A new upper bound of diameter in undirected double loop networks has been obtained. We also proved the connectivity of undirected double loop networks four.
湖泊沉积物储存着长时期的流域环境信息,是百年尺度湖泊环境演变研究的高分辨率记录器。本文通过210Pbex、137Cs组合定年法,分析放射性核素剖面特征,研究沉积速率时空变化规律,并解析沉积物粒度指示的环境信息,揭示百年尺度湖泊沉积环境演变。结果表明:洞庭湖210Pbex剖面分布呈现多段衰变型、混合-衰变-平衡型、衰变-本底型和无沉积型4种类型。3大湖体200 cm的沉积深度对应的年代跨度为127~162年,构建了洞庭湖沉积物深度-年代关系。洞庭湖平均质量沉积速率为1.47~3.11 g/(cm2·a),整体上呈现南洞庭湖>西洞庭湖>东洞庭湖的空间特征;自1900年后,质量沉积速率增加了3~7倍,呈时序增加趋势。沉积年代上,沉积物粒度特征表明洞庭湖沉积环境呈3个阶段2个过渡期的演变特征,与同时期区域内毁林开荒、水利建设、植树造林等人为活动有较好的对应关系,说明近百年来人为活动是洞庭湖泥沙沉积的重要因素。本研究结果可为强化湖泊及其流域生态环境保护提供理论参考。;Lake sediments record large amount of environmental information from the watershed over a long time, and has become a high-resolution archives for investigating the lake environmental change at the centennial scale. In this paper, we analyzed the features of radionuclide profiles through the combined dating method of 210Pbex and 137Cs, investigated the temporal and spatial changes of sedimentation rate, and extracted the environmental information indicated by sediment particle size to reveal the patterns of the sedimentary environment during the last century. The results showed that 210Pbex profiles presented four stages:a multi-stage decay, a mixed-decay-equilibrium, a decay-background and a no-deposition. The sediment depths of 200 cm in the three major lake bodies covered a chronological span of 127-162 years, and a depth-age relationship was constructed for Lake Dongting sediments. The mean mass accumulative rate (MAR) ranged from 1.47 to 3.11 g/(cm2·a), demonstrating an overall spatial characteristic of the highest MARs in the south, and higher in the west than the east. Since 1900, MARs in Lake Dongting increased 3-7 times, showing an upward trend in the chronology. The depositional change of sediment grain size in Lake Dongting was characterized by three stages and two transitional periods, which corresponded well with anthropogenic activities such as deforestation, hydrological construction, and afforestation in the basin in the period. These findings suggested that anthropogenic activities had been an important factor for sediment deposition in Lake Dongting. The foundings in our study can provide a theoretical reference for enhancing the ecological protection of the lake and its watershed.
The adsorption, distribution, and migration behaviors of heavy metals in soil particles are critically reviewed from the perspectives of research method, soil type, and comparison of results.
In order to analyze the fracture of bolts at the flange of TBM cutterhead, the load distribution of the thread in the standard thread connection structure was studied.Based on theoretical analysis and Finite element simulation analysis, starting from the connection structure of bolt, the load distribution of a thread connection structure under axial load was studied, and the maximum stress at the root of each whorl was obtained by Abaqus simulation.Finally, the fatigue test of double threaded studs with standard thread connection structure was carried out.The load distribution law of each circle screw thread was verified through a set of stud fatigue fracture position.Found that the bolt fracture problem was not due to the excessive load on the flange, but due to the nouniform distribution of the axial force of the thread.
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