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CONTENTS
Volume 20, Number 4, April 2015
 

Abstract
In this paper, a novel methodology is proposed to obtain optimum location of outriggers. The method utilizes genetic algorithm (GA) for shape and size optimization of outrigger-braced tall structures. In spite of previous studies (simplified methods), current study is based on exact modeling of the structure in a computer program developed on Matlab in conjunction with OpenSees. In addition to that, exact wind loading distribution is calculated in accordance with ASCE 7-10. This is novel since in previous studies wind loading distributions were assumed to be uniform or triangular. Also, a new penalty coefficient is proposed which is suitable for optimization of tall buildings .Newly proposed penalty coefficient improves the performance of GA and results in a faster convergence. Optimum location and number of outriggers is investigated. Also, contribution of factors like central core and outrigger rigidity is assessed by analyzing several design examples. According to the results of analysis, exact wind load distribution and modeling of all structural elements, yields optimum designs which are in contrast of simplified methods results. For taller frames significant increase of wind pressure changes the optimum location of outriggers obtained by simplified methods. Ratio of optimum location to the height of the structure for minimizing weight and satisfying serviceability constraints is not a fixed value. Ratio highly depends on height of the structure, core and outriggers stiffness and lateral wind loading distribution.

Key Words
tall buildings; optimization; outrigger-braced structures; exact wind loading; optimum design

Address
Farshid Nouri and Payam Ashtari: Department of Civil Engineering, University of Zanjan, Zanjan, Iran

Abstract
A three-dimensional aerodynamic model and a vehicle dynamics model are established to investigate the effect of noise barrier on the dynamic performance of a high-speed train running on an embankment in crosswind in this paper. Based on the developed model, flow structures around the train with and without noise barrier are compared. Effect of the noise barrier height on the train dynamic performance is studied. Then, comparisons between the dynamic performance indexes of the train running on the windward track and on the leeward track are made. The calculated results show that the noise barrier has significant effects on the structure of the flow field around the train in crosswind and thus on the dynamic performance of the high-speed train. The dynamic performance of the train on the windward track is better than that on the leeward track. In addition, various heights of the noise barrier will have different effects on the train dynamic performance. The dynamic performance indexes keep decreasing with the increase of the noise barrier height before the height reaches a certain value, while these indexes have an inverse trend when the height is above this value. These results suggest that optimization on the noise barrier height is possible and demonstrate that the designed noise barrier height of the existing China Railway High-speed line analysed in this article is reasonable from the view point of the flow field structure and train dynamic performance although the noise barrier is always designed based on the noise-related standard.

Key Words
high-speed railway; noise barrier; crosswind; train dynamic performance; aerodynamics

Address
Hai Zhao: Qingdao Sifang Rolling Stock Research Institute Co., Ltd., Qingdao 266031, P. R. China
Wanming Zhai and Zaigang Chen: Train and Track Research Institute, State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu 610031, P. R. China

Abstract
Aeroelastic wind tunnel experiments were conducted for conventional and tapered super-tall building models to investigate the effect of taper on fundamental aeroelastic behaviors in various incident flows. Three incident flows were simulated: a turbulent boundary-layer flow representing urban area; a low-turbulent flow; and a grid-generated flow. Results were summarized focusing on the effect of taper and the effect of incident flows. The suppression of responses by introducing taper was profound in the low-turbulence flow and boundary-layer flow, but in the grid-generated flow, the response becomes larger than that of the square model when the wind is applied normal to the surface. The effects of taper and incident flows were clearly shown on the normalized responses, power spectra, stability diagrams and probability functions.

Key Words
wind tunnel experiment; aeroelastic behavior; taper; low-turbulence flow; grid-generated flow; boundary-layer flow

Address
Yong Chul Kim: Wind Engineering Research Center, Tokyo Polytechnic University, 1583 Iiyama, Atsugi 242-0297, Japan
Yukio Tamura: School of Civil Engineering, Beijing Jiaotong University, 3 Shangyuancun, Xizhimenwai, Beijing 100044, P.R. China
Sung-won Yoon: School of Architecture, Seoul National University of Science and Technology, 232 Gongneung-ro Nowon-gu, Seoul 139-743, Korea

Abstract
The three-dimensional unsteady incompressible Reynolds-averaged Navier-Stokes equations and k-e double equations turbulent model were used to investigate the effect on the measurements of anemometers due to a passing high-speed train. Sliding mesh technology in Fluent was utilized to treat the moving boundary problem. The high-speed train considered in this paper was with bogies and inter-carriage gaps. Combined with the results of the wind tunnel test in a published paper, the accuracy of the present numerical method was validated to be used for further study. In addition, the difference of slipstream between three-car and eight-car grouping models was analyzed, and a series of numerical simulations were carried out to study the influences of the anemometer heights, the train speeds, the crosswind speeds and the directions of the induced slipstream on the measurements of the anemometers. The results show that the influence factors of the train-induced slipstream are the passing head car and tail car. Using the three-car grouping model to analyze the train-induced flow is reasonable. The maxima of horizontal slipstream velocity tend to reduce as the height of the anemometer increases. With the train speed increasing, the relationship between Vtrain and Vinduced slipstream can be expressed with linear increment. In the absence of natural wind conditions, from the head car arriving to the tail car leaving, the induced wind direction changes about 330, while under the crosswind condition the wind direction fluctuates around -90. With the crosswind speed increasing, the peaks of VX, |VXY -Vwind| of the head car and that of VX of the tail car tend to enlarge. Thus, when anemometers are installed along high-speed railways, it is important to study the effect on the measurements of anemometers due to the train-induced slipstream.

Key Words
train-induced slipstream; anemometer; crosswind; direction; numerical simulation

Address
Jie Zhang, Guangjun Gao, Sha Huang and Tanghong Liu: Key Laboratory of Traffic Safety on Track of Ministry of Education, School of Traffic & Transportation Engineering, Central South University, Changsha 410075, Hunan, China

Abstract
A wind energy assessment is an integrated analysis of the potential of wind energy resources of a particular area. In this work, the wind energy potentials for Mauritius have been assessed using a Computational Fluid Dynamics (CFD) model. The approach employed in this work aims to enhance the assessment of wind energy potentials for the siting of large-scale wind farms in the island. Validation of the model is done by comparing simulated wind speed data to experimental ones measured at specific locations over the island. The local wind velocity resulting from the CFD simulations are used to compute the weighted-sum power density including annual directional inflow variations determined by wind roses. The model is used to generate contour maps of velocity and power, for Mauritius at a resolution of 500 m.

Key Words
Computational Fluid Dynamics (CFD); WindSim; complex terrains; wind power map; Mauritius Island

Address
Asma Z. Dhunny, Michel R. Lollchund and Soonil D.D.V. Rughooputh: Department of Physics, University of Mauritius, Reduit., Mauritius

Abstract
A Combination Random Flow Generation (CRFG) technique for obtaining the fluctuating inflow boundary conditions for Large Eddy Simulation (LES) is proposed. The CRFG technique was developed by combining the typical RFG technique with a novel calculation of k and e to estimate the length- and time-scales (l, t) of the target fluctuating turbulence field used as the inflow boundary conditions. Through comparatively analyzing the CRFG technique and other existing numerical/experimental results, the CRFG technique was verified for the generation of turbulent wind velocity fields with prescribed turbulent statistics. Using the turbulent velocity fluctuations generated by the CRFG technique, a series of LESs were conducted to investigate the wind flow around S-, R-, L- and U-shaped building models. As the pressures of the models were also measured in wind tunnel tests, the validity of the LES, and the effectiveness of the inflow boundary generated by the CRFG techniques were evaluated through comparing the simulation results to the wind tunnel measurements. The comparison showed that the LES accurately and reliably simulates the wind-induced pressure distributions on the building surfaces, which indirectly validates the CRFG technique in generating realistic fluctuating wind velocities for use in the LES. In addition to the pressure distribution, the LES results were investigated in terms of wind velocity profiles around the building models to reveal the wind flow dynamics around bluff bodies. The LES results quantitatively showed the decay of the bluff body influence when the flow moves away from the building model.

Key Words
computational wind engineering; large eddy simulation; inflow boundary condition; random turbulence generation technique; wind pressure characteristic; wind velocity profile

Address
Dayang Wang and Y. Zho: School of Civil Engineering, Guangzhou University, Guangzhou 510006, P.R. China
X.J. Yu and K.T. Tse: Department of Civil and Environmental Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong


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