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Search Results: 1 - 10 of 464926 matches for " <br>张娜 "
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Simulating the spatial pattern of annual mean evapotranspiration of a heterogeneous landscape
异质景观年平均蒸发量空间格局模拟

ZHANG Na,<br>
生态学报 , 2004,
Abstract: Understanding how landscape pattern affects ecosystem processes is a central issue in landscape ecology. In this study, we examined the effects of landscape heterogeneity on ecosystem evapotranspiration in the Changbai Mountain Nature Reserve in northeastern China, using a process-based, spatially explicit model, EPPML (Ecosystem Productivity Process-based Model at Landscape Scale). We developed EPPML to investigate the spatiotemporal patterns of carbon and water cycling and ecosystem productivity, and th...
Scale issues in ecology: upscaling
生态学中的尺度问题——尺度上推

ZHANG Na,<br>
生态学报 , 2007,
Abstract: Scaling means transferring information between or across spatial and temporal scales or organizational levels. Transferring from finer scale to broader scale is called as upscaling, and transferring from broader scale to finer scale is called as downscaling. In both basic ecology and its applications, scaling is the essence of predicting and understanding a phenomenon, and is at the core of ecological theories and applications. However, scaling is often very complex because scaling has to overcome constraints and critical thresholds between different systems, non-linear interactions among different components always occur within the same scale and among different scales, and especially spatial heterogeneity always exists. Therefore, scaling across heterogeneous ecosystems remains an unresolved puzzle, greatly challenging current ecologists devoting to studying environmental problems under global change. As a whole, four general scaling approaches can be distinguished: spatial analysis, similarity-based scaling, local dynamic model-based scaling, and random model approaches. Spatial analysis approach is based on spatial pattern analyzing, such as fractal and wavelet analysis method. Similarity-based scaling approach is an important approach and has been widely used in physics, earth science, hydrology, meteorology, and biology. The similarity-based biological allometry reveals the relationships between biological characteristics and body sizes, while spatial allometry reveals the relationships between landscape characteristics (such as species richness, natural river network, landform features, ecological variables, and landscape metrics) and spatial scales. The simple power law in allometric relations might be the integration of extremely complex underlying processes and mechanisms, possibly related to ubiquitous fractal structure of biological body and landscape, but the validation of this hypothesis will has to be conducted in developing scaling theory. However, allometric relations may only exist within a limited range of scales, beyond which some new processes will occur and the allometric relations at these scales cannot be extrapolated to other scales. Three key problems have to be addressed in local dynamic model-based approach: building a dynamic model at local scale, accurately defining and quantifying spatial heterogeneity of model parameters and input variables at local scale, aggregating or integrating heterogeneous information of output variables at local scale to object scale. The differences in both quantifying heterogeneity and aggregating information decide the merits and lacks of each method within this approach. Firstly, if scaling is conducted between adjacent scales, and the interactions among different spatial units can be ignored, lumping, extrapolation by effective parameters, direct extrapolation, extrapolation by expected value, and explicit integration methods can be used. Secondly, if scaling is conducted between adjac
Scale issues in ecology: concepts of scale and scale analysis
生态学中的尺度问题:内涵与分析方法

ZHANG Na,<br>
生态学报 , 2006,
Abstract: Scale issues are fundamental to all ecological investigations,and have become a central topic in ecology in recent decades with the increasing recognition of broad-scale environmental issues(such as global warming) and land-management problems,rapid development of digital technologies(remote sensing,GIS, desktop computers,etc.),and emergence of landscape ecology.However,there are still many ecological studies in which scale is treated simplistically or superficially.Scale issues remain a key challenge for ecologists in the 21~(st) century.Scale issues include conceptualization of scale,scale analysis and scaling.This paper focuses on the former two issues.A three-tiered conceptualization of scale is introduced: dimensions,kinds,and components.Dimensions of scale are most general,including space,time,and organizational levels.Kings of scales can be distinguished among phenomenon scale(including structure and process scale,also referred to as characteristic or intrinsic scale),observation scale(also referred to as sampling or measurement scale),analysis or modeling scale and policy scale.The most specific and measurable definitions are components of scale,including grain,extent,spacing(or lag),coverage,cartographic scale,and support.Scale is often expressed as grain and extent in landscape ecology.Because each type of scale concept includes multiple terms and definitions, it is necessary to carefully discriminate,classify and unify them.Scale analysis is to analyze scale effects and identify multiple-scale spatial patterns(especially characteristic scale(s)).Scale effects may occur in each of the following three situations: changing grain size only,changing extent only,and changing both grain and extent.When scale of observation,analysis,modeling,or experimentation change,statistical results(for example,mean,variance and multivariate relationships) are expected to change.Scale influences the results of examining spatial pattern,such as spatial heterogeneity,spatial distribution,spatial autocorrelation,spatial anisotropy,and patch and gap sizes. When scale changes,new ecological processes and patterns may occur,and the rate or frequency of processes,control factors,and correlations between processes may also change.Some landscape properties(for example,landscape openness, equilibrium,predictability,species richness and diversity) also exhibit scale effects.Because of scale effects,it is extremely important to conduct research at multiple scales.Identifying characteristic scale(s) means examining hierarchical structure of pattern or process,and distribution of patch sizes and spacing between patches,which are the basis for studying scale effects and scaling.Some specific methods need to be developed for scale analysis,mainly including spatial statistics methods,landscape metrics and fractal analysis.Landscape metrics are the most popular methods which are both simple and potentially misleading.Spatial statistics methods and fractal analysis have been
不同职业路径下基金经理变更对基金业绩的影响
THE IMPACT OF FUND MANAGER CHANGES ON THE PERFORMANCE OF MUTUAL FUNDS FROM DIFFERENT CAREER PATH#br#

,杨佳佳,田澍#br#
- , 2018,
Abstract: 摘要 本文对2003—2013年基金经理更换事件采用事件分析和面板数据回归进行研究。研究发现基金经理更换后基金业绩得到显著提升,此结论在牛市更显著。当基金历史业绩较好时更换基金经理对业绩没有影响;而当历史业绩较差时更换基金经理则会显著提升业绩。从职业发展路径来看,基金管理经验能有效提升基金经理的投资能力;之前具备资管经验的继任基金经理能够显著提高业绩;并且管理经验来自同一基金公司的继任基金经理更能改善业绩。
Abstract: This paper analyzes all mutual fund manager changes during 2003 to 2013 in China by using events study method and panel regressions We find that manager changes can improve the fund performance in the medium term, and this result is robust in both stock market boom and recession situations There is a significantly positive effect of manager changes on fund performance when the historical fund performance is below the market When the historical fund performance is above the market, there is no significant effect of manager changes on fund performance Regarding the manager career path, this study suggests that previous fund managing experience can improve managers ability New managers with previous fund managing experience are able to improve fund performance significantly while those without previous experience cant Last, the previous fund managing experience from the same mutual fund company can bring the improvement quicker than outside fund managing experience
The effect of arid climate on the structure and above-ground growth of Bothrichloa ischaemum community
干旱气候对白羊草群落土壤水分和地上部生长的初步观察

ZHANG Na,LIANG Yi-Min,<br>
生态学报 , 2000,
Abstract: 对大旱之年白羊草(Bothrichloa ischaemum)群落的善上壤水分状况和地上部的生长状况进行了研究。结果表明,白羊草群落在整个生长期都处于水分严重亏缺状态,植物生长被大大地抑制。种类少而单调,群落地上生物量、高度和盖度受上壤水分的影响沿升-降-升-降的方向变化。地上年净初级生产量仅为83.8g/m^2,比正常年份低2~4倍。干物质含量则显著高于正常年份。
Metadata Registries: Basic Concepts and Structure
元数据登记系统:基本概念与基本结构*

Zhang Xiaolin Liang Na,<br>晓林,
现代图书情报技术 , 2003,
Abstract: The paper describes the roles, categories, and basic functional structures of metadata registries, and explores ISO/IEC11179 standards as the basic supporting framework for metadata registries.
Hydrogen production by an anaerobic sequencing batch reactor (ASBR)
厌氧序批式反应器产氢

Zhang Na,Yuan Linjiang,<br>,袁林江
环境工程学报 , 2012,
Abstract: 以啤酒厂废水处理厂UASB中的厌氧污泥为种泥,葡萄糖为基质,研究了厌氧序批式反应器产氢。控制反应器内pH为4.0~4.5,温度为(36±1)℃,水力停留时间为8 h,当进水葡萄糖浓度为4 000 mg/L,容积负荷为12 kg/(m3.d)条件下,该厌氧序批式反应器实现了连续高效厌氧产氢。生物气中的氢气含量约为48%~53%,基质产氢率为1.1 mol/mol葡萄糖,COD去除率为15%~25%,最大比产氢速率为84.5 mol/(kg VSS.d)。液相末端发酵产物中乙醇和乙酸的含量占液相末端发酵产物总量的80%以上,表明该反应器内进行的是乙醇型发酵厌氧产氢。厌氧序批式反应器完全可以实现连续高效厌氧产氢,比较适用于日处理量较小的高浓度含糖废水。
MEASUREMENT OF WELDING RESIDUAL STRESS IN THE HEAVY CONVERTER USING BLIND-HOAL METHORD
基于大型炉体焊接残余应力测试的盲孔法研究

ZHANG Xiaopeng,WANG Na,<br>王,小鹏
力学与实践 , 2007,
Abstract: To enhance the measurement accuracy for welding residual stress in the converter,the traditional blind-hole method is improved by widening hole and increasing depth more than once after drilling hole on one measuring point.Strain relief parameters A and B for welding residual stress of Steel SM400ZL measured by the blind-hole method are verified by test,applying multi-group strain relief parameters for calculation.Welding residual stress estimations in different conditions are obtained,and compared with test results.In the force tensile test,it is found that the plastic deformation around the blind hole influences the measurement precision and plastic correction formulation.The tests show that the improved blind-hole method could measure the residual stress more accurately,and by plastic correction the largest measurement error is reduced with in 2%. The test method and results can be directly used in welding residual stress measurement of Steel SM400ZL.
高速列车经过跨线天桥脉动风压分析
Fluctuating wind pressure analysis of the bridge while High-speed train passing by

,,<br>YANG Na, ZHANG Shuai
- , 2017,
Abstract: 高速列车通过跨线天桥结构时在天桥与高速列车之间的狭小空间内产生较大的局部风压,跨线天桥结构对局部风压的高频脉动成分极为敏感。为了探究脉动成分的大小,采用CFD数值模拟,运用滑移网格技术,大涡模拟湍流模型(LES)对高速列车经过跨线天桥结构周围的三维、非定常、可压缩流场进行模拟,并与跨线天桥实测结果吻合。对保留脉动成分的LES模拟结果在时域上进行分析,探究脉动成分分布的位置,脉动极值空间分布,以及脉动风能量在列车风压中所占比例,同时在频域上采用小波包分析的方法对LES结果进行分解,分析各频段风压能量分布规律。研究结果表明:跨线天桥表面风压脉动成分极小。
High-speed train passing through the bridge structure causes a great local pressure in the small space between the bridge and the high-speed train, while the bridge structure across the line is extremely sensitive to the fluctuating wind pressure . In order to study the size of the fluctuating wind pressure, we simulate the three-dimensional, unsteady, compressible flow field while high-speed train passing through the bridge across the line using numerical simulation CFD, sliding mesh technique, large eddy simulation turbulence model (LES) and the results squares with the measured data. Via analyzing the LES simulation results in the time domain, we explore the position of the fluctuating part, pulsating extreme spatial distribution, as well as the proportion of fluctuating wind energy in total pressure energy.While in the frequency domain we decompose LES results using wavelet packet analysis, analyzing the energy distribution of the band pressure. The results showed that: the fluctuating wind pressure across the bridge is very small
面向青少年的机器人技术创新能力培养基地建设研究

祁伟, 李玉, 卢旭, 送英<br>祁 伟, 李玉, 卢 旭, 送英
- , 2016,
Abstract: “基于创造的学习”是青少年科技创新能力培养的原始动力.针对青少年科技创新能力训练缺少渠道,动手能力渐弱,对现代电子产品依赖成瘾等问题,有效利用大学实验室,借鉴大学生创新能力培养成熟经验,建设“面向青少年的机器人技术创新培养基地”,是青少年创新能力培养的有效渠道.本文利用大学科技含量较高的实验室平台及大学生科创成果,构建青少年创客平台,架起大学生与中小学生工程实践活动桥梁,以“基于创造的学习”为学生学习、创造途径,激发其主动思考、主动设计、主动制作,在创造过程中提升科学、技术、工程、数学、艺术等学科学习中的自信、创造力与兴趣.
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