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Dynamic stiffness for thin-walled structures by power series

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Journal of Zhejiang University-Science A(Applied Physics & Engineering) 2006年第8期7卷 1351-1357页

作者： ZHU Bin leung A.Y.T. Department of Civil Engineering Zhejiang University Hangzhou 310027 China Department of Building and Construction City University of Hong Kong Hong Kong China

The dynamic stiffness method is introduced to analyze thin-walled structures including thin-walled straight beams and spatial twisted helix beam. A dynamic stiffness matrix is formed by using frequency dependent shape functions which are exact solutions of the governing differential equations. With the obtained thin-walled beam dynamic stiffness matrices, the thin-walled frame dynamic stiffness matrix can also be formulated by satisfying the required displacements compatibility and forces equilib-rium, a method which is similar to the finite element method (FEM). Then the thin-walled structure natural frequencies can be found by equating the determinant of the system dynamic stiffness matrix to zero. By this way, just one element and several elements can exactly predict many modes of a thin-walled beam and a spatial thin-walled frame, respectively. Several cases are studied and the results are compared with the existing solutions of other methods. The natural frequencies and buckling loads of these thin-walled structures are computed.

关键词： Dynamic stiffness method Thin-wall structures Power series Buckling

Profiling selected paths with loops

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Science China(Information Sciences) 2014年第7期57卷 74-88页

作者： LI BiXin WANG LuLu leung Hareton School of Computer Science and Engineering Southeast University Department of Computing Hong Kong Polytechnic University

Path profiling records the frequency of each path in an executed *** accomplish profiling,probes are instrumented in the program and executed as the program ***,the number of probes has a significant impact on the efficiency of a profiling *** profiling only the interesting paths,existing techniques try to improve the profiling efficiency by reducing the number of probes and optimize path encodings for efficient ***,they lack accuracy,waste time on running uninteresting paths,and can mainly deal with acyclic *** this article,a novel technique called Profiling Selected Paths(PSP)is introduced to profile selected paths,which enables custom selection for both acyclic and cyclic paths and increases the execution efficiency by early termination on uninteresting *** analysis and experimental evaluation indicate that PSP performs better than existing techniques.

关键词： path profiling interesting paths dynamic analysis

Failure strength and fracture characteristics of rock with discontinuity under indirect tension

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Journal of Rock Mechanics and Geotechnical Engineering 2022年第6期14卷 1810-1822页

作者： Dongya Han Jianbo Zhu Yat-Fai leung School of Resources and Safety Engineering Central South UniversityChangshaChina Department of Civil and Environmental Engineering The Hong Kong Polytechnic UniversityHong KongChina Guangdong Provincial Key Laboratory of Deep Earth Sciences and Geothermal Energy Exploitation and Utilization Institute of Deep Earth Sciences and Green EnergyCollege of Civil and Transportation EngineeringShenzhen UniversityShenzhenChina

Large-scale discontinuities can significantly affect the mechanical properties of rock ***,the tensile behavior of rock discontinuities is often less *** study the statistical characteristics of failure strength and fracture characteristics of rock discontinuities,Brazilian disc tests were conducted on limestone specimens with a single natural discontinuity at different load-discontinuity angles(β).In this study,β=0°andβ=90°correspond to the discontinuity parallel and perpendicular to loading direction,*** results show that Brazilian failure strength(BFS)can reasonably represent the tensile strength of rock with discontinuities,by comparing the BFS and tensile stress in the disc center at peak *** two-parameter Weibull distribution can capture the statistical BFS characteristics of rock discontinuities parallel to loading direction(β=0°)and at different loaddiscontinuity angles(β≠0°).All specimens with discontinuity at different load-discontinuity angles show more plastic deformational behaviour than intact rock *** increasingβ,the mean BFS of limestone with discontinuity increases before reaching a plateau atβ=45°.The single plane of weakness theory best explains the BFS of fractured limestone withβ.Only a specific segment of preexisting rock discontinuity could affect the fracture ***β=0°,interfacial cracks and alternative cracks ***β≠0°,mixed failure mode with shear and tensile failure occurred,particularly whenβ=30°andβ=60°.The findings can contribute to better understanding the failure and fracture characteristics of rock with discontinuities,particularly the interaction of pre-existing discontinuities with stress-induced fracturing.

关键词： Brazilian failure strength(BFS) Weibull distribution Rock discontinuity Anisotropy

Theoretical and experimental studies of heat transfer with moving phase-change interface in freezing and thawing of porous potting soil

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Journal of Zhejiang University-Science A(Applied Physics & Engineering) 2009年第1期10卷 1-6页

作者： Michael K.H.leung Kit-ying CHAN Department of Mechanical Engineering the University of Hong Kong

Soil in a cold region is subject to frequent freezing and thawing *** frozen for a prolonged period may cause adverse freeze damage to the plants due to cell dehydration or root cell *** is important to understand the detailed heat transfer behaviors of the freezing and thawing processes to prevent freeze damage,and to devise proper mitigation measures for effective pot planting in cold regions.A theoretical model was developed to analyze the transient moving phase-change interface heat transfer in the freezing and thawing of porous potting *** theoretical derivation is based on the assumption that the soil freezes completely at a single *** poromechanic effects on heat transfer behavior were *** spatial domain of the problem was simplified to a 1D spherical coordinate system with variation in the radial ***'s function was applied to solve for the time-dependent body *** were conducted for validation of the theoretical *** agreement between the theoretical predictions and experimental measurements was *** theoretical model developed can be easily used to determine the sensitivity of various parameters in the freezing/thawing processes,e.g.,thermal properties of soil,ambient temperature,and planting pot size.

关键词： Porous soil Freezing Thawing Moving phase-change interface Theoretical model

Numerical study on the ozone formation inside street canyons using a chemistry box model

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Journal of Environmental Sciences 2008年第7期20卷 832-837页

作者： Chun-Ho Liu Dennis Y.C.leung Department of Mechanical Engineering The University of Hong KongHong Kong.

Tropospheric ozone is a secondary air pollutant produced in the presence of nitrogen oxides (NO_x),volatile organic compounds (VOCs),and solar *** an urban environment,ground-level vehicular exhaust is the major anthropogenic source of ozone *** the cases of street canyons,pollutant dilution is weakened by the surrounding buildings that creates localized high concentration of NO_x and VOCs,and thus leads to high potential of ozone *** considering the major physical and chemical p...

关键词： volatile organic compounds VOCs-to-NO_x ratio air quality vehicular emission chemistry box model

Impacts of environmental factors on urban heating

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Journal of Environmental Sciences 2010年第12期22卷 1903-1909页

作者： Rizwan Ahmed Memon Dennis Y.C.leung Department of Mechanical Engineering Mehran University of Engineering & Technology Department of Mechanical Engineering The University of Hong Kong School of Energy and Environment City University of Hong Kong

This study investigated the impact of important environmental variables （i.e., wind speed, solar radiation and cloud cover） on urban heating. Meteorological parameters for fifteen years （from 1990 to 2005）, collected at a well developed and densely populated commercial area （Tsim Sha Tsui, Hong Kong）, were analyzed in details. Urban heat island intensity （UHII）, a well known indicator of urban heating, has been determined as the spatially averaged air-temperature difference between Tsim Sha Tsui and Ta Kwu Ling （a thinly populated rural area with lush vegetation）. Results showed that the UHII and cloud cover have increased by around 9.3% and 4%, respectively, whereas the wind speed and solar radiation have decreased by around 24% and 8.5%, respectively. The month of December experienced the highest UHII （10.2℃） but the lowest wind speed （2.6 m/sec） and cloud cover （3.8 oktas）. Conversely, the month of April observed the highest increases in the UHII （over 100%） and the highest decreases in wind speed （over 40 %） over fifteen years. Notably, the increases in the UHII and reductions in the wind speed were the highest during the night-time and early morning. Conversely, the intensity of solar radiation reduced while the intensity of urban cool island （UCII） increased during solar noon-time. Results demonstrated strong negative correlation between the UHII and wind speed （coefficient of determination, R^2 = 0.8） but no negative correlation between UCII and solar radiation attenuation. A possible negative correlation between UHII and cloud cover was investigated but could not be substantiated.

关键词： urban heat island urban cool island wind speed cloud cover

Interface formation energy,bonding,energy band alignment in α-NaYF4 related core shell models:For future multi-layer core shell luminescence materials

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Journal of Rare Earths 2017年第4期35卷 315-334页

作者：黄勃龙董浩 Wong Ka-leung 孙聆东严纯华 Department of Applied Biology and Chemical Technology The Hong Kong Polytechnic University Beijing National Laboratory for Molecular Sciences State Key Laboratory of Rare Earth Materials Chemistry and ApplicationsPKU-HKU Joint Laboratory in Rare Earth Materials and Bioinorganic ChemistryCollege of Chemistry and Molecular EngineeringPeking University Department of Chemistry Hong Kong Baptist University

To break through the bottle-neck of quantum yield in upconversion （UC） core-shell system, we elucidated that the energy transfer efficiency in core-shell system had an evident contribution from the charge transfer of interface with related to two factors：（i） band offsets and （2） binding energy area density. These two variables were determined by material intrinsic properties and core-shell thickness ratio. We further unraveled the mechanism of non-radiative energy transfer by charge transfer induced dipole at the inter- face, based on a quasi-classical derivation from F6rster type resonant energy transfer （FRET） model. With stable bonding across the interface, the contributions on energy transfer in both radiative and non-radiative energy transfer should also be accounted together in Auzel＇s energy transfer （ETU） model in core-shell system. Based on the discussion about interface bonding, band offsets, and forma- tion energies, we figured out the significance of interface bonding induced gap states （IBIGS） that played a significant role for influ- encing the charge transfer and radiative type energy transfer. The interface band offsets were a key factor in dominating the non-radiative energy transfer, which was also correlated to core-shell thickness ratio. We found that the energy area density with re- lated to core/shell thickness ratio followed the trend of Boltzman sigmoidal growth function. By the physical trend, this work contrib- uted a reference how the multi-layered core-shell structure was formed starting from the very beginning within minimum size. A route was paved towards a systematic study of the interface to unveil the energy transfer mechanism in core-shell systems.

关键词： interface band-offset core-shell energy-transfer rare earths

Within-Cluster Variability Exponent for Identifying Coherent Structures in Dynamical Systems

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Communications in Computational Physics 2023年第3期33卷 824-848页

作者： Wai Ming Chau Shingyu leung Department of Mathematics The Hong Kong University of Science and TechnologyClear Water BayHong Kong

We propose a clustering-based approach for identifying coherent flow structuresin continuous dynamical systems. We first treat a particle trajectory over a finitetime interval as a high-dimensional data point and then cluster these data from differentinitial locations into groups. The method then uses the normalized standarddeviation or mean absolute deviation to quantify the deformation. Unlike the usualfinite-time Lyapunov exponent (FTLE), the proposed algorithm considers the completetraveling history of the particles. We also suggest two extensions of the method. To improvethe computational efficiency, we develop an adaptive approach that constructsdifferent subsamples of the whole particle trajectory based on a finite time interval. Tostart the computation in parallel to the flow trajectory data collection, we also developan on-the-fly approach to improve the solution as we continue to provide more measurementsfor the algorithm. The method can efficiently compute the WCVE over adifferent time interval by modifying the available data points.

关键词： Dynamical system visualization finite time Lyapunov exponent numerical methods for differential equations k-means clustering

Cluster-Based Advantages in the Innovation Process： The Diffusion of Nanotechnology in Two Chinese Cities

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Frontiers of Business Research in China 2012年第1期6卷 25-54页

作者： Ricky leung Department of Health Management and Informatics University of Missouri Columbia MO65212 USA

Researchers have shown that ＂cluster＂ is a more useful unit of analysis than ＂nation＂ in innovation studies. Clusters are characterized by interconnected organizations, shared resources and frequent knowledge flows. Within national boundaries, multiple clusters with different relative advantages may coexist, and exhibit very different innovation patterns. Yet, in studies of innovation in emerging economies, there remains little attention on cluster-based advantages, and how these advantages generate distinct patterns of innovation. To bridge this gap, this study analyzes the diffusion patterns of nanotechnology in two Chinese clusters--Beijing and Shanghai. With different relative advantages, the diffusion of nanotechnology has been oriented by different levels of ＂imitation＂ and ＂innovation＂ forces in the two Chinese clusters. This study applies the Bass Model to quantify imitation and innovation forces, and compare the resultant diffusion patterns of nanotechnology in the two clusters with other technologies. Supplementary qualitative data is also provided to show how Chinese scientists perceive their relative advantages in different clusters. Among other things, the findings suggest that scientists of emerging economies favor the learning-by-doing principle while utilizing external networks.

关键词： clusters emerging economies diffusion of innovation technology management China