Study On Virtual Urban Model Based On Complexity Theory

Cities are viewed as complex systems. There hasn't a clear definition of complex system in science, but characteristics of complex system are tangible, which consist of fractal structure, self-organized critical, complex network. Based on the theory of complex science and modeling tool of complex system, the paper has studied city system's complex characteristics, including city's fractal, city's self-organized and self-organized criticality, and city's complex network. The paper has concentrated on the following subjects:1. Study on City Fractal StructureFractals are hierarchically ordered structures revealing self-similarity across scales, which widely exist in the nature, and besides, they have advantages of high space utilization as well as better expansibility. Fractal structures come into being consciously or unconsciously in designing human activity space from architecture to inhabitant environment, community to city, and city to city system. Not only does city's apace have fractal structure, but also all integer things in a city have fractal structures, both in hardware infrastructures such as traffic, communications, energy source transport etc., and service establishments such as police, bank, school, and hospital etc. To do research on hierarchy and self-similarity in city fractal characteristics can help to better space utilization in city planning and optimize resource allocation in city management.The paper has offered a city fractal planning model, in which fractal mechanism is applied. The planning process includes an iterative process and a fractal process. Iterative process is a planning process of space layout with the characteristics of similarity on the same hierarchy, whereas fractal process is a recursive iterative process, which realizes hierarchies of fractal structure. With the interaction of two processes, city space is continuously plotted and fractal, it finally forms a fractal city with self-similarity across scales. The paper has further compared similarities and differences between virtual fractal city model and central place system, and validated that the virtual fractal city obeys Zipf's law. Virtual fractal city model has put forward a creative method for city modeling and offers both theoretical and practical significance to study of city phenomena and city planning.2. Study on Self-Organized City and City's Self-Organized CriticalitySince another characteristic of complexity is self-organized, city is considered to be a complex system, and meanwhile it is also called a Self-organized system. There are many self-organized mechanism in a city system, and relationship among these mechanism is very complex. The paper combines Hoteling model and DLA model to construct city's forming and evolving model based on self-organized mechanism of space entities'site-selection. Contrary to fractal planning process, self-organized process is a bottom-up process. In spite of the fact that the two processes are distinctly different, they achieve the same result in simulating cities'phenomenon.The self-organized process in our model has used an evolvement mechanism of city space layout which is based on actions of individual's site-selection. Individuals choose their behaviors (transition, migration, or stand) according to their circumstances. Virtual city's space layout is a result of all individual's decisions. With the aid of multi-agent technology, we can build corresponding computer model to simulate the process, and analyze the simulating results. Comparing with space layout features of real city, we can study long-term developing trend of city system.Through the analysis of results, we can draw many conclusions about circumstance factors influencing city's development. For instance, when system develops to a balanced state, we can observe habitant-site and commerce-site are assembling together, and assembling degree of commerce-site is greater than that of habitant-site. Furthermore, the more prosperous city's commerce is, the weaker city's stability is, and vice versa. The paper has validated the theoretical analysis through analysis of results of simulating experiments. In the mean time, it has also validated that virtual city model is in the self-organized critical state.3. Study on Formation and Evolvement of Road NetworkComplex network is an important field in the complex science. The characteristic of complex network includes small world, cluster, and power-law of degree distribution. Animal's blood vessel, neural system, plant's twig and vein, water system of rivers are all complex network. City traffic system can also be treated as a complex network. Based on J. Sun model, this paper has put forward a traffic auto-generation system model. The model can automatically generate new road system according to local traffic situation. The demand for traffic determines the magnitude of road network. The demand for local roads is positively correlated with population density, while the existing road layout has an impact on the distribution of population since people will move towards main lines of communication. The interaction between road layout and population density causes the traffic system to be in a dynamic evolving state. The forming and evolving law of cities'road networks can be studies with the help of model.Comparing with 5 features of complex network, the paper has got some revelations about city road network programming:â‘ Current road system has its own inner rationality. City road network planner should make the most of current road system to open up or rebuild road sections based on old road. One can use complex network model to simulate traffic and analyze load ability, flux, undulation of road system to get more knowledge of current road system.;â‘¡It needs to find the balanced relations between population distribution and road network layout;â‘¢Hierarchy in road system embodies the balance between resource utility and efficiency.;â‘£Bionic model of Animal's blood vessel or plant vein can be employed in planning logistics.4. Study on City EcosystemThe paper use a metaphor that city is a kind of life-form. City system is a city ecosystem in which there exist mechanisms of competition and cooperation among cities. The concept of ecological niche can help us to analyze strategy of city development and evolvement, such strategy as city development scale and city feature orientation. Cities'ecological niche is a result of a long evolvement and each ecological niche has formed an interdependent service chain, by which substance, energy and information are passed. City planner should describe the city system's ecological niche map and determine the position in which he is, in order to figure out strategy of city development. Every city should have its own ecological niche, namely survival room. If development of a city deviates from its ecological niche, malignant competitions among cities and cities'abnormal development will yield.It has been a hot topic for many years to apply complex science to the study of city space structure and city management. The topic still remains great significant at present, since many cities in our country are all the while in a dynamic state. These cities'developing orientations are not clear and city space structures keep changing to balance the confliction of utility of all kinds of resource. Therefore, study on law of city development is of great importance to planning and management of cities.