676 research outputs found
Fractal dynamics analysis of the VHF radiation pulses during initial breakdown process of lightning
No full textAuthor name used in this publication: Chen, Mingli.Author name used in this publication: Du, Yaping.2009-2010 > Academic research: refereed > Publication in refereed journalVersion of RecordRGCPolyU 512908EPublishedVoR allowe
Multi-objective imperfect maintenance for dependent competing risk systems with multiple degradation processes
No full textMultiple competing risks are one of the important topics in reliability field, especially degradation processes and random shocks. This research aims to relax the independent assumption by considering that there exist dependent relationships not only among multiple degradation measures but also between degradation measure and random shocks. In reality, many systems have multiple components with more than one degradation measure which is dependent with each other due to their interplaying functions or common usage history. Independent assumption may underestimate system reliability estimation under many cases. Random shocks will also contribute to the system failure through two ways: (1) one is working directly on the degradation processes; (2) the other is causing immediate failure to the system. We develop a new methodology to formulate the reliability prediction model for the gradually degradating systems subject to multiple dependent competing risks of degradation processes and random shocks. Two kinds of random shocks are considered: (1) fatal shocks, which fail the system immediately; (2) non-fatal shocks, which exhibit two effects on the system degradation process, including sudden degradation increment and degradation rate acceleration. The dependency between degradation processes and random shocks are modulated by a time-scaled covariate factors while the dependency among degradation processes are fitted by copula method. Also the reliability and state probability estimation for the systems are derived under the research scope of multi-state system using both analytical and Monte Carlo simulation for the dependent competing-risk systems. Different maintenance policy models involving imperefect preventive maintenance for this dependent model are introducted and compared with each other. Multi-objective optimization is applied to consider two important targets simultaneously in maintenance issues, including long-run expected cost rate and system availability.Ph. D.Includes bibliographical referencesIncludes vitaby Yaping Wan
Simultaneous observations of optical and electrical signals in altitude-triggered negative lightning flashes
No full textAuthor name used in this publication: Chen, Mingli.Author name used in this publication: Du, Yaping.2002-2003 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishedVoR allowe
Selection of the Lateral Distance for the Assessment Area in a Monte Carlo Procedure Under Indirect Lightning for Overhead Distribution Lines
No full textLightning Protection with a Differentiated Arrester Configuration for Distribution Networks Using a Multi-objective Optimization Procedure
No full textLightning Protection with a Differentiated Configuration of Arresters in a Distribution Network
No full text
Spatiotemporal dynamics of carbon cycling and thermokarst in response to climate and fire-regime changes in the Arctic tundra biome
No full textAmplified climate change and fire regime shifts in the northern high latitudes are posing growing threat to key properties and functions of tundra ecosystems, including soil carbon stock, permafrost stability and vegetation types. However, it remains poorly understood how tundra ecosystems will feedback to the combined forces of changing climate and fire disturbance. In this study, I integrated paleoecology, numerical modeling and remote sensing observation to address (1) the resilience and sensitivity of tundra carbon stocks to shifting fire regimes, (2) the consequences of climate change and fire disturbance on thermokarst disturbance (e.g. collapse of ground surface after permafrost thaw), and (3) the patterns of shrub expansion in heterogenous tundra landscape in response to accelerated warming and fire disturbance. My results indicate that fire disturbance has threshold effects on tundra carbon stocks. Variation in fire return intervals from 5000 to 900 years causes minimal carbon stock loss (<5%). However, increasing fire frequency beyond every 800 years is projected to trigger sustained mobilization of ancient soil organic matter that leads to irreversible carbon stock loss from permafrost. Multi-decade remote sensing observations revealed that tundra fires resulted in pervasive thermokarst formation, and that this impact lasted more than four decades. Nevertheless, substantial spatial heterogeneity exists regarding thermokarst formation and the greatest amount of thermokarst appears in severely burned tundra ecosystems in ice-rich areas. Although fire disturbance is a strong force exacerbating permafrost degradation, widespread warming surpasses sporadic burning as the primary driver responsible for ~90% of thermokarst growth in northern Alaskan tundra over the past ~70 yrs. Permafrost thawing strongly influences shrub cover dynamics in tundra ecosystems, but the net outcome is largely contingent on topographical positions. In poorly drained tundra lowlands, thermokarst-induced water impounding resulted in massive shrub cover loss throughout three decades following fire. In contrast, shrub expansion was significantly enhanced in well-drained upland tundra after fire disturbance, especially in area burned of high severity fire. In unburned tundra, a general increase of shrub cover was detected, driven primarily by warming temperature in the lowland but by increased precipitation in the upland. Overall my research yields new insights into the complex responses of tundra ecosystems to climate and fire-regime changes, and suggests the importance of incorporating such information into earth system models for improving our understanding of land-atmosphere feedback processes.Submission published under a 24 month embargo labeled 'Closed Access', the embargo will last until 2022-08-01The student, Yaping Chen, accepted the attached license on 2020-06-17 at 13:59.The student, Yaping Chen, submitted this Dissertation for approval on 2020-06-17 at 14:01.This Dissertation was approved for publication on 2020-06-24 at 16:07.DSpace SAF Submission Ingestion Package generated from Vireo submission #15453 on 2020-10-02 at 15:49:26Made available in DSpace on 2020-10-07T22:48:07Z (GMT). No. of bitstreams: 3
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sj-docx-1-hol-10.1177_09596836221138336 – Supplemental material for Synchronous change in the intensified millet cultivation and ecological environment from the early to middle Holocene on the Inner Mongolia Plateau, northern China
No full textSupplemental material, sj-docx-1-hol-10.1177_09596836221138336 for Synchronous change in the intensified millet cultivation and ecological environment from the early to middle Holocene on the Inner Mongolia Plateau, northern China by Keliang Zhao, Huiping Wei, Zhanhu Zhao, Yaping Zhang, Wenqing Liu, Jian Wang, Guanhan Chen, Hui Shen, Hua Du, Peng Cheng, Shan Chen, Peter Weiming Jia, Xinying Zhou and Xiaoqiang Li in The Holocene</p
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