三角洲岸线的变迁是研究地貌冲淤最直接的要素。黄河三角洲是世界上发育最快的三角洲,研究其海岸线变化规律和演化趋势对地区生态环境保护、海洋资源开发、基础设施建设等至关重要。本文在前人研究的基础上,采用修正归一化水体指数(Modi...三角洲岸线的变迁是研究地貌冲淤最直接的要素。黄河三角洲是世界上发育最快的三角洲,研究其海岸线变化规律和演化趋势对地区生态环境保护、海洋资源开发、基础设施建设等至关重要。本文在前人研究的基础上,采用修正归一化水体指数(Modified Normalized Difference WaterIndex,MND-WI)与多年水频率指数(Multi-year Water Frequency Index,MWFI)相结合的方法对1976–2021年期间典型年份的207幅遥感影像进行岸线提取,使获取到的海岸线更具科学性和代表性,并在此基础上通过定量计算分析了海岸线的时空演变及其稳定性特征,以此探讨黄河清水沟亚三角洲海岸线自1976年以来的演化机制。研究结果表明:(1)总体上,清水沟亚三角洲海岸线演化呈现先快速向海淤积后波动稳定的趋势,以1996年和2002年为节点划分为“快速发育”“缓慢发育”“动态平衡”3个阶段;(2)45年来,研究区海岸线稳定性持续增强,其中孤东海堤段岸线与黄河清水沟亚三角洲南部岸线保持相对稳定,而清8汊河口段岸线与清水沟废弃河口段岸线较为活跃,其岸线稳定性指数基本低于0.5;(3)陆上三角洲淤积、侵蚀中心的迁移同河口位置变动相对应,尤其是淤积中心的迁移与河口位置变动之间在经度向存在明显的正向关系,R^(2)=0.6904;(4)黄河入海泥沙减少、河口位置迁移以及人类活动对三角洲海岸线的发育演化影响显著。从长远来看,在黄河入海泥沙持续减少的背景下,三角洲的未来仍面临侵蚀的威胁。展开更多
Based on the microscopic phase-field model, ordered domain interfaces formed between D022 （Ni3V） phases along [001] direction in Ni75AlxV25-x alloys were simulated, and the effects of atomic structure on the migrati...Based on the microscopic phase-field model, ordered domain interfaces formed between D022 （Ni3V） phases along [001] direction in Ni75AlxV25-x alloys were simulated, and the effects of atomic structure on the migration characteristic and solute segregation of interfaces were studied. It is found that the migration ability is related to the atomic structure of interfaces, and three kinds of interfaces can migrate except the interface （001）//（002） which has the characteristic of L12 （Ni3Al） structure. V atoms jump to the nearest neighbor site and substitute for Ni, and vice versa. Because of the site selectivity behaviors of jumping atoms, the number of jumping atoms during the migration is the least and the jumping distance of atoms is the shortest among all possible modes, and the atomic structures of interfaces are unchanged before and after the migration. The preferences and degree of segregation or depletion of alloy elements are also related to the atomic structure of interface.展开更多
A lattice Boltzmann model of two dimensions is used to simulate the movement of a single rigid particle suspended in a pulsating flow in micro vessel The particle is as big as a red blood cell, and the micro vessel is...A lattice Boltzmann model of two dimensions is used to simulate the movement of a single rigid particle suspended in a pulsating flow in micro vessel The particle is as big as a red blood cell, and the micro vessel is four times as wide as the diameter of the particle. It is found that Segrd-Silberberg effect will not respond to the pulsation of the flow when the Reynolds number is relatively high. However, when the Reynolds number is low enough, Segrd-Silberberg effect disappears. In the steady flow, different initial position leads to different equilibrium positions. In a pulsating flow, different frequencies of pulsation also cause different equilibrium positions. Particularly, when the frequency of pulsation is closed to the human heart rate, Segrd-Silberberg effect presents again. The evolutions of velocity, rotation, and trajectory of the particle are investigated to find the dynamics of such abnormal phenomenon.展开更多
One of the greatest feats of avian migration is the non-stop crossing of extensive areas of inhospitable habitat such as deserts and seas. Differences in spring and autumn migration routes have been reported in specie...One of the greatest feats of avian migration is the non-stop crossing of extensive areas of inhospitable habitat such as deserts and seas. Differences in spring and autumn migration routes have been reported in species that cross such barriers, and are thought to have evolved in response to seasonal variation in prevailing wind direction. We tested the hypothesis that migration routes vary seasonally with respect to the Gulf of Mexico in the tree swallow Tachycineta bicolor using solar geolocators attached and retrieved at 4 breeding sites in central North America. We found that 100 % of birds （n = 10） made a trans-Gulf flight of 〉850 km from Louisiana south to their wintering grounds in the Yucatan Peninsula in 12-36 hours, achieving minimum ground speeds as high as 32 m/s. Although most days during autumn migration were characterized by unfavorable headwinds blowing to the northwest, migration over the Gulf mostly occurred on days with strong winds blowing to the south. In contrast, in 8 of 9 （88 %） birds on spring migration returned from the wintering grounds towards Louisiana following a clockwise loop pat- tern flying over land to the west around the Gulf. During this spring period there were few days with prevailing winds from the south to assist northward migration. Results suggest that, despite being up to three times further （ca. 2,700 kin）, a coastal circum-Gulf spring migration represents the less risky route when wind conditions are not favorable. These findings also help to resolve a long-standing dispute in the literature concerning migration patterns between the US Gulf coast and Mexico, and provide insight into the factors shaping migration strategies of small songbirds migrating across large bodies of water [Current Zoology 60（5）： 653-559, 2014].展开更多
基金National Natural Science Foundation of China (50671084 and 50875217)Natural Science Foundation of Shaanxi Province (2003E106, SJ08-ZT05)China Postdoctoral Science Foundation (20070420218)
文摘三角洲岸线的变迁是研究地貌冲淤最直接的要素。黄河三角洲是世界上发育最快的三角洲,研究其海岸线变化规律和演化趋势对地区生态环境保护、海洋资源开发、基础设施建设等至关重要。本文在前人研究的基础上,采用修正归一化水体指数(Modified Normalized Difference WaterIndex,MND-WI)与多年水频率指数(Multi-year Water Frequency Index,MWFI)相结合的方法对1976–2021年期间典型年份的207幅遥感影像进行岸线提取,使获取到的海岸线更具科学性和代表性,并在此基础上通过定量计算分析了海岸线的时空演变及其稳定性特征,以此探讨黄河清水沟亚三角洲海岸线自1976年以来的演化机制。研究结果表明:(1)总体上,清水沟亚三角洲海岸线演化呈现先快速向海淤积后波动稳定的趋势,以1996年和2002年为节点划分为“快速发育”“缓慢发育”“动态平衡”3个阶段;(2)45年来,研究区海岸线稳定性持续增强,其中孤东海堤段岸线与黄河清水沟亚三角洲南部岸线保持相对稳定,而清8汊河口段岸线与清水沟废弃河口段岸线较为活跃,其岸线稳定性指数基本低于0.5;(3)陆上三角洲淤积、侵蚀中心的迁移同河口位置变动相对应,尤其是淤积中心的迁移与河口位置变动之间在经度向存在明显的正向关系,R^(2)=0.6904;(4)黄河入海泥沙减少、河口位置迁移以及人类活动对三角洲海岸线的发育演化影响显著。从长远来看,在黄河入海泥沙持续减少的背景下,三角洲的未来仍面临侵蚀的威胁。
基金Projects (50941020, 10902086, 50875217, 20903075) supported by the National Natural Science Foundation of ChinaProjects (SJ08-ZT05, SJ08-B14) supported by the Natural Science Foundation of Shaanxi Province, ChinaProject (CX200905) supported by the Doctorate Foundation of Northwestern Polytechnical University, China
文摘Based on the microscopic phase-field model, ordered domain interfaces formed between D022 （Ni3V） phases along [001] direction in Ni75AlxV25-x alloys were simulated, and the effects of atomic structure on the migration characteristic and solute segregation of interfaces were studied. It is found that the migration ability is related to the atomic structure of interfaces, and three kinds of interfaces can migrate except the interface （001）//（002） which has the characteristic of L12 （Ni3Al） structure. V atoms jump to the nearest neighbor site and substitute for Ni, and vice versa. Because of the site selectivity behaviors of jumping atoms, the number of jumping atoms during the migration is the least and the jumping distance of atoms is the shortest among all possible modes, and the atomic structures of interfaces are unchanged before and after the migration. The preferences and degree of segregation or depletion of alloy elements are also related to the atomic structure of interface.
基金Supported by the National Natural Science Foundation of China under Grant Nos.10747004,11065006,and 81060307
文摘A lattice Boltzmann model of two dimensions is used to simulate the movement of a single rigid particle suspended in a pulsating flow in micro vessel The particle is as big as a red blood cell, and the micro vessel is four times as wide as the diameter of the particle. It is found that Segrd-Silberberg effect will not respond to the pulsation of the flow when the Reynolds number is relatively high. However, when the Reynolds number is low enough, Segrd-Silberberg effect disappears. In the steady flow, different initial position leads to different equilibrium positions. In a pulsating flow, different frequencies of pulsation also cause different equilibrium positions. Particularly, when the frequency of pulsation is closed to the human heart rate, Segrd-Silberberg effect presents again. The evolutions of velocity, rotation, and trajectory of the particle are investigated to find the dynamics of such abnormal phenomenon.
文摘One of the greatest feats of avian migration is the non-stop crossing of extensive areas of inhospitable habitat such as deserts and seas. Differences in spring and autumn migration routes have been reported in species that cross such barriers, and are thought to have evolved in response to seasonal variation in prevailing wind direction. We tested the hypothesis that migration routes vary seasonally with respect to the Gulf of Mexico in the tree swallow Tachycineta bicolor using solar geolocators attached and retrieved at 4 breeding sites in central North America. We found that 100 % of birds （n = 10） made a trans-Gulf flight of 〉850 km from Louisiana south to their wintering grounds in the Yucatan Peninsula in 12-36 hours, achieving minimum ground speeds as high as 32 m/s. Although most days during autumn migration were characterized by unfavorable headwinds blowing to the northwest, migration over the Gulf mostly occurred on days with strong winds blowing to the south. In contrast, in 8 of 9 （88 %） birds on spring migration returned from the wintering grounds towards Louisiana following a clockwise loop pat- tern flying over land to the west around the Gulf. During this spring period there were few days with prevailing winds from the south to assist northward migration. Results suggest that, despite being up to three times further （ca. 2,700 kin）, a coastal circum-Gulf spring migration represents the less risky route when wind conditions are not favorable. These findings also help to resolve a long-standing dispute in the literature concerning migration patterns between the US Gulf coast and Mexico, and provide insight into the factors shaping migration strategies of small songbirds migrating across large bodies of water [Current Zoology 60（5）： 653-559, 2014].
