简介:摘要网络教育越来越广泛地应用于各种教育领域,Web课件的制作是当前网络化CAI的重点之一。本文先介绍了Web课件具有共享性、即时更新性、时空不限性、人机优势互补性、整合性、结构灵活性等特点,并从网络CAI课件的特点出发,结合现代教育理念,阐述了Web课件制作的一般方法和过程。
简介:为了满足面向服务的空间数据框架对空间访问控制的需求,提出多粒度的时空相关访问控制模型MSTAC。此模型在基于角色的访问控制模型基础上,进行属性约束扩展。属性约束包括上下文时间属性、用户的位置属性、角色的时间属性约束、地图类的图层向量约束、图层的尺度及制图时间约束、地物要素间的拓扑约束、地物要素的语义属性约束以及要素视图的字段约束。通过此模型,授权用户将受控访问不同粒度的空间数据集。这些粒度包括地图粒度、图层粒度、要素对象粒度和要素视图粒度。最后,将MSTAC模型在webGIS中实施。该实例显示了在不同的数据粒度上和不同的时间段内,系统可以对不同粒度服务进行肯定和否定授权。
简介:Theobjectiveofthisworkistoanalyzethefatiguereliabilityofcomplexweldedstructurescomposedofmultipleweb-framejoints,accountingforcorrelationeffects.Athree-dimensionalfiniteelementmodelusingthe20-nodesolidelementsisgenerated.Alinearelasticfiniteelementanalysiswasperformed,hotspotstressesinaweb-framejointwereanalyzedandfatiguedamagewasquantifiedemployingtheS-Napproach.Thestatisticaldescriptorsofthefatiguelifeofanon-correlatedweb-framejointcontainingseveralcriticalhotspotswereestimated.Thefatiguereliabilityofaweb-framejointwasmodeledasaseriessystemofcorrelatedcomponentsusingtheDitlevsenbounds.Thefatiguereliabilityoftheentireweldedstructurewithmultipleweb-framejoints,modeledasaparallelsystemofnon-correlatedweb-framejointswasalsocalculated.
简介:Soilfoodwebstructureisfundamentaltoecosystemprocessandfunction;moststudiesonsoilfoodwebstructurehavefocusedonagro-ecosystemsunderdifferentmanagementpracticesandnaturalterrestrialecosystems,butseldomongreenhouses.ThisstudyexploredthestaticandtemporalvariabilityofsoilfoodstructureintwogreenhousesofShandongProvince,NorthChinaoveratwo-yearperiod.Thestaticpropertiesweremeasureddirectlybysurveyingfunctionalgroupcompositionandaseriesofparametersportrayingthespeciesproperties,linkproperties,chainpropertiesandomnivorypropertiesoftheweb,aswellasindirectlythroughcalculationofnematodeindices,enrichmentindex(EI),structureindex(SI),andchannelindex(CI).Thedynamicvariabilityofgreenhousesoilfoodstructurewasdescribedbythedynamicsoffunctionalgroups,Bray-Curtis(BC)similarityandclusteranalysis.Theresultsshowedthatthegreenhousesoilfoodwebcontained14functionalgroups,withmicrobeshavingthehighestmeanbiomass,followedbyprotozoa.Ofthethreefunctionalgroupsofprotozoa,flagellateswerethedominantgrouponmostsamplingdates,amoebaeonlybecamethedominantgroupduringthesummer,whileciliatesweretheleastprevalentgroup.Allnematodeswereassignedintooneofthefourfunctionalgroups,bacterivorous,fungivorous,herbivorousandomnivorous,andthefungivorousnematodeshadthelowestmeanbiomass.Miteswereassignedintothreefunctionalgroupsandtheomnivorousnoncryptostigmaticmiteswerethedominantgroup.Allthefunctionalgroupsshowedsignificantseasonalchanges.Thesoilfoodwebconnectancewas0.15,themaximumfoodchainlengthwas5,andtheaveragefoodchainlengthwas3.6.TheprofilesoftheEIandSIshowedthatthefoodwebwasresourcedepletedwithminimalstructure.TheresultsofCIindicatedthatthebacterialdecompositionpathwaywasthedominantpathwayinthefoodwebofthegreenhousesoilsstudiedandtheresultsofBCsimilarityshowedthatthesoilfoodwebhadhig
简介:为满足天马望远镜Q波段(35-50GHz)双波束致冷接收机的需求,介绍了一种Q波段宽带圆极化器的设计。圆极化器采用90°移相器与正交模式转换器组合的方式,其中,90°移相器利用双壁波纹移相结构,仿真结果表明,工作带宽内可以达到90°±3°;正交模式转换器采用十字转门结构,仿真结果表明各端口反射系数好于-25dB,输出端口隔离度好于-60dB。文章还给出了对实际加工90°移相器与正交模式转换器具有指导意义的容差分析,并且给出了90°移相器与正交模式转换器组合的仿真结果。组合而成的圆极化器性能满足天马望远镜Q波段致冷接收机的设计指标:端口反射系数好于-20dB,输出端口隔离度好于-32dB。