801 research outputs found
Pipeline and cable stability: updated state of the art
No full textAt OMAE in 2008 the ‘state of the art’in pipeline on-bottom stability engineering was summarized, providing an overview of the current available knowledge for addressing pipeline stability. The aim of that work was to summarise key aspects of the pipeline stability design process and to include some historical perspective. The paper discusses the advantage and shortfalls of the different design approaches with a view to consolidate understanding, rather than to provide a ready-made solution to a complex design problem [1]. Since that time, a decade of research and further methodology refinement has extended the boundaries of the industry's knowledge and understanding of the behaviour of subsea pipelines and cables, including geotechnics, hydrodynamics, oceanography and structural response modelling. In particular, progress has been made in: • The response of pipelines to sediment transport and scour; • Understanding the behaviour of small diameter pipelines and cables within wave and current boundary layers; and • The behaviour of cables on rocky seabeds in high energy marine environments. This paper summarises these innovations to enable the application of new paradigms in engineering practice and improved outcomes for initial project capital cost, reliability and operational integrity, as well as better models to predict the longterm behaviour where pipes are decommissioned in-situ. While a relatively widely studied field of engineering, there remain areas of active ongoing research to improve our understanding and ability to model and predict subsea pipeline on-bottom behavior, with<br/
Estimating the effects of functional diversity and composition on the spatial variability of ecosystem multifunctionality in a large temperate forest region
No full textAbstract Functional diversity and composition have been shown to significantly influence the temporal variability of ecosystem functioning. However, their impact on the spatial variability of multiple ecosystem functions (multifunctionality) is still unknown. Therefore, this study aims to explore how functional diversity and composition affect the spatial variability of ecosystem multifunctionality (EMF-SV) under different environmental conditions in a temperate forest region. Regional communities representing specific spatial scales were established by assembling different numbers of sample plots. The EMF-SV was represented by the ratio of the standard deviation to the mean value of ecosystem multifunctionality within each regional community. Linear mixed-effects models were used to evaluate the effects of functional diversity and composition on the EMF-SV at different spatial scales. Structural equation models were applied to explore the direct and indirect pathways of functional diversity and composition influencing the EMF-SV. Our results show that functional diversity and composition had significant effects on the EMF-SV, and these effects changed with spatial scales and environmental conditions. They affected the EMF-SV directly or indirectly through species asynchrony and population stability. Our results demonstrate the role of functional traits in regulating the EMF-SV across spatial scales and explore the main impact mechanisms. This will contribute to our understanding and protection of ecosystem multifunctionality in temperate forests.the National Key R&D Program of Chinathe Program of National Natural Science Foundation of China http://dx.doi.org/10.13039/50110000180
Reduced soil moisture caused by human disturbance mediates the biodiversity effects on ecosystem multifunctionality across stand ages in the temperate forests of North-Eastern China
No full texthttp://dx.doi.org/10.13039/501100001809 National Natural Science Foundation of Chinahttp://dx.doi.org/10.13039/501100002855 Ministry of Science and Technology of the People's Republic of Chinahttp://dx.doi.org/10.13039/501100012166 National Key Research and Development Program of Chin
Suppression of vortex shedding in the wake of a circular cylinder through high-frequency in-line oscillation
No full textThis paper presents a new flow control approach to suppress the vortex shedding in the wake of a circular cylinder through high-frequency oscillation. The circular cylinder is forced to oscillate in the streamwise direction at high-frequency and low amplitude, corresponding to a high Stokes number (b ¼ 100–1000) and low Keulegan–Carpenter number (KC¼ 0.001–4). Two-dimensional (2-D) and three-dimensional (3-D) direct numerical simulations of an oscillating circular cylinder in steady current have been carried out in the parameter space of KC, Rec, and b. Our numerical results show that when the flow remains in the two-dimensional vortex shedding regime, the cylinder wake sequentially experiences transitions from the vortex shedding regime to the suppression of the vortex shedding regime and finally to the symmetry breaking regime, with increasing KC. Corresponding wake characteristics and variations of hydrodynamic forces over the three wake regimes are explored. Three quantities that represent shear-layer characteristics, including the length of separating shear layers, the circulation of shear layers and wake recirculation length, reach maxima at the onset of suppression. The physical mechanisms for the suppression of vortex shedding and occurrence of symmetry breaking are also explained. Once the flow becomes 3-D, vortex shedding from the cylinder cannot be suppressed, primarily because the outer shear layers induced by the steady approaching flow are enhanced in 3-D flows. The cylinder oscillation over the frequency range investigated in the present study delays wake transition to 3-D. The cylinder oscillation alters the 3-D vortical structure and its spanwise wavelength significantly.Dan Pang, Liang Cheng, Hongyi Jiang, Feifei Tong, Hongwei A
Population Development, Fecundity, and Flight of Spodoptera frugiperda (Lepidoptera: Noctuidae) Reared on Three Green Manure Crops: Implications For an Ecologically Based Pest Management Approach in China
Get PDFThe fall armyworm, Spodoptera frugiperda (Smith), is an invasive pest of cereal crops that now inhabits southern China year-round. Cultivation of crops unsuitable as host plants has been an effective pest management strategy for some insect pests, but the effects of green manure crops on S. frugiperda have not been investigated. An age-stage two-sex life table and tethered flight performance of S. frugiperda reared on different green manure species were obtained, and a population dynamics model established. Developmental durations of stages, survival rates, and fecundities of S. frugiperda differed significantly depending on host plant. Larvae fed Astragalus sinicus L. did not complete development. Although some larvae fed Vicia villosa Roth and Vicia sativa L. completed development, generation time was significantly prolonged, egg production was halved, and net reproductive rate decreased to 31% and 3% of those reared on corn, respectively. Survival rates of early-instars fed V. villosa and V. sativa were significantly lower than those fed corn. Population dynamics projections over 90 d showed the number of generations of S. frugiperda fed on V. villosa and V. sativa was reduced compared to those reared on corn. Flight performance of S. frugiperda reared on V. villosa decreased significantly compared to corn. Our results show that the three green manure species are unsuitable host plants for S. frugiperda. Therefore, reduction of corn production in southern China through rotation with these green manure crops may be a feasible method of ecological management of this major corn pest in China.This article is published as Wu, Feifei, Lei Zhang, Yueqiu Liu, Yunxia Cheng, Jianya Su, Thomas W. Sappington, and Xingfu Jiang. "Population Development, Fecundity, and Flight of Spodoptera frugiperda (Lepidoptera: Noctuidae) Reared on Three Green Manure Crops: Implications For an Ecologically Based Pest Management Approach in China." Journal of Economic Entomology (2021). doi:10.1093/jee/toab235.
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Mining author relationship in scholarly networks based on tripartite citation analysis.
No full textFollowing scholars in Scientometrics as examples, we develop five author relationship networks, namely, co-authorship, author co-citation (AC), author bibliographic coupling (ABC), author direct citation (ADC), and author keyword coupling (AKC). The time frame of data sets is divided into two periods: before 2011 (i.e., T1) and after 2011 (i.e., T2). Through quadratic assignment procedure analysis, we found that some authors have ABC or AC relationships (i.e., potential communication relationship, PCR) but do not have actual collaborations or direct citations (i.e., actual communication relationship, ACR) among them. In addition, we noticed that PCR and AKC are highly correlated and that the old PCR and the new ACR are correlated and consistent. Such facts indicate that PCR tends to produce academic exchanges based on similar themes, and ABC bears more advantages in predicting potential relations. Based on tripartite citation analysis, including AC, ABC, and ADC, we also present an author-relation mining process. Such process can be used to detect deep and potential author relationships. We analyze the prediction capacity by comparing between the T1 and T2 periods, which demonstrate that relation mining can be complementary in identifying authors based on similar themes and discovering more potential collaborations and academic communities
Hydrodynamic Damping of an In-Line Oscillating Cylinder In-Line in Steady Flow
No full textThis study conducts a series of analysis to reconstruct the time-dependent in-line forces on an in-line oscillating cylinder in steady flow. The analysis involves reconstructing these forces from several approaches and comparing them with those obtained from the three-dimensional numerical simulations conducted at a fixed Reynolds number of 500 based on the incoming steady flow velocity. A range of velocity ratio r* (cylinder-oscillation-velocity to current velocity) from 0.1 to 1.83 at amplitude-to-diameter ratio A* = 1/π are considered. To quantify their performance, the reconstructed in-line force and damping force from the conventional Morison Equation (MOJS) with the relative and independent velocity models are examined. The results show that the independent velocity model outperforms the relative velocity model and provides a good fit at large r* where the wake is controlled by the cylinder oscillation. The study also demonstrates that the independent velocity model using a more general form of MOJS is superior to the conventional formula in predicting the in-line force. Recommendations are provided on selecting velocity models in force estimation at different r* ranges.Chengjiao Ren, Liang Cheng, Feifei Ton
Investigation on micro-cutting mechanics with application to micro-milling
Get PDFThis thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University LondonNowadays technology development places increasing demands on miniature and micro components and products, and micro-milling is one of the most flexible machining processes in manufacturing 3D structures and complex structured surfaces. A thorough and scientific understanding on fundamentals of the micro-milling process is essential for applying it in an industrial scale. Therefore, in-depth scientific understanding of the micro-cutting mechanics is critical, particularly on size effect, minimum chip thickness, chip formation, tool wear and cutting temperature, etc. so as to fulfil the gap between fundamentals and industrial scale applications. Therefore, three key fundamental research topics are determined for this research, and a comprehensive study on those topics is conducted by means of modeling, simulation, experiments. The topics include chip formation process in micro-milling, novel cutting force modeling in multiscale and study on the tool wear and process monitoring. The investigation into chip formation process in micro-milling consists of three stages; the micro-cutting process is firstly simulated by means of FEA with a primary focus on finding the minimum chip thickness for different tool/material pair and explaining the size effect; the simulation results are then validated by conducting micro-cutting experiment on the ultra-precision lathe. Experiments are carried out on aluminium 6082-T6 with both natural diamond and tungsten carbide tool. By knowing the minimum chip thickness for different tool/material pair, the chip formation process is investigated by performing comparative study by using the diamond and tungsten carbide micro-milling tools. As the minimum chip thickness for diamond micro-milling tool is smaller than that for tungsten carbide tool compared to nominal chip thickness, MCT is ignored in diamond micro-milling. Thus the comparative study is conducted by utilizing both tools with perfectly sharpened cutting edge and tools with the rounded cutting edge in micro-milling. The chips are inspected and associated with cutting force variations in the micro-milling process. The findings are further consolidated by comparing with research results by other researchers. The cutting force modeling is developed in three different aspects, e.g. cutting force on the unit length or area and cutting force on the unit volume in order to better understand the micro-cutting mechanics in aspects of size effect, tool wear mechanism and the cutting energy consumption. The mathematical modeling firstly starts with a novel instantaneous chip thickness algorithm, in which the instantaneous chip thickness is computed by taking account of the change of tool geometry brought about by the tool runout; then the collected cutting forces are utilized to calibrate the model coefficients. For accurate measurement on cutting forces, the Kalman Filter technique is employed to compensate the distortion of the measured cutting force. Model calibration is implemented using least-square method. The proposed cutting force model is then applied in micro-milling to represent the conditions of tool wear and the cutting energy consumption. Further study on the surface generation simulation is based on force model and its comparison with the machined surface is also performed.
Cutting experiments using the new tungsten carbide tool are carried out and the tool wear is monitored offline at different machining stages. The dominant tool wear types are characterised. Tool wear is investigated by mainly analysing cutting force at different tool wear status. Frequency analysis by Fourier Transform and Wavelet Transform are carried out on the force signals, and features closely related to the tool wear status are identified and extracted. The potential of applying these features to monitoring the tool wear process is then discussed. Experimental studies to machine the structured surface and nano-metric level surface roughness are presented, the machining efficiency, dimensional accuracy and tool-path strategies are optimised so as to achieve the desired outcomes. Moreover, investigation on cutting temperature in micro-cutting is also studied to some extent by means of simulation; the influence of cutting edge radius on cutting temperature is particularly investigated. Investigation on above aspects provides systematic exploration into the micro-milling process and can contribute substantially to future micro-milling applications.China Scholarship Council and Korean Institute of Machinery and Material
The community-based partnership approach for affordable housing development : a case in Shenzhen, China
Get PDFThesis (S.M.)--Massachusetts Institute of Technology, Dept. of Architecture, 2009.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student submitted PDF version of thesis.Includes bibliographical references (p. 101-103).Affordable housing has been a crucial urban issue in China. Given its fast-paced urbanization process, China is experiencing a dramatic increase in the demand for affordable housing. At the same time, the affordable housing is in short supply due to the demolition of existing low-income housing during the urban renewal process, and the lack of funding for housing construction. In the United States, the affordable housing industry is relatively well-developed with a comprehensive system and a mature research framework. The literature in this area is complete with policy, financing and design. However, in China, research on affordable housing has not yet been conducted on a comprehensive basis, with existing research mostly focused on physical planning and design. This thesis recognizes that in order to solve the problems, a comprehensive system for affordable housing development should be established. A comprehensive affordable system needs to consist of six major components: policy, planning, design, financing, and partnership. However, the scope of this research will deal with only the partnership aspect, which is about how a variety of entities collaborate in each stage of affordable housing development. This thesis will investigate the feasibility and potential of community-based partnership approach in providing affordable housing in the context of China's urbanization and urban renewal. It will first examine the housing affordability in China. It will then look at the current affordable housing system in China. The third part presents and companies the affordable housing system in the United States and its use of community-based partnerships.(cont.) This will be followed by a case study of Shenzhen's urban villages and urban village corporations. Finally, the paper will make suggestions for the Shenzhen case, and provide examples of potential community-based partnerships as well as future steps of researchby Feifei Zhao.S.M
Hydrodynamic forces of a prescribed oscillating cylinder in steady currents
No full textPredicting hydrodynamic forces on an obliquely oscillating riser in current poses challenges, especially as existing practices rely on a linear summation of forces induced by the current and riser motion, which overlooks crucial nonlinear interactions. The present study, using three-dimensional numerical simulations at a fixed Reynolds number of 500 and a small oscillation amplitude of 0.4 cylinder diameters, evaluates these nonlinear effects over a range of oscillation wavelengths (λ* from 1.0 to 25.0) and orientation angles (θ from 0° to 90°). The results reveal significant nonlinear interactions at intermediate λ*=1.5–10, forming multiple resonant wake states that either amplify or reduce the hydrodynamic forces. For instance, time-averaged force coefficients in inline and transverse directions increase by more than 2.5 times compared to the no-oscillation case, while the root-mean-squared force coefficient in the motion direction can drop to a minimum (0.1–0.3) at λ*=4–10. Physical explanations on the influences of inclination angle on the wake dynamics and hydrodynamic force coefficients are provided. These findings indicate the inadequacy of linear summation method and propose a new framework incorporating data at 0°, 45°, and 90°, offering improved predictive capability for oblique angles.Chengjiao Ren, Liang Cheng, Feifei Tong, Fei H
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