历史时期黄土高原的植被及其变迁

距今３０００－２０００年前，黄土高原六盘山以东，吕梁山以西，长城以南地区显域生境植被为疏林灌丛草原，并存在一定的区域差异唐宋时期，黄土高原的植被表现出显著的变化，主要是草原带的南移和某些耐旱植物分布范围的扩大。自北宋以后，人类活动对植被的破坏越来越强烈。掩盖了植被由于自然原因发生的变化。     

闽赣自然植被覆盖格局及其驱动机制分析

自然植被覆盖格局的研究对生态环境可持续发展具有十分重要的意义。以福建和江西2省为研究区,基于30 m分辨率的植被指数数据,运用地理加权回归模型,结合自然环境和人类活动因素,探索分析闽赣地区自然植被覆盖格局特征及其驱动机制。研究表明:①地形是影响闽赣自然植被覆盖程度的主要因素,江西和福建2省自然植被覆盖程度分别在高程[100,300)m和[300,500)m区间,与高程相关性最大;在坡度25°和10°以上,与坡度的相关性减弱。②江西主干河流附近农田和城镇交错,福建主干河流附近城镇和林地嵌套,2省自然植被覆盖程度与距河流距离的相关性差异显著。③距离城镇居民点5 km和4 km分别为江西和福建2省自然植被覆盖程度与距居民点距离相关性最弱处。     

SWAT模型的参数灵敏度分析——以贡嘎山海螺沟不同植被类型流域为例

目前分布式水文模型在我国各种流域的水文过程模拟中已得到广泛的应用,为了深入了解不同分布式水文模型参数在水文循环过程中的影响程度,文中针对贡嘎山海螺沟4种不同植被类型流域,采用扰动分析方法定义模型参数的灵敏度并根据灵敏度的取值分为5级,基于SWAT(Soil&Water Assessment Tool)分布式水文模型研究了模型参数灵敏度在年内夏季和冬季的变化及其对水文过程模拟结果的影响。计算结果表明:冬季模型参数的灵敏度普遍偏高,会出现枯水期径流过程模拟误差较大的现象,而夏季模型参数的灵敏度相对降低;但在同时模拟汛期和枯期的径流过程情况下,前者的误差会小于后者。通过对SWAT模型参数在不同植被类型流域的灵敏度分析,将会有助于减少水文模拟的不确定性或能获得更好的估计结果。     

黑河流域生态输水对下游植被变化影响研究

为挽救黑河下游额济纳地区严重退化的生态环境，本世纪初开始向下游实施生态输水工程。本文通过对2000-2009年的MODIS/NDVI序列分析，揭示了生态输水以来额济纳地区植被时空变化格局；基于不同时空分辨率的遥感影像和实地考察资料，分析了植被变化空间差异的主要原因。研究表明：（1）研究区内80.4 %的绿洲区植被和91.5 %的荒漠区植被呈恢复趋势。绿洲区内年累积植被指数序列趋势度大于0.14 a-1 的区域都存在农田开垦或弃耕地复播现象；荒漠植被的显著恢复主要发生在西河中、下游和东河下游地区。（2）研究区内19.6 %的绿洲区植被和5.1 %的荒漠区植被进一步退化，退化区主要分布在衬砌渠系两侧和一些干涸或径流减少的自然河流两岸，植被退化主要是乔木林的退化。（3）大范围的植被恢复主要原因在于地下水环境的整体好转，以及生态保护措施的实施；地下水环境的整体好转取决于黑河中游来水量的增加和下游输水方案的实施。（4）研究区植被退化的直接原因在于目前输水方案实施引起的局部地表水环境恶化。基于植被动态监测和局部植被对全局生态的重要性评价，确定合理的输水方案，对于区域生态环境演变进入良性循环意义重大。     

Effect of Additives on Melting Point of LATS Refining Ladle Slag

  To avoid slag sticking on the ladle immersion cover during the LATS refining and alloying process, the effect of Al2O3 on the melting point of the ladle slag was studied and the additives including CaF2, B2O3, Li2O, and CaO were used to decrease the melting point of the ladle slag. The melting point was measured using the hemisphere method. The results show that the addition of Al2O3 to the ladle slag increases the melting point. The fluxing action is not remarkable if only CaF2 or CaO is used as the additive. The fluxing action of the composite additive obtained by the mixing of CaO and CaF2 in the mass proportion of wCaO∶wCaF2=2∶1 is preferred. The fluxing action of B2O3 is also notable. When the B2O3 content in mass percent is in the range from 2% to 10%, the corresponding melting point is 1 380 ℃ to 1 290 ℃. The fluxing action of Li2O is the most remarkable. When the Li2O content is up to 5%, the melting point of the slag is lower than 1 300 ℃.     

飞行平台罩爆炸拉深实验研究及数值模拟

在简单介绍爆炸拉深基本原理的基础上应用ANSYS软件中的LS-DYNA模块,对具有复杂曲面的航天飞行平台罩建立了显性动力学弹塑性力学模型,并做了相应简化后,成功地进行了爆炸拉深成形过程的有限元数值模拟,获得了爆炸变形过程中不同时刻的应力、应变分布规律,以及变形过程中的危险截面等重要结果.该模拟结果与实验结果基本一致,对制定工艺参数、改进模具结构和坯料尺寸提供了重要依据.     

散热器盖压铸模设计

介绍了散热器盖铸件的成型工艺,分型面的选择,浇注系统设计,脱模方式设计对铸件质量的影响分析,以及模具的总装结构设计。     

地铁车站盖挖逆作钢管立柱受力机理及工程应用

以深圳地铁11号线与6号线换乘站松岗站的盖挖逆作法施工为工程研究对象,在介绍盖挖逆作设计方案的基础上,对钢管立柱—围护结构体系进行了受力分析计算,提出了盖挖钢管立柱施工方案,分析了施工的重点难点,最后提出了有效的施工处置措施。     

Meso-scale numerical investigation on cracking of cover concrete induced by corrosion of reinforcing steel

Concrete cover cracking induced by corrosion of steel reinforcement is a major influencing factor for durability and serviceability of reinforced concrete structures. Here in this study, the influence of concrete meso-structure on the failure pattern of concrete cover is accounted for. The concrete is assumed to be a three-phase composite composed of aggregate, mortar matrix and the interfacial transition zone (ITZ). And a concrete random aggregate structure is established for the study on the mechanical behavior of radial corrosion expansion. In the present simulations, the plasticity damaged model is used to describe the mechanical behavior of the mortar matrix and the ITZ, and it is assumed that the corrosion of steel reinforcement is uniform. The cracking of concrete cover due to steel reinforcement corrosion is numerically simulated. The simulation results have a good agreement with the available test data and they are between the two analytical results. The failure patterns obtained from the macro-scale homogeneous model and the meso-scale heterogeneous model are compared. Furthermore, the influences of ratio of cover thickness and reinforcement diameter (i.e. c/d), the location of the steel reinforcement (i.e., placed at the middle and corner zones) and the concrete tensile strength on the steel corrosion rate when the concrete cover cracks are investigated. Finally, some useful conclusions are drawn.     

Single Machine Precedence Constrained Scheduling Is a Vertex Cover Problem

In this paper we study the single machine precedence constrained scheduling problem of minimizing the sum of weighted completion time. Specifically, we settle an open problem first raised by Chudak and Hochbaum and whose answer was subsequently conjectured by Correa and Schulz. As shown by Correa and Schulz, the proof of this conjecture implies that the addressed scheduling problem is a special case of the vertex cover problem. This means that previous results for the scheduling problem can be explained, and in some cases improved, by means of vertex cover theory. For example, the conjecture implies the existence of a polynomial time algorithm for the special case of two-dimensional partial orders. This considerably extends Lawler’s result from 1978 for series-parallel orders.