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CONTENTS
Volume 29, Number 6, December 2019
 

Abstract
In the present paper, a simple refined nth-higher-order shear deformation theory is applied for the free vibration analysis of laminated composite plates. The proposed displacement field is based on a novel kinematic in which include the undetermined integral terms and contains only four unknowns, as against five or more in case of other higher-order theories. The present theory accounts for adequate distribution of the transverse shear strains through the plate thickness and satisfies the shear stress-free boundary conditions on the top and bottom surfaces of the plate, therefore, it does not require problem dependent shear correction factor. The governing equations of motion are derived from Hamilton\'s s principle and solved via Navier-type to obtain closed form solutions. The numerical results of non-dimensional natural frequencies obtained by using the present theory are presented and compared with those of other theories available in the literature to verify the validity of present solutions. It can be concluded that the present refined theory is accurate and efficient in predicting the natural frequencies of isotropic, orthotropic and laminated composite plates.

Key Words
nth-higher-order theory; free vibration; laminated composite; Hamilton\'s principle

Address
I. Klouche Djedid: Departement de Genie Civil, Universite Ibn Khaldoun Tiaret, BP 78Zaaroura, 1400 Tiaret, Algerie
Kada Draiche: Departement de Genie Civil, Universite Ibn Khaldoun Tiaret, BP 78Zaaroura, 1400 Tiaret, Algerie;
Material and Hydrology Laboratory, University of Sidi Bel Abbes, Faculty of Technology, Civil Engineering Department, Algeria
B. Guenaneche:Material and Hydrology Laboratory, University of Sidi Bel Abbes, Faculty of Technology, Civil Engineering Department, Algeria
Abdelmoumen Anis Bousahla: Laboratoire de Modélisation et Simulation Multi-echelle, Departement de Physique, Faculte des Sciences Exactes,
Departement de Physique, Universite de Sidi Bel Abbes, Algeria;
Centre Universitaire Ahmed Zabana de Relizane, Algeria
Abdelouahed Tounsi: 2Material and Hydrology Laboratory, University of Sidi Bel Abbes, Faculty of Technology, Civil Engineering Department, Algeria;
Department of Civil and Environmental Engineering, King Fahd University of Petroleum & Minerals,
31261 Dhahran, Eastern Province, Saudi Arabia
E.A. Adda Bedia: Centre of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, 21589, Saudi Arabia





Abstract
In this study, two original spectral representations of stationary stochastic fields, say the continuous proper orthogonal decomposition (CPOD) and the frequency-wavenumber spectral representation (FWSR), are derived from the Fourier-Stieltjes integral at first. Meanwhile, the relations between the above two representations are discussed detailedly. However, the most widely used conventional Monte Carlo schemes associated with the two representations still leave two difficulties unsolved, say the high dimension of random variables and the incompleteness of probability with respect to the generated sample functions of the stochastic fields. In view of this, a dimension-reduction model involving merely one elementary random variable with the representative points set owing assigned probabilities is proposed, realizing the refined description of probability characteristics for the stochastic fields by generating just several hundred representative samples with assigned probabilities. In addition, for the purpose of overcoming the defects of simulation efficiency and accuracy in the FWSR, an improved scheme of non-uniform wavenumber intervals is suggested. Finally, the Fast Fourier Transform (FFT) algorithm is adopted to further enhance the simulation efficiency of the horizontal stochastic wind velocity fields. Numerical examples fully reveal the validity and superiority of the proposed methods.

Key Words
stochastic wind velocity field; continuous proper orthogonal decomposition; frequency-wavenumber spectral representation; dimension reduction; non-uniform wavenumber intervals; FFT algorithm

Address
Zhangjun Liu, Zenghui Liu and Hailin Lu: School of Civil Engineering and Architecture, Wuhan Institute of Technology, Wuhan 430074, P.R. China
Chenggao He: College of Civil Engineering & Architecture, China Three Gorges University, Yichang 443002, P.R. China

Abstract
Aerodynamic characteristic of a small scale wind turbine under the influence of an incoming uniform wind field is studied using k-w Shear Stress Transport turbulence model. Firstly, the lift and drag characteristics of the blade section consisting of S826 airfoil is studied using 2D simulations at a Reynolds number of 1X 10. After that, the full turbine including the rotational effects of the blade is simulated using Multiple Reference Frames (MRF) and Sliding Mesh Interface (SMI) numerical techniques. The differences between the two techniques are quantified. It is then followed by a detailed comparison of the turbine\'s power/thrust output and the associated wake development at three tip speeds ratios ( = 3, 6, 10). The phenomenon of blockage effect and spatial features of the flow are explained and linked to the turbines power output. Validation of wake profiles patterns at multiple locations downstream is also performed at each . The present work aims to evaluate the potential of the numerical methods in reproducing wind tunnel experimental results such that the method can be applied to full-scale turbines operating under realistic conditions in which observation data is scarce or lacking.

Key Words
wind energy; aerodynamics; wind tunnel tests; computational fluid dynamics; high fidelity simulations

Address
M. Salman Siddiqui and Trond Kvamsdal: Department of Mathematical Sciences, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway
Adil Rasheed: Department of Mathematics and Cybernetics, SINTEF Digital, NO-7465 Trondheim, Norway

Abstract
To study the wake influence of an upstream bridge on the wind-resistance performance of a downstream bridge, two adjacent long-span cable-stayed bridges are taken as examples. Based on wind tunnel tests, the static aerodynamic coefficients and the dynamic response of the downstream bridge are measured in the wake of the upstream one. Considering different horizontal and vertical distances, the flutter derivatives of the downstream bridge at different angles of attack are extracted by Computational Fluid Dynamics (CFD) simulations and discussed, and the change in critical flutter state is further studied. The results show that a train passing through the downstream bridge could significantly increase the lift coefficient of the bridge which has the same direction with the gravity of the train, leading to possible vertical deformation and vibration. In the wake of the upstream bridge, the change in lift coefficient of the downstream bridge is reduced, but the dynamic response seems to be strong. The effect of aerodynamic interference on flutter stability is related to the horizontal and vertical distances between the two adjacent bridges as well as the attack angle of incoming flow. At large angles of attack, the aerodynamic condition around the downstream girder which may drive the bridge to torsional flutter instability is weakened by the wake of the upstream bridge, and the critical flutter wind speed increases at this situation.

Key Words
two adjacent bridges; horizontal and vertical distances; aerodynamic interference; static coefficients; dynamic response; flutter stability

Address
Zhenhua Chen, Haojun Tang, Yongle Li and Bin Wang: Department of Bridge Engineering, Southwest Jiaotong University, Chengdu 610031, China
Zhenyun Lin: Department of Bridge Engineering, Southwest Jiaotong University, Chengdu 610031, China;
HaiXia (Fujian) Transportation Engineering Design Co., Ltd., Fuzhou 350004, China


Abstract
The aim of this study was to explore the influence of structure coupling effect on structural damping of blade based on the blade vibration characteristic. For this purpose, the scaled blade model of NREL 5 MW offshore wind turbine was processed and employed in the wind tunnel test to validate the reliability of theoretical and numerical models. The attenuation curves of maximum displacement and the varying curves of equivalent damping coefficient of the blade under the rated condition were respectively compared and analyzed by constructing single blade model and whole machine model. The attenuation law of blade dynamic response was obtained and the structure coupling effect was proved to exert a significant influence on the equivalent damping coefficient. The results indicate that the attenuation trend of the maximum displacement response curve of the single blade varies more obviously with the increase of elastic modulus as compared to that under the structure coupling effect. In contrast to the single blade model, the varying curve of equivalent damping coefficient with the period is relatively steep for the whole machine model. The findings are of great significance to guide the structure design and material selection for wind turbine blades.

Key Words
offshore wind turbines; damping coefficient; structure coupling effect; wind tunnel test; elastic modulus

Address
Jianping Zhang and Haolin Li: School of Mechanical Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China
Zhen Gong, Mingqiang Wang and Fengfeng Shi: College of Energy and Mechanical Engineering, Shanghai University of Electric Power, Shanghai 200090, China
Zhiwei Zhang: Shanghai Green Environmental Protection Energy Co., Ltd, Shanghai 200433, China

Abstract
In this study, vortex induced vibrations of a cylinder mounted on a flexible rod are analyzed. This simple configuration represents the key element of new conception bladeless wind turbine (Whitlock 2015). In this study the structure oscillations equation coupled to the wake oscillation equation for this configuration are solved using analytical perturbation method, for the first time. An analytical expression that predicts the lock-in phenomena range of wind speed is derived. The discretized equations of motion are also solved using RKF45 numerical method. The equations of motion are discretized by Galerkin method. Free vibration mode shape of the structure taking into account the discontinuity of the cross section are used as comparison function. Numerical results are compared to the analytical results, and they show a satisfying agreement. The effect of system parameters on the oscillations of structure and wake as well as on the lock-in domain are presented. Moreover, it is shown that the values of wind speed triggering the start and the stop of the lock-in phenomenon, for increasing wind speed are different from those values obtained during the reverse process, i.e., when the wind speed decreases.

Key Words
vortex induced vibration; lock-in; van der Pol wake oscillator; circular cylinder; perturbation method

Address
Mehdi Zamanian: Department of Mechanical Engineering, Faculty of Engineering, Kharazmi University, P.O. Box 15719-14911, Tehran, Iran
Luigi Garibaldi: Dipartimento di Ingegneria Meccanica e Aerospaziale, Politecnico di Torino, C.so Duca degli Abruzzi, 24, 10129, Torino, Italy

Abstract
In this paper, a numerical solution is presented for supersonic flutter analysis of cantilever non-symmetric functionally graded (FG) sandwich plates. The plate is considered to be composed of two different functionally graded face sheets and an isotropic homogeneous core made of ceramic. Based on the first order shear deformation theory (FSDT) and linear piston theory, the set of governing equations and boundary conditions are derived. Dimensionless form of the governing equations and boundary conditions are derived and solved numerically using generalized differential quadrature method (GDQM) and critical velocity and flutter frequencies are calculated. For various values of the yaw angle, effect of different parameters like aspect ratio, thickness of the plate, power law indices and thickness of the core on the flutter boundaries are investigated. Numerical examples show that wings and tail fins with larger length and shorter width are more stable in supersonic flights. It is concluded for FG sandwich plates made of Al-Al2O3 that increase in volume fraction of ceramic (Al2O3) increases aeroelastic stability of the plate. Presented study confirms that improvement of aeroelastic behavior and weight of wings and tail fins of aircrafts are not consistent items. It is shown that value of the critical yaw angle depends on aspect ratio of the plate and other parameters including thickness and variation of properties have no considerable effect on it. Results of this paper can be used in design and analysis of wing and tail fin of supersonic airplanes.

Key Words
aeroelasticity; flutter; yawed flow; cantilever plate; sandwich plate

Address
Mohammad Hosseini and Mohammad Reza Karamizadeh: Department of Mechanical Engineering, Sirjan University of Technology, 78137-33385 Sirjan, Iran
Ali Ghorbanpour Arani: Faculty of Mechanical Engineering, Department of Solid Mechanics, University of Kashan, Kashan, Iran;
Institute of Nanoscience & Nanotechnology, University of Kashan, Kashan, Iran
Hassan Afshari: Department of Mechanical Engineering, Khomeinishahr Branch, Islamic Azad University, Khomeinishahr/Isfahan, Iran
Shahriar Niknejad: Faculty of Mechanical Engineering, Department of Solid Mechanics, University of Kashan, Kashan, Iran


Abstract
Wind pressure is a critical argument for the wind-resistant design of structures. The attempt, however, to explore the wind pressure field on buildings still encounters challenges though a large body of researches utilizing wind tunnel tests and wind field simulations were carried out, due to the difficulty in logical treatments on the scale effect and the modeling error. The full-scale measurement has not yet received sufficient attention. By performing a field measurement, the present paper systematically addresses wind pressures on the rectangular attic of a double-tower building. The spatial and temporal correlations among wind speed and wind pressures at measured points are discussed. In order to better understand the wind pressure distribution on the attic facades and its relationship against the approaching flow, a full-scale CFD simulation on the similar rectangular attic is conducted as well. Comparative studies between wind pressure coefficients and those provided in wind-load codes are carried out. It is revealed that in the case of wind attack angle being zero, the wind pressure coefficient of the cross-wind facades exposes remarkable variations along both horizontal and vertical directions; while the wind pressure coefficient of the windward facade remains stable along horizontal direction but exposes remarkable variations along vertical direction. The pattern of wind pressure coefficients, however, is not properly described in the existing wind-load codes.

Key Words
wind pressure; wind speed; spatial and temporal correlations; field measurement; CFD simulation; rectangular attic

Address
Yongbo Peng: State Key Laboratory of Disaster Reduction in Civil Engineering & Shanghai Institute of Disaster Prevention and Relief,
Tongji University, 1239 Siping Road, Shanghai 200092, P.R. China
Weijie Zhao: College of Civil Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, P.R. China
Xiaoqiu Ai: Shanghai Institute of Disaster Prevention and Relief, Tongji University, 1239 Siping Road, Shanghai 200092, P.R. China


Abstract
Wind-adaptable design (WAD) provides a new method for super-tall buildings to lessen design conflicts between architectural prerequisites and aerodynamic requirements, and to increase the efficiency of structural system. Compared to conventional wind-resistant design approach, the proposed new method is to design a building in two consecutive stages: a stage in normal winds and a stage during extreme winds. In majority of time, the required structural capacity is primarily for normal wind effects. During extreme wind storms, the building\'s capacity to wind loads is reinforced by on-demand operable flow control measures/devices to effectively reduce the loads. A general procedure for using WAD is provided, followed by an exploratory case study to demonstrate the application of WAD.

Key Words
wind-adaptable design; wind-resistant design; operable flow control; building aerodynamic optimization; wind effects; super-tall buildings

Address
Jiming Xie and Xiao-yue Yang: College of Civil Engineering and Architecture, Zhejiang University, 866 Yuhangtang Road, Hangzhou, China

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