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CONTENTS
Volume 22, Number 1, January 2016
 

Abstract
The paper presents the state-of-the-art in aerodynamic performance of the modern horizontal axis wind turbine. The study examines the different complexities involved with wind turbine blade aerodynamic performance in open atmosphere and turbine wakes, and highlights the issues which require further investigations. Additionally, the latest concept of smart blades and frequently used wind turbine design analysis tools have also been discussed. The investigation made through this literature survey shows significant progress towards wind turbine aerodynamic performance improvements in general. However, still there are several parameters whose behavior and specific role in regulating the performance of the blades is yet to be elucidated clearly; in particular, the wind turbulence, rotational effects, coupled effect of turbulence and rotation, extreme wind events, formation and life time of the wakes.

Key Words
horizontal axis wind turbine; aerodynamic performance; complexities; natural wind; turbine wakes; design analysis tools

Address
Muhammad Ramzan Luhur and Abdul Latif Manganhar: Department of Mechanical Engineering, Quaid-e-Awam University of Engineering, Science and Technology, Nawabshah, Sindh, Pakistan
K.H. Solangi: Department of Mechanical Engineering, University of Malaya, Kuala Lumpur, Malaysia
Abdul Qayoom Jakhrani, Kishan Chand Mukwana and Saleem Raza Samo: Department of Energy and Environment Engineering, Quaid-e-Awam University of Engineering, Science and Technology, Nawabshah, Sindh, Pakistan



Abstract
To address the uncertainty of the flight trajectories caused by the turbulence and gustiness of the wind field over the roof and in the wake of a building, a 3-D probabilistic trajectory model of flat-type wind-borne debris is developed in this study. The core of this methodology is a 6 degree-of-freedom deterministic model, derived from the governing equations of motion of the debris, and a Monte Carlo simulation engine used to account for the uncertainty resulting from vertical and lateral gust wind velocity components. The influence of several parameters, including initial wind speed, time step, gust sampling frequency, number of Monte Carlo simulations, and the extreme gust factor, on the accuracy of the proposed model is examined. For the purpose of validation and calibration, the simulated results from the 3-D probabilistic trajectory model are compared against the available wind tunnel test data. Results show that the maximum relative error between the simulated and wind tunnel test results of the average longitudinal position is about 20%, implying that the probabilistic model provides a reliable and effective means to predict the 3-D flight of the plate-type wind-borne debris.

Key Words
probabilistic trajectory model; plate-type wind-borne debris; turbulent wind field; random gust; Monte-Carlo simulation

Address
Peng Huang, Feng Wang, Anmin Fu and Ming Gu: State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China

Abstract
In conventional buffeting theory, it is assumed that the aerostatic coefficients along a bridge deck follow the strip assumption. The validity of this assumption is suspect for a cable-stayed bridge in the construction stages, due to the effect of significant aerodynamic interference from the pylon. This situation may be aggravated in skew winds. Therefore, the most adverse buffeting usually occurs when the wind is not normal to bridge axis, which indicates the invalidity of the traditional \"cosine rule\". In order to refine the studies of static wind load on the deck of cable-stayed bridge under skew wind during its most adverse construction stage, a full bridge \'aero-stiff\' model technique was used to identify the aerostatic loads on each deck segment, in smooth oncoming flow, with various yaw angles. The results show that the shelter effect of the pylon may not be ignored, and can amplify the aerostatic loading on the bridge deck under skew winds (10-30) with certain wind attack angles, and consequently results in the \"cosine rule\" becoming invalid for the buffeting estimation of cable-stayed bridge during erection for these wind directions.

Key Words
aero-static coefficients; skew wind; shelter effect; cosine rule; full aero-stiff model; CFD simulation; cable-stayed bridge

Address
Shaopeng Li and Jiadong Zeng: Research Centre for Wind Engineering, Southwest Jiaotong University, Chengdu,Sichuan610031, China
Mingshui Li and Haili Liao: Key Laboratory for Wind Engineering of Sichuan Province, Chengdu, Sichuan 610031, China

Abstract
The wind velocity profile over the height of a structure in high intensity wind (HIW) events, such as downbursts, differs from that associated with atmospheric boundary layer (ABL) winds. Current design codes for lattice transmission structures contain only limited advice on the treatment of HIW effects, and structural design is carried out using wind load profiles and response factors derived for ABL winds. The present study assesses the load-deformation curve (capacity curve) of a transmission tower under modeled downburst wind loading, and compares it with that obtained for an ABL wind loading profile. The analysis considers nonlinear inelastic response under simulated downburst wind fields. The capacity curve is represented using the relationship between the base shear and the maximum tip displacement. The results indicate that the capacity curve remains relatively consistent between different downburst scenarios and an ABL loading profile. The use of the capacity curve avoids the difficulty associated with defining a reference wind speed and corresponding wind profile that are adequate and applicable for downburst and ABL winds, thereby allowing a direct comparison of response under synoptic and downburst events. Uncertainty propagation analysis is carried out to evaluate the tower capacity by considering the uncertainty in material properties and geometric variables. The results indicated the coefficient of variation of the tower capacity is small compared to those associated with extreme wind speeds.

Key Words
transmission towers; downbursts; extreme winds; nonlinear analysis; Monte Carlo technique

Address
T.G. Mara and T.C.E. Ho: The Boundary Layer Wind Tunnel Laboratory, University of Western Ontario, London,
ON, Canada, N6A 5B9
H.P. Hong: Department of Civil and Environmental Engineering, University of Western Ontario, London, ON, Canada, N6A 5B9
C.S. Lee: Department of Civil and Environmental Engineering, University of Western Ontario, London, ON, Canada, N6A 5B9;
Rowan Williams Davies & Irwin Inc., 650 Woodlawn Road West, Guelph, ON, Canada, N1K 1B8

Abstract
With an aim to assess the wind damage to urban trees in more realistic conditions, the nonlinear dynamics of structured trees subjected to strong winds with different levels is investigated in the present paper. For the logical treatment of dynamical behavior of trees, material nonlinearities of green wood associated with tree biomechanics and geometric nonlinearity of tree configuration are included. Applying simulated fluctuating wind velocity to the numerical model, the dynamical behavior of the structured tree is explored. A comparative study against the linear dynamics analysis usually involved in the previous researches is carried out. The failure wind velocity of urban trees is then defined, whereby the failure percentages of the tree components are exposed. Numerical investigations reveal that the nonlinear dynamics analysis of urban trees results in a more accurate solution of wind-induced response than the classical linear dynamics analysis, where the nonlinear effect of the tree behavior gives rise to be strengthened as increasing of the levels of wind velocity, i.e., the amplitude of 10-min mean wind velocity. The study of relationship between the failure percentage and the failure wind velocity provides a new perspective towards the vulnerability assessment of urban trees likely to fail due to wind actions, which is potential to link with the practical engineering.

Key Words
nonlinear dynamics; failure wind velocity; urban trees; geometric nonlinearity; tree biomechanics; wind damage

Address
Xiaoqiu Ai and Yingyao Cheng: Shanghai Institute of Disaster Prevention and Relief, Tongji University, China
Yongbo Peng: Shanghai Institute of Disaster Prevention and Relief, Tongji University, China;
State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, China


Abstract
The need for a more affordable, reliable, clean and secure energy has led to explorations in non-traditional sources, particularly renewable energies. Wind is one of the cleanest energy sources that plays a significant role in augmenting sustainability. Wind turbines, as energy convertors, are usually tall and slender structures, and depending on their location (inland or offshore), they can be subject to high wind and/or strong wave loadings. These loads can cause severe vibrations with detrimental effects on energy production, structural lifecycle and initial cost. A dissipativity analysis study was carried out to know whether wind turbine towers require damping enhancement or rigidity modifications for vibration suppression. The results suggest that wind turbines are lightly damped structures and damping enhancement is a potential solution for vibration lessening. Accordingly, the paper investigates different damping enhancement techniques for vibration mitigation. The efficacy of tuned mass damper (TMD), tuned liquid column damper (TLCD), tuned sloshing damper (TSD), and viscous damper (VD) to reduce vibrations is investigated. A comparison among these devices, in terms of robustness and effectiveness, is conducted. The VD can reduce both displacement and acceleration responses of the tower, better than other types of dampers, for the same control effort, followed by TMD, TSD, and finally TLCD. Nevertheless, the use of VDs raises concerns about where they should be located in the structure, and their application may require additional design considerations.

Key Words
dissipativity analysis; wind turbine; viscous dampers; vibration control; tuned mass dampers; liquid dampers; high-rise structures

Address
Milad Rezaee and Aly Mousaad Aly: Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, 70803, USA


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