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CONTENTS
Volume 23, Number 6, December 2016
 

Abstract
To investigate the nonlinear aerostatic stability of the Hutong cable-stayed rail-cum-road bridge with ultra-kilometer main span, a FEM bridge model is established. The tri-component wind loads and geometric nonlinearity are taken into consideration and discussed for the influence of nonlinear parameters and factors on bridge resistant capacity of aerostatic instability. The results show that the effect of initial wind attack-angle is significant for the aerostatic stability analysis of the bridge. The geometric nonlinearities of the bridge are of considerable importance in the analysis, especially the effect of cable sag. The instable mechanism of the Hutong Bridge with a steel truss girder is the spatial combination of vertical bending and torsion with large lateral bending displacement. The design wind velocity is much lower than the static instability wind velocity, and the structural aerostatic resistance capacity can meet the requirement.

Key Words
cable-stayed bridge; long span; aerostatic stability; geometric nonlinearity; wind attack-angle; tri-component wind loads

Address
Man Xu, Weiwei Guo, He Xia and Kebing L: School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China

Abstract
The objective of this study is to investigate numerically the effect of building roof shaps on wind flow and pollutant dispersion in a street canyon with one row of trees of pore volume, Pvol = 96%. A three-dimensional computational fluid dynamics (CFD) model is used to evaluate air flow and pollutant dispersion within an urban street canyon using Reynolds-averaged Navier–Stokes (RANS) equations and the Explicit Algebraic Reynolds Stress Models (EARSM) based on k-e turbulence model to close the equation system. The numerical model is performed with ANSYS-CFX code. Vehicle emissions were simulated as double line sources along the street. The numerical model was validated by the wind tunnel experiment results. Having established this, the wind flow and pollutant dispersion in urban street canyons (with six roof shapes buildings) are simulated. The numerical simulation results agree reasonably with the wind tunnel data. The results obtained in this work, indicate that the flow in 3D domain is more complicated; this complexity is increased with the presence of trees and variability of the roof shapes. The results also indicated that the largest pollutant concentration level for two walls (leeward and windward wall) is observed with the upwind wedge-shaped roof. But the smallest pollutant concentration level is observed with the dome roof-shaped.

Key Words
street canyon; pollutant dispersion; trees; building configuration; numerical simulation; EARSM

Address
Lakhdar Bouarbi, Bouabdellah Abed and Mohamed Bouzit: Laboratoire de mécanique appliquée, Faculté de génie mécanique, Université des sciences et de la technologie d\'Oran – Mohamed-Boudiaf, BP 1505, Oran El M\'Naouer, 31000, Oran, Algeria

Abstract
Snowdrift formation on roofs should be considered in snowy and windy areas to ensure the safety of buildings. Presently, the prediction of snowdrifts on roofs relies heavily on field measurements, wind tunnel tests and numerical simulations. In this paper, a new snowdrift modeling method by using CFD (Computational Fluid Dynamics) coupled with DEM (Discrete Element Method) is presented, including material parameters and particle size, collision parameters, particle numbers and input modes, boundary conditions of CFD, simulation time and inlet velocity, and coupling calculation process. Not only is the two-way coupling between wind and snow particles which includes the transient changes in snow surface topography, but also the cohesion and collision between snow particles are taken into account. The numerical method is applied to simulate the snowdrift on a typical stepped flat roof. The feasibility of using coupled CFD with DEM to study snowdrift is verified by comparing the simulation results with field measurement results on the snow depth distribution of the lower roof.

Key Words
snowdrift; CFD; DEM; snow depth distribution; flow field characteristics

Address
Lei Zhao, Fu Zhu, Xin Qi and Shichun Zhao: School of Civil Engineering, Southwest Jiaotong University, Chengdu 610031, China
Zhixiang Yu: School of Civil Engineering, Southwest Jiaotong University, Chengdu 610031, China;
Key Laboratory of High-speed Railway Engineering, Ministry of Education, Chengdu 610031, China


Abstract
The response of functionally graded ceramic-metal plates is investigated using theoretical formulation, Navier\'s solutions, and a new displacement based on the high-order shear deformation theory are presented for static analysis of functionally graded plates. The theory accounts for a quadratic variation of the transverse shear strains across the thickness, and satisfies the zero traction boundary conditions on the top and bottom surfaces of the plate without using shear correction factors. The plates are assumed to have isotropic, two-constituent material distribution through the thickness, and the modulus of elasticity of the plate is assumed to vary according to a power-law distribution in terms of the volume fractions of the constituents. Numerical results of the new refined plate theory are presented to show the effect of the material distribution on the deflections, stresses and fundamental frequencies. It can be concluded that the proposed theory is accurate and simple in solving the static and free vibration behavior of functionally graded plates.

Key Words
theoretical formulation; Navier\'s solutions; FGM plate; static

Address
Lazreg Hadji: Département de Génie Civil, Université Ibn Khaldoun, BP 78 Zaaroura, 14000 Tiaret, Algérie;
Laboratoire des Matériaux & Hydrologie, Université de Sidi Bel Abbes, 22000 Sidi Bel Abbes, Algérie
Nafissa Zouatnia and Amar Kassoul: Laboratoire de Structures, Géotechnique et Risques, Université Hassiba Benbouali de Chlef, Algérie


Abstract
In order to investigate the influence of different blade positions on aerodynamic load and wind loads and load-effects of large scale wind turbine tower under the halt state, we take a certain 3 MW large scale horizontal axis three-blade wind turbine as the example for analysis. First of all, numerical simulation was conducted for wind turbine flow field and aerodynamic characteristics under different halt states (8 calculating conditions in total) based on LES (large eddy simulation) method. The influence of different halt states on the average and fluctuating wind pressure coefficients of turbine tower surface, total lift force and resistance coefficient, circular flow and wake flow characteristics was compared and analysed. Then on this basis, the time-domain analysis of wind loads and load-effects was performed for the wind turbine tower structure under different halt states by making use of the finite element method. The main conclusions of this paper are as follows: The halt positions of wind blade could have a big impact on tower circular flow and aerodynamic distribution, in which Condition 5 is the most unfavourable while Condition 1 is the most beneficial condition. The wind loads and load-effects of disturbed region of tower is obviously affected by different halt positions of wind blades, especially the large fluctuating displacement mean square deviation at both windward and leeward sides, among which the maximum response occurs in 350 to the tower top under Condition 8; the maximum bending moment of tower bottom occurs in 330 under Condition 2. The extreme displacement of blade top all exceeds 2.5 m under Condition 5, and the maximum value of windward displacement response for the tip of Blade 3 under Condition 8 could reach 3.35 m. All these results indicate that the influence of halt positions of different blades should be taken into consideration carefully when making wind-resistance design for large scale wind turbine tower.

Key Words
wind turbine tower; blade position; large eddy simulation; aerodynamic load; wind-induced response

Address
Shitang Ke: Jiangsu Key Laboratory of Hi-Tech Research for Wind Turbine Design, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China;
State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China
Wei Yu and Tongguang Wang: Jiangsu Key Laboratory of Hi-Tech Research for Wind Turbine Design, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
Lin Zhao and Yaojun Ge: State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China



Abstract
numerical analyses of small wind turbine operation, with particular emphasis on variable speed generator modelling in various wind speed conditions. The following characteristics are determined using the available computer tools: the tip speed ratio as a function of the generator constant (under the assumption of constant wind speed), the turbine coefficient of power as a function of the tip speed ratio (the torque curve is modified accordingly and generator speed and power curves are plotted), turbine power curves and coefficient of power curve as functions of the incoming wind speed. The last stage is to determine forces and torques acting on rotor blades and turbine tower for specific incoming wind speeds in order to examine the impact of the stall phenomena on these values (beyond the rated power of the turbine). It is shown that the obtained results demonstrate a valuable guideline for small wind turbines design process.

Key Words
small wind turbines; numerical analysis; FAST; generator; swt; wind energy

Address
Jakub Bukala, Krzysztof Damaziak and Jerzy Malachowski: Department of Mechanics and Applied Computer Science, Military University of Technology, gen. Sylvester Kaliski 2, 00-908 Warsaw, Poland
Hamid Reza Karimi: Department of Engineering, Faculty of Technology and Sciences, University of Agder, Jon Lilletuns vei 9, 4879 Grimstad, Norway



Abstract
Large amplitude oscillation of steepled main cables usually presents during construction of a long-span bridge. To study this phenomenon, six typical main cables with different cross sections during construction are investigated. Two main foci have been conducted. Firstly, aerodynamic coefficients of a main cable are obtained and compared through simulation and wind tunnel test: (1) to ensure the simulation accuracy, influences of the numerical model\'s grid size, and the jaggy edges of main cable\' s aerodynamic coefficients are investigated; (2) aerodynamic coefficients of main cables at different wind attack angles are obtained based on the wind tunnel test in which the experimental model is made by rigid plastic using the 3D Printing Technology; (3) then numerical results are compared with wind tunnel test results, and they are in good agreement. Secondly, aerodynamic coefficients of the six main cables at different wind attack angles are obtained through numerical simulation. Then Den Hartog criterion is used to analyze the transverse galloping of main cables during construction. Results show all the six main cables may undergo galloping, which may be an important reason for the large amplitude oscillation of steepled main cables during construction. The flow structures around the main cables indicate that the characteristic of the airflow trajectory over a steepled main cable may play an important role in the galloping generation. Engineers should take some effective measures to control this harmful phenomenon due to the big possibility of the onset of galloping during the construction period.

Key Words
main cable; galloping; wind tunnel test; suspension bridge; construction period

Address
Yonghui An: Department of Civil Engineering, State Key Laboratory of Coastal and Offshore Engineering,
State Key Laboratory of Structural Analyses for Industrial Equipment, Dalian University of technology, Dalian, China
Chaoqun Wang, Shengli Li and Dongwei Wang: School of Civil Engineering, Zhengzhou University, Zhengzhou, China


Abstract
Although the underlying mechanism of rain-wind induced vibrations (RWIVs) of stay cables has not been fully understood, some countermeasures have been successfully applied to mitigating this kind of vibration. Among these, installing dampers near the bridge deck was widely adopted, and several field observations have shown its effectiveness. In this study, the effectiveness of dampers to RWIVs of stay cables is numerically investigated comprehensively by means of finite difference method (FDM). Based on the free vibration analysis of a taut string, it is found that the 3-points triangle scheme, which can be easily implemented in FDM, can offer an excellent approximation of the concentrated damping coefficient (expressed as a Dirac delta function) at the location where the damper is installed. Then, free vibration analysis of a 3-D continuous stay cable attached with two dampers is carried out to study the relationship of modal damping ratio and damping coefficient of the dampers. The effects of orientation of the dampers and cable sag on the modal damping ratio are investigated in detail. Finally, the RWIV response of a 3-D continuous stay cable attached with two dampers is examined. The results indicate that 0.5% of damping ratio is sufficient to reduce the RWIV vibration of the Cable A20 on the No.2 Nanjing Bridge over Yangtze River.

Key Words
3-D continuous stay cable; Rain-wind induced vibration; dampers; numerical simulations; finite difference method

Address
Shouying Li: Hunan Provincial Key Laboratory of Wind Engineering and Bridge Engineering, Hunan University,
Changsha 410082, China
Teng Wu: Department of Civil, Structural and Environmental Engineering, University at Buffalo,
State University of New York, Buffalo, NY 14126, USA
Shouke Li: School of Civil Engineering, Hunan University of Science and Technology, Xiangtan 411201, China
Ming Gu:State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University, Shanghai, 200092, China



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