At the beginning of the 1980s, the Central Place Theory(CPT) was introduced into China from the West. The authors of this article are the initiators of applying this model to China's urban and regional planning as...At the beginning of the 1980s, the Central Place Theory(CPT) was introduced into China from the West. The authors of this article are the initiators of applying this model to China's urban and regional planning as well as to the allocation of commercial activities. Since the commodity economy has been developing in China for thousands of years,most of the areas in eastern plains and inland basins of the country show the spatial structure of typical central places scene. During some six years, the authors participatd in a lot of rural, urban and regional planning projects in which the spatial phenomena and relevant internal mechanism proved this medel and its deformation. This paper gives only a few examples of the application of CPT in China. 1. The distribution of rural markets: the empirical study is based mainly on the situation in Shengfang Town and its vicinal area, 2. The spatial combination of urban Systems; problems about city allocation in North China are emphasized, 3. Commercial centres in city and the networks: a crucial study on commercial distribution of the metropolis Beijing is done by using the central place model.展开更多
The UWA channel is characterized as a time-dispersive rapidly fading channel, which in addition exhibits Doppler instabilities and limited bandwidth. To eliminate inter- symbol interference caused by multipath propaga...The UWA channel is characterized as a time-dispersive rapidly fading channel, which in addition exhibits Doppler instabilities and limited bandwidth. To eliminate inter- symbol interference caused by multipath propagation, spatial diversity equalization is the main technical means. The paper combines the passive phase conjugation and spatial processing to maximize the output array gain. It uses signal-to-noise-plus-interference to evaluate the quality of signals received at different channels. The amplitude of signal is weighted using Sigmoid function. Second order PLL can trace the phase variation caused by channel, so the signal can be accumulated in the same phase. The signals received at different channels need to be normal- ized. It adopts fractional-decision feedback diversity equalizer （FDFDE） and achieves diversity equalization by using different channel weighted coefficients. The simulation and lake trial data processing results show that, the optimized diversity receiving equalization algorithm can im- prove communication system＇s ability in tracking the change of underwater acoustic channel, offset the impact of multipath and noise and improve the performance of communication system. The performance of the communication receiving system is better than that of the equal gain combination. At the same time, the bit error rate （BER） reduces 1.8%.展开更多
Land use/cover change(LUCC)and climate change have important influences on ecosystem services(ESs)and their interactions,particularly in regions with rapid socioeconomic development.However,little research has disting...Land use/cover change(LUCC)and climate change have important influences on ecosystem services(ESs)and their interactions,particularly in regions with rapid socioeconomic development.However,little research has distinguished the impacts of these 2 factors on ES interactions.Therefore,the impacts of LUCC and climate change on water-related ecosystem service(WES)trade-offs in the Yangtze River Economic Belt(YREB)were measured by combining spatial analysis with ES valuation.These results indicated that water yield,soil retention,and water purification presented varying degrees of increase from 1990 to 2020,with rates of 4.53%,21.80%and 5.40%,respectively.The order of the mean WES tradeoffs in the upstream,midstream,and downstream regions remained stable at the grid scale from 1990 to 2020,while there were important changes at the county scale.Climate change had a greater impact on WES trade-offs than LUCC,with climate change dominating 88.17%of the total area.The effect of climate change on WES trade-offs across areas was in the order of downstream>midstream>upstream,while the effect of LUCC was not obvious.The relative importance of drivers on WES trade-offs from 1990 to 2020 was dominated by climate dominance(71.50%),followed by LUCC to climate dominance(13.66%)and least by LUCC dominance(0.65%).This research emphasized the importance of LUCC and climate change on ES trade-offs in heavily disturbed areas,providing important guidance for multi-objective land management that sustainably provides ESs.展开更多
By the biological construction of a bird neck,a bionic bird-neck multilevel rigid-flexible structure is proposed and some biometric properties are explained.The proposed structure can flexibly deform in six directions...By the biological construction of a bird neck,a bionic bird-neck multilevel rigid-flexible structure is proposed and some biometric properties are explained.The proposed structure can flexibly deform in six directions,which inspires the study of its mechanical properties for flexible deformations.First,the structural configuration and composition are determined based on the study of the anatomical characteristics of the woodpeckers.Since the skeletons and muscles have very different values for the elasticity modulus and the deformation is mostly dependent on the muscle tension,the bionic structure consists of rigid units and bio-syncretic components.For combined deformations,the mechanical model is established by the connectivity matrix to describe the connection of each level.Second,based on the principle of minimum potential energy,an integral form-finding method is proposed for flexible combination deformations.All of the integral forms obtained with the theoretical analysis are compared with the results with Finite Element Analysis.The structural parameters of the bionic structure were then tightly fixed to the actual shape of the bird’s neck and the corresponding overall form took on an"S"shape,which perfectly matched the construction of the bird’s neck.In addition,for the pre-deformation form,by analyzing the potential energy of the bionic structure,due to the adjustable dynamic stiffness property,an explanation is provided for the significant dynamic stability of the bird neck in bending.This study not only proposes a bionic rigid-flexible structure with high spatial accessibility but also explains biological properties of a bird neck based on the study of its mechanics characteristics.Based on the modeling and the mechanical properties of the bionic structure in flexible spatial combination deformations,the multi-steady state,and the variable dynamic stiffness,the bird-neck bionic rigid-flexible structure has significant applications such as aeronautical deployable systems,manipulator positioning,and dynamic stability fields.展开更多
文摘At the beginning of the 1980s, the Central Place Theory(CPT) was introduced into China from the West. The authors of this article are the initiators of applying this model to China's urban and regional planning as well as to the allocation of commercial activities. Since the commodity economy has been developing in China for thousands of years,most of the areas in eastern plains and inland basins of the country show the spatial structure of typical central places scene. During some six years, the authors participatd in a lot of rural, urban and regional planning projects in which the spatial phenomena and relevant internal mechanism proved this medel and its deformation. This paper gives only a few examples of the application of CPT in China. 1. The distribution of rural markets: the empirical study is based mainly on the situation in Shengfang Town and its vicinal area, 2. The spatial combination of urban Systems; problems about city allocation in North China are emphasized, 3. Commercial centres in city and the networks: a crucial study on commercial distribution of the metropolis Beijing is done by using the central place model.
基金supported by National Natural Science Foundation of China(61531018)
文摘The UWA channel is characterized as a time-dispersive rapidly fading channel, which in addition exhibits Doppler instabilities and limited bandwidth. To eliminate inter- symbol interference caused by multipath propagation, spatial diversity equalization is the main technical means. The paper combines the passive phase conjugation and spatial processing to maximize the output array gain. It uses signal-to-noise-plus-interference to evaluate the quality of signals received at different channels. The amplitude of signal is weighted using Sigmoid function. Second order PLL can trace the phase variation caused by channel, so the signal can be accumulated in the same phase. The signals received at different channels need to be normal- ized. It adopts fractional-decision feedback diversity equalizer （FDFDE） and achieves diversity equalization by using different channel weighted coefficients. The simulation and lake trial data processing results show that, the optimized diversity receiving equalization algorithm can im- prove communication system＇s ability in tracking the change of underwater acoustic channel, offset the impact of multipath and noise and improve the performance of communication system. The performance of the communication receiving system is better than that of the equal gain combination. At the same time, the bit error rate （BER） reduces 1.8%.
基金supported by the Youth Innovation Promotion Association of the Chinese Academy of Sciences(grant number 2021194)
文摘Land use/cover change(LUCC)and climate change have important influences on ecosystem services(ESs)and their interactions,particularly in regions with rapid socioeconomic development.However,little research has distinguished the impacts of these 2 factors on ES interactions.Therefore,the impacts of LUCC and climate change on water-related ecosystem service(WES)trade-offs in the Yangtze River Economic Belt(YREB)were measured by combining spatial analysis with ES valuation.These results indicated that water yield,soil retention,and water purification presented varying degrees of increase from 1990 to 2020,with rates of 4.53%,21.80%and 5.40%,respectively.The order of the mean WES tradeoffs in the upstream,midstream,and downstream regions remained stable at the grid scale from 1990 to 2020,while there were important changes at the county scale.Climate change had a greater impact on WES trade-offs than LUCC,with climate change dominating 88.17%of the total area.The effect of climate change on WES trade-offs across areas was in the order of downstream>midstream>upstream,while the effect of LUCC was not obvious.The relative importance of drivers on WES trade-offs from 1990 to 2020 was dominated by climate dominance(71.50%),followed by LUCC to climate dominance(13.66%)and least by LUCC dominance(0.65%).This research emphasized the importance of LUCC and climate change on ES trade-offs in heavily disturbed areas,providing important guidance for multi-objective land management that sustainably provides ESs.
基金the National Natural Science Foundation of China(12122208,11972254 and 11932015).
文摘By the biological construction of a bird neck,a bionic bird-neck multilevel rigid-flexible structure is proposed and some biometric properties are explained.The proposed structure can flexibly deform in six directions,which inspires the study of its mechanical properties for flexible deformations.First,the structural configuration and composition are determined based on the study of the anatomical characteristics of the woodpeckers.Since the skeletons and muscles have very different values for the elasticity modulus and the deformation is mostly dependent on the muscle tension,the bionic structure consists of rigid units and bio-syncretic components.For combined deformations,the mechanical model is established by the connectivity matrix to describe the connection of each level.Second,based on the principle of minimum potential energy,an integral form-finding method is proposed for flexible combination deformations.All of the integral forms obtained with the theoretical analysis are compared with the results with Finite Element Analysis.The structural parameters of the bionic structure were then tightly fixed to the actual shape of the bird’s neck and the corresponding overall form took on an"S"shape,which perfectly matched the construction of the bird’s neck.In addition,for the pre-deformation form,by analyzing the potential energy of the bionic structure,due to the adjustable dynamic stiffness property,an explanation is provided for the significant dynamic stability of the bird neck in bending.This study not only proposes a bionic rigid-flexible structure with high spatial accessibility but also explains biological properties of a bird neck based on the study of its mechanics characteristics.Based on the modeling and the mechanical properties of the bionic structure in flexible spatial combination deformations,the multi-steady state,and the variable dynamic stiffness,the bird-neck bionic rigid-flexible structure has significant applications such as aeronautical deployable systems,manipulator positioning,and dynamic stability fields.
