The large eddy simulation (LES) technique can provide detailed information about time-dependent and three-dimensional turbulent flow fields at high Reynolds number. The application of LES to practical problems and in the basic study of turbulence require investigation. More studies are needed on the boundary conditions and on the subgrid-scale (SGS) model in order to make LES practical. In this paper, the laws of the wall (two-layer model or Spalding's law) are applied as a special approach to the solid boundary in LES. This wall boundary condition is adapted to plane channel flow and the suitability of this method is tested. Further, improvements of the SGS model in which the Smagorinsky model coefficient C_S is not constant are attempted. Recently, Yoshizawa [Phys. Fluids A1(7), 1293 (1989)] derived the form of the variable C_S from a statistical analysis. Here, we optimize this new model in both the decay of isotropic turbulence and plane channel flow simultaneously.     

Computer simulation of a material flow system

This paper presents a computer simulation model of a material flow system. The flow system is a conveyor fed system for sorting and packing different types of bundles of cigarettes. Several bundle-types of cigarettes are produced at different rates by different machines and then released onto a common belt conveyor. The conveyor transports the bundles to a marshalling area. Because of the nature of the flow system different types of bundles get mixed up in the conveyor and several workers are stationed in the marshalling area to sort the different bundle-types and then pack into different crates. The model is used to answer questions relating to assignment of workers to particular bundle-types, whether workers should help each other in picking up bundles, and manpower levels in the marshalling area.This paper illustrates the application of computer simulation to a real world problem in a small factory.     

Real-time simulation of physically based on-surface flow

Although many papers have been published in the field of fluid simulation, little attention has been paid to on-surface flow involving wetting and stain transportation as well as erosion and deposition phenomena. In this paper, we introduce nonzero divergence in the mass equation of Navier–Stokes equations to simulate water penetration from on-surface flow into substrate material. Also, the volume of fluid method is adopted to track the free surface. With a computation of the actual amount of absorbed water we render the wetting effects with fully dry and fully wet texture images simultaneously. Using our model, the on-surface flow that accompanies water absorption can be simulated realistically in real time with OpenGL preview rendering. Experimental results illustrate that our model can be widely applied to solve various problems related to on-surface flow.     

Large-eddy simulation of streamwise-rotating turbulent channel flow

In many engineering and industrial applications the investigation of rotating turbulent flow is of great interest. Whereas some research has been done concerning channel flows with a spanwise rotation axis, only few investigations have been performed on channel flows with a rotation about the streamwise axis. In the present study an LES of a turbulent streamwise-rotating channel flow at Re_τ = 180 is performed using a moving grid method. The three-dimensional structures and the details of the secondary flow distribution are analyzed and compared with experimental data. The numerical-experimental comparison shows a convincing agreement as to the overall flow features. The results confirm the development of a secondary flow in the spanwise direction, which has been found to be correlated to the rotational speed. Furthermore, the findings show the distortion of the main flow velocity profile, the slight decrease of the streamwise Reynolds stresses in the vicinity of the walls, and the pronounced increase of the spanwise Reynolds stresses at higher rotation rates near the walls and particularly in the symmetry region. As to the numerical set-up it is shown that periodic boundary conditions in the spanwise direction suffice if the spanwise extent of the computational domain is larger than 10 times the channel half width.     

A computer simulation of turbulent jet flow

The evolution of a round turbulent jet is simulated numerically under the constraint of axial symmetry. The vortex sheet shed from the orifice is represented by vortex ring elements, with a velocity field cut-off to control close encounters. Large-scale vortex clusters form in the model jet, similar to those inferred from laboratory flow visualization experiments. However, comparisons of statistical properties reveal significant differences between the axisymmetric model flow and real turbulent jets.