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CONTENTS
Volume 33, Number 1, July 2021
 

Abstract
The overturning issues in a strong wind are extremely critical to the railway vehicles, which have attracted a great deal of attention over the years. To address such problems, this paper introduces a dynamic reliability approach to evaluate the overturning risk of vehicles in crosswinds. Starting from the aerodynamic model, a novel prediction formula of unsteady crosswind forces with a consideration of the complete turbulent field effect is derived. Using the pseudo-excitation method (PEM), the power spectrum of vehicle responses is then calculated by the established vehicle model, and finally the corresponding results are used to assess the probabilistic overturning of vehicles in terms of the dynamic reliability analysis. It is found from the calculations that the time-dependent failure probability curves are related to the vehicle speed, wind speed, and crosswind direction, and the probabilistic characteristic wind curves (PCWCs) at different failure probabilities are more useful and reasonable for evaluating the overturning risk in comparison with the traditional characteristic wind curves (CWCs) in previous investigations. Furthermore, the probabilistic characteristic wind surface (PCWS) that considers the effect of crosswind direction, is developed, and it reveals that the vehicle is most vulnerable at a condition of perpendicular crosswind direction.

Key Words
cross wind; railway vehicles; overturning risk; failure probability; PCWCs

Address
Zhiyong Yao:School of Civil Engineering, Guangzhou University, Guangzhou 510006, PR China/ School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, PR China

Nan Zhang:School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, PR China/ Beijing Key Laboratory of Structural Wind Engineering and Urban Wind Environment, Beijing 100044, PR China

Abstract
In this study, three representative configurations of tornado-like vortices, i.e., single vortex, vortex breakdown and multi-vortex, are numerically simulated using large-eddy simulation (LES). Proper orthogonal decomposition (POD) is firstly employed to decompose flow-field snapshots into a series of orthogonal flow patterns (POD modes) and time-dependent coefficients. Then, a conditional-average analysis is conducted to obtain the four kinds of conditionally-averaged flow fields, which are then compared with instantaneous and ensemble-averaged flow fields. Next, a quadruple POD analysis is performed to decompose the instantaneous flow field into mean, coherent, transition and noise components. Finally, a qualitative analysis is implemented for unsteady vortex motions in horizontal and vertical planes. Results show that the conditional average shows larger-scale coherent structures than the classical ensemble average, while it loses the small-scale turbulent fluctuations present in instantaneous flow. The tornado vortex structure is controlled by the mean component in the single-vortex stage. With increase in swirl ratio, the tornado vortex evolves from single-vortex, to vortex-breakdown to multi-vortex, companied by kinetic energy transference to coherent and transition components. The horizontal and vertical vortex motions are essentially the results of horizontal and vertical velocity perturbations.

Key Words
tornado-like vortex; Computational Fluid Dynamic (CFD); Proper Orthogonal Decomposition (POD); conditional average; vortex motion

Address
Mengen Wang:College of Civil Engineering, Tongji University, Shanghai, 200092, China

Shuyang Cao:College of Civil Engineering, Tongji University, Shanghai, 200092, China/ State Key Lab of Disaster Reduction in Civil Engineering, Tongji University, Shanghai, 200092, China

Jinxin Cao:College of Civil Engineering, Tongji University, Shanghai, 200092, China/ State Key Lab of Disaster Reduction in Civil Engineering, Tongji University, Shanghai, 200092, China

Abstract
The current paper presents an investigation, which has its main objective in the verification of outdoor wind flow CFD simulation results (ANSYS R Fluent) with real environment measurements in urban setting. The details of the simulation set-up are discussed in the paper including the inlet boundary conditions, surface roughness parameters and the source/sink terms to represent the effect of trees. The simulation results are compared with the high-resolution on-site wind velocity measurements from the Measurement of Turbulence in an Urban Setup (MoTUS) project of Ecole Polytechnique Federale de Lausanne (EPFL) in Switzerland. Multiple simulations were conducted to evaluate the performance of the model based on different wind direction and speeds. At the end of the current study, the highly consistent and accurate results were observed from all 18 verification cases; with the RMSEs of the simulated wind velocities in range of 0.21 and 0.59 m/s only.

Key Words
Computational Fluid Dynamics; field verification; urban wind simulation; urban canopy

Address
Daniel Sang-Hoon Lee:Institute of Architecture and Technology, The Royal Danish Academy, Schools of Architecture, Design and Conservation, Philip de Langes Alle 10, Copenhagen, 1435, Denmark

Dasaraden Mauree:Solar Energy and Building Physics Laboratory, École polytechnique fédérale de Lausanne ENAC IIC LESO-PB,LE 2 201 (Batiment LE), Station 18, CH-1015, Lausanne, Switzerland/ BG Ingénieurs Conseils SA Route de Montfleury 3 1214 Vernier, Switzerland

Abstract
Constructing combined highway–railway bridge brings concerns regarding the aerodynamic interference between train and road vehicle. Research on the interaction mechanism can help calculate the vehicle response for the assessment of travelling safety. In this work, computational fluid dynamics (CFD) verified by a moving model test was applied on researching the aerodynamic characteristics of a moving van under the influence of train-induced wind. Two processes - encounter process (train and van drive towards each other) and chase process (train surpasses the van), are compared. The aerodynamic forces and pressure distribution of the van as well as the flow fields around the vehicles during the interaction are analyzed coherently. The results reveal that the adjacent positive and negative pressure zones around the nose and tail of the train bring moving and centralized high-pressure zone on the van's flank and generate significant aerodynamic variations, each variation contains at least two peak/valley values, and the middle carriage provide a stable transition between. Different superposition effect of the pressure zones results in difference between the encounter process and chase process, the variation trend of drag force and lift in the two processes are similar while the encounter has larger variation amplitude, in terms of pitching moment and yawing moment, more inversions of force happen in the encounter process but the variation amplitude is smaller. When the van runs near the nose of the train in the encounter, it gets the largest variation of drag force, lift force and rolling moment, while the largest variation of yawing moment and pitching moment happens when it runs near the nose of the train in the chase process.

Key Words
combined highway-railway bridge; aerodynamic interference; CFD; aerodynamic force; flow field

Address
Jiajun He:Department of Bridge Engineering, Southwest Jiaotong University,111 The north section of the second ring road, Jinniu District, Chengdu, Sichuan, PR China

Huoyue Xiang:Department of Bridge Engineering, Southwest Jiaotong University,111 The north section of the second ring road, Jinniu District, Chengdu, Sichuan, PR China/ Wind Engineering Key Laboratory of Sichuan Province, Southwest Jiaotong University,999 Xian road, Pidu district, Chengdu, Sichuan, PR China

Wenyuan Ren:Department of Bridge Engineering, Southwest Jiaotong University,111 The north section of the second ring road, Jinniu District, Chengdu, Sichuan, PR China

Yongle Li: Department of Bridge Engineering, Southwest Jiaotong University,111 The north section of the second ring road, Jinniu District, Chengdu, Sichuan, PR China/ Wind Engineering Key Laboratory of Sichuan Province, Southwest Jiaotong University,999 Xian road, Pidu district, Chengdu, Sichuan, PR China

Abstract
Spectral representation method (SRM) is the most classical one for the simulation of wind velocity. It is inefficiency when applied to large-scale non-stationary wind velocities with large simulation points. There are two reasons: numerous Cholesky decomposition and summation of Trigonometric terms. In order to improve the efficiency while ensuring accuracy, two aspects of work have been in this paper. (1) An adaptive interpolation-enhanced scheme is devised, which uses "average resolution" as the quantization index. This scheme can automatically realize the non-uniform distribution of interpolation points in two dimensions of time and frequency simultaneously, and improve the accuracy of interpolation. (2) The non-stationary wind velocities were reconstructed in time, frequency and space domain. Firstly, interpolation in time and frequency domain is directly applied to the H matrix, then proper orthogonal decomposition (POD) technology is introduced to decouple the wind velocities at spatial interpolation points, so as to obtain the time-dependent principal coordinates and space-dependent intrinsic mode function (IMF). Finally, IMF is reconstructed in the space domain to obtain the complete wind velocities. The above methodology is carried out to a super high-rise building containing 100 wind velocities simulation points and, results show that the proposed approach saves about 88% of the computational time compared with the classical SRM; saves about 47% of the computational time compared with the time-frequency interpolation based method. This paper achieves the rapid construction of large-scale non-stationary wind velocities.

Key Words
non-stationary; spectral representation method; proper orthogonal decomposition; Cholesky decomposition; time-frequency-space reconstruction; adaptive scheme

Address
Hui Han: Department of Civil Engineering, School of Mechanics and Engineering Science, Shanghai University,333 Nanchen Road, Shanghai 200444, China

Chunxiang Li: Department of Civil Engineering, School of Mechanics and Engineering Science, Shanghai University,333 Nanchen Road, Shanghai 200444, China

Jinhua Li: Department of Civil Engineering, East China Jiaotong University, Nanchang 330013, China

Abstract
In classical buffeting analysis theory, aerodynamic forces are usually expressed by a linear quasi-steady formula, and they are improved by aerodynamic admittances suitable for streamlined bridge girders. Recent studies have shown that admittances change obviously with incoming flow characteristics and aerodynamic nonlinearity, such as the frequency multiplication phenomenon, and motion-induced amplitude-related aerodynamic effects cannot be ignored in some cases. To address these problems, a nonlinear condensed subsystem equation (NCSE) suitable for wind-induced aerodynamic force modeling is established in the time domain. It characterizes aerodynamic nonlinearity with series of nonlinear differential equations and data-driven parameters. The proposed framework can be used for complex aerodynamic re-illustration related to the strong nonlinearity of streamlined box girders. To validate the precision and feasibility of the framework, sectional model experiments performed on a streamlined box girder were carried out in an active turbulence generated wind tunnel in which an adjustable array of multiple fans was assisted by actively controlled vibrating wings for a 2D turbulence condition. The case study shows that the NCSE model can be used to predict nonlinear aerodynamic forces in the time and frequency domains, even under complex stochastic flow conditions. The proposed method provides an alternative way to predict possible aerodynamics based on the condition of incoming flow with sufficient accuracy, and it can illustrate multifrequency components of aerodynamic forces.

Key Words
aerodynamic force; nonlinear condensed subsystem equation; streamlined box girder; multifrequency effect; actively controlled turbulence

Address
Lin Zhao: State Key Lab of Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China/ Key Laboratory of Transport Industry of Wind Resistant Technology for Bridge Structures, Tongji University, Shanghai 200092, China/ State Key Laboratory of Mountain Bridge and Tunnel Engineering, Chongqing Jiaotong University, Chongqing 400074, China

Shengyuan Liu: State Key Lab of Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China

Junfeng Yan: China Railway Siyuan Survey and Design Group Co., Ltd., Wuhan 430063, China

Yaojun Ge: State Key Lab of Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China/ Key Laboratory of Transport Industry of Wind Resistant Technology for Bridge Structures, Tongji University, Shanghai 200092, China

Abstract
Wind turbines are commonly used power generation systems around the world and their application is becoming increasingly widespread. Traditionally, they have been mounted on circular towers, but their recent upsizing has exposed weaknesses of these structures, including problems related to manufacturing and insufficient strength. Thus, the concept of site-assembled modular towers with polygonal cross-sections has been proposed, but their aerodynamic performances have not been properly investigated. In the present study, the aerodynamic performances of a wind turbine with seven polygonal towers were investigated. Wind tunnel tests have shown that the forces on the upper structure (rotor and nacelle) are larger than those on the tower, which makes the effect of cross-sectional shape of tower relatively small. Drag forces decrease with increasing number of sides of the tower, and lift forces on the square helical tower are quite small. For the power spectra, there are peaks in high reduced frequency for oblique wind directions at azimuth angles of 60° and 90°, which were considered to result from vortices that were formed and shed behind the blade in front of the tower.

Key Words
wind turbine; polygonal tower; pressure measurement; force measurement; drag force; lift force; power spectrum; time history analysis

Address
Y.C. Kim:Department of Architecture, Tokyo Polytechnic University, 2430297 Atsugi, Japan

Y. Tamura: Department of Civil Engineering, Chongqing University, 400045 Chongqing, Chin

Abstract
Intensive attention has been given to mitigating the dynamic responses of wind turbine towers (WTs) under wind and seismic excitations to ensure their safety and serviceability. This study details the damping mechanisms of a suspended particle damper (suspended PD) on the vibration control of a horizontal-axis WT. This damper combines the benefits of a tuned mass damper (TMD) and fixed PD, and can be effective without an external damping system. It therefore is a more practical solution for the vibration control of a WT. In this study, a finite element WT is built, and two damper systems with a TMD and suspended PD are modeled and compared. Ground motions and strong lateral winds are applied as external excitations to the operational and parked turbines, respectively. A full factorial study using a statistical method is conducted to determine the interaction effects of key parameters of the suspended PD. Results show that the damping effectiveness of a suspended PD is not sensitive to the external damping system under specific parameters, and it can be effective in detuned cases. Finally, a comparison between the optimal TMD and suspended PD on the vibration control of a WT is performed. The comparative results indicate that the performance of the suspended PD is considerably more robust than the TMD in wind-seismic excitations.

Key Words
vibration control; passive control; particle damper; tuned mass damper; wind turbine tower; parametric study

Address
Chenzhi Ma: College of Civil Engineering, Tongji University, Shanghai, 200092, China

Zheng Lu:College of Civil Engineering, Tongji University, Shanghai, 200092, China/ State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, Shanghai, 200092, China

Dianchao Wang: College of Civil Engineering, Tongji University, Shanghai, 200092, China

Zixin Wang: College of Civil Engineering, Tongji University, Shanghai, 200092, China

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