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通过一维粒子模拟研究了利用相对论少周期强激光与固体密度等离子体表面相互作用实现单个孤立阿秒光脉冲产生的参数条件。主要研究描述相互作用的多维参数，如激光强度、入射角和等离子体标尺长度等，对相对论高次谐波能量转换效率和孤立阿秒光脉冲分离度的影响。研究发现，虽然激光等离子体参数对阿秒光脉冲产生的影响是复杂的，但是存在着能够实现大能量孤立阿秒光脉冲的最佳等离子体标尺长度和最佳入射角。当其他相互作用条件确定时，使用中等强度的相对论强激光可以在较宽的参数范围内实现孤立的阿秒光脉冲。大角度入射时，孤立阿秒光脉冲的分离度较高，能够实现孤立阿秒光脉冲的相互作用参数范围也较宽。The generation of single isolated attosecond light pulses from the interaction of relativistic few-cycle lasers with solid density plasma surfaces is investigated using one dimensional particle-in-cell simulations. The primary subject of the study is the effects of the multi-parameter combinations which uniquely define the laser plasma interactions, on the laser to relativistic high-order harmonic energy conversion efficiencies, and also on the single attosecond light pulse isolation degrees. Here these multi-parameters include laser intensities, incidence angles, plasma scale lengths, etc. The impact of laser-plasma interaction parameters on attosecond light pulse generations is generally complicated. However, there exist an optimal plasma scale length and an optimal incidence angle to efficiently generate high-order harmonics and intense attosecond light pulses. When other parameters are fixed, a moderately intense relativistic laser is more advantageous to realize isolated attosecond light pulses with a broad controlling parameters range. And a larger incidence angle favors a higher isolation degree as well as a broader range of controlling parameters towards the generation of intense isolated attosecond light pulses. Copyright © 2022 High Power Laser and Particle Beams. All rights reserved.

2002-2003 > Academic research: refereed > Publication in refereed journal ; Version of Record ; Published

The health of a river ecosystem is the foundation of the sustainable development of a basin, while the natural variability of the hydrological regime is a key factor for maintaining and protecting the ecological health of a river. Therefore, variation attribution of the hydrological regime in the changing environment becomes more important. In this study, the Laohahe basin was selected as a case study area and its hydro-meteorological data during 1964-2016 were collected to achieve the study aim. Firstly, the study period was divided according to the trend and change points test results of Mann-Kendall, Pettitt and precipitation-streamflow double cumulative curve method. Then, the natural streamflow series was reconstructed using the Variable Infiltration Capacity (VIC) model. After that, the principal component analysis (PCA) was used to select the most ecologically relevant hydrological indicators (ERHIs). Then the degree of the change of the hydrological indicators was evaluated by the range of variability approach. Finally, the ' simulated-observed' comparison method was adapted to quantify the impacts of climate change and human activities on the variations of streamflow and ERHIs. The results showed that the annual streamflow series of the basin had a significant downward trend (α < 0.05) and its change points appeared in 1979 and 1999, respectively. Then the whole study period could be divided into base period, changed period I and II based on the above two change points. Then, 7 ERHIs were selected, including February flow, June flow, annual maximum 7-day flow, Julian day of each annual 1-day minimum, number of low pulse each year, mean duration of high pulse within each year, and rise rate. The 6 ERHIs, February flow, June flow, annual maximum 7-day flow, number of low pulse each year, mean duration of high pulse within each year, and rise rate, had a downward trend. While the Julian day of each annual 1-day minimum showed a slight upward trend. Also, the comprehensive degree of change of the 7 ERHIs ...

碰撞是在自然、生活、运动和工程中广泛存在的力学现象。在简要回顾研究历史之后，本文概述了物体对心正碰撞的基本力学规律；同时指出理论力学的动力学部分提供的理论工具仅限于动量守恒和动量–冲量定理，对撞击力、撞击脉冲和能量损失这些重要的力学量均不能给出任何信息。接着介绍了在线弹性力学、接触力学和弹塑性力学等不同框架下对碰撞问题建立的力学模型，强调碰撞之后的恢复系数并不是材料常数，它同撞击速度密切相关。文章最后呼吁在力学基础教学改革中要弥补碰撞力学知识，帮助学生树立正确的概念。Collision is a kind of mechanical phenomena which widely exist in nature, people’s live, sports and engineering. After a brief review of historical development, the article summarizes the fundamental laws of collinear collision between two particles/bodies, and points out that the particle dynamics only provides us the law of momentum conservation and the momentum-impulse theorem, with no information on the magnitude and pulse shape of colliding force, or on energy loss during collision. The mechanical models of collision based on linear elasticity, contact mechanics and elastoplastic theory are respectively illustrated, indicating that the coefficient of restitution (COR) is not a material constant. Finally, the article calls for making up the knowledge of collision dynamics in the teaching reform of basic mechanics, so as to help students to establish correct concepts. © 2022, Chinese Academy of Mechanics. All rights reserved.

2004-2005 > Academic research: refereed > Publication in refereed journal ; Version of Record ; Published

Causality is an important question across many scientific fields.In climate change research,a crucial issue is whether and to what extent the change of weather and extreme events can be attributed to anthropogenic warming.Attribution analysis is predominantly using climate models,which have inherent limitations in describing the non-linear systems.We investigate this problem using an alternative approach on long-term observational data.More specifically,we examine the cause effect of global CO2 concentration and urbanization effect on Hong Kong temperature evolution using Granger causality test.Based on temperature data for Hong Kong from 1886 to 2012,we first illustrate the increasing trend of summer mean,maximum and minimum temperatures and the change in the number of very hot days and hot nights.A distributional change of daily temperatures can also be detected.The results of the Granger causality analysis suggest that both the global CO2 concentration and an urbanization effect are driving the evolution of Hong Kong’s temperature,and the effect holds with different time lags.By comparing with a non-urban site,we find that the urbanization effect is likely to contribute about 40%of the summer temperature increase.The preliminary results indicate that the approach can offer a useful alternative way to assess the cause of local temperature rises.Our analysis could be expanded to investigate on the causal effect of other greenhouse gases on extreme weather events,as well as the link between climate change and societal impacts. 因果关系在许多科学领域中是一个重要课题。在气候变化研究中,一个关键的问题是天气和极端事件的变化是否或在何种程度上可以归因于全球变暖的人为因素。归因分析主要依赖于气候模型,但其在描述非线性系统中存在一定的局限性。本文应用长期观测数据并使用统计方法来探讨此问题。更具体地,本文应用格兰杰因果关系检验方法分析全球二氧化碳浓度与城市化效应对香港气温演变的影响。基于香港1886年至2012年的温度数据,我们发现其夏季平均、最高和最低气温以及酷热天气、热夜日数皆增加趋势。同时可以发现日温度的统计分布变化。格兰杰因果关系分析的结果表明,全球二氧化碳浓度的变化和城市化效应皆对香港气温的演变有所贡献,对于不同的时间滞后性这种效应同样存在。通过与非城市地区的比较,我们发现城市化效应对于夏季气温的升高大概贡献40%。根据初步的结果,文本所引入的方法可以作为一种新的角度来评估局部地区温度升高的原因。分析方法可以扩展至研究其他温室气体对极端天气事件的影响,以及气候变化和社会影响之间的联系。

2011-2012 > Academic research: refereed > Publication in refereed journal ; Version of Record ; Published

综述了绿色基础设施的起源发展,总结了推动其概念形成的发展脉络,分别是人居环境视角、生态保护视角和绿色技术视角。提出了绿色基础设施在空间、功能、要素上的内涵,阐述了它与生态系统服务的外延关系。通过文献研究,综述了绿色基础设施在气候变化、人体健康、空气质量、雨洪管理、公众认知和社区参与、评价研究等领域的国际研究进展。结合我国绿色基础设施的研究现状和问题进行评述,并对未来发展提出展望。 This paper first reviews the origin and development of green infrastructure (GI) and then summarizes the context, formation and development from the perspectives of human settlements, ecological protection, and green technology. Then, it describes GI from spatial, functional, and elemental aspects. The relationship of GI with ecological services is also elucidated. Through a literature review, the international research frontiers of GI are assessed for the following topics: climate change, human health, air quality, stormwater management, public awareness, community participation, and evaluation research. Finally, this paper predicts future developments in GI based on the current status of GI research and problems in China. © 2017, Ecological Society of China. All rights reserved.

目前铂（Pt）及其合金仍是氢燃料电池首选催化剂，但是Pt高价格、低储量及循环稳定性差等缺点严重阻 碍了氢燃料电池商业化，因此发展低成本、高性能的新型非Pt催化剂和低Pt催化剂是实现氢燃料电池商业化的 关键。本文围绕燃料电池催化开发及使用过程中存在的成本、稳定性和毒化问题，回顾了近年来阴离子交换膜燃 料电池和质子交换膜燃料电池催化剂分别在提高阳极催化剂活性、降低阴极催化剂成本领域的最新研究进展，包 括催化剂的组成、结构以及颗粒尺寸等对催化活性、稳定性的影响。最后针对燃料电池催化剂存在的问题，指出 未来应基于原位观测和表征技术加强对碱性氢氧化机理的研究，同时开发高温制备小尺寸高有序度的有序铂合金 阴极催化剂的方法是未来的研究重点。 Currently, the carbon supported Pt and Pt alloy nanoparticles (NPs) are still the primary electrocatalysts for hydrogen fuel cells. However, the large-scale commercialization of hydrogen fuel cells is impeded due to the poor durability, high cost and low reserves of Pt. Hence, the development of low-cost, high-performance non-Pt or low-Pt electrocatalysts is a key for the commercialization of hydrogen fuel cells. With the focus on the cost, durability and poisoning of electrocatalysts, this paper reviews the latest research progress in improving the activity of anode catalysts and reducing the cost of cathode catalysts. Moreover, this paper also summarizes the influence of the composition, structure and size of nanoparticles on the activity and durability of the catalysts. Finally, we provide guidance for the development of fuel cell catalysts, that is, the application of in-situ characterization techniques to unravel the hydroxide (HOR) mechanism in alkaline medium, and the development of facile and efficient strategies to fabricate the small size and highly order intermetallic Pt alloys catalysts. © 2021, Chemical Industry Press Co., Ltd. All right reserved.

2007-2008 > Academic research: refereed > Publication in refereed journal ; Version of Record ; Published