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CONTENTS
Volume 16, Number 5, May 2013
 

Abstract
A computational study of vortex-induced transverse vibrations of a cylinder with low mass-damping is presented. An Arbitrary Lagrangian-Eulerian (ALE) formulation of the Unsteady Reynolds-Averaged Navier-Stokes equations (URANS), along with the Spalart-Allmaras (SA) one-equation turbulence model, are coupled conservatively with rigid body motion equations of the cylinder mounted on elastic supports in order to study the amplitude and frequency response of a freely vibrating cylinder, its flow-induced motion, Vortex Street, near-wake flow structure, and unsteady loading in a moderate range of Reynolds numbers. The time accurate response of the cylinder from rest to its limit cycle is studied to explore the effects of Reynolds number on the start of large displacements, motion amplitude, and frequency. The computational results are compared with published physical experiments and numerical studies. The maximum amplitudes of displacements computed for various Reynolds numbers are smaller than the experimental values; however, the overall agreement of the results is quite satisfactory, and the upper branch of the limit-cycle displacement amplitude vs. reduced velocity response is captured, a feature that was missed by other studies. Vortex shedding modes, lock-in phenomena, frequency response, and phase angles are also in agreement with experiments.

Key Words
vortex-induced vibrations; URANS; fluid-structure interaction; low mass-damping; turbulent flows

Address
Amir Borna, Wagdi G. Habashi and Siva K. Nadarajah : Computational Fluid Dynamics Laboratory, Department of Mechanical Engineering, McGill University, Montreal, QC, Canada H3A 2S6;
Ghyslaine McClure : Department of Civil Engineering and Applied Mechanics, McGill University, Montreal, QC, Canada H3A 2S6

Abstract
A phase delay spectrum model towards the representation of spatial coherence of stochastic wind fields is proposed. different from the classical coherence functions used in the spectral representation methods, the model is derived from the comprehensive description of coherence of fluctuating wind speeds and from the thorough analysis of physical accounts of random factors affecting phase delay, building up a consistent mapping between the simulated fluctuating wind speeds and the basic random variables. It thus includes complete probabilistic information of spatial stochastic wind fields. This treatment prompts a ready and succinct scheme for the simulation of fluctuating wind speeds, and provides a new perspective to the accurate assessment of dynamic reliability of wind-induced structures. Numerical investigations and comparative studies indicate that the developed model is of rationality and of applicability which matches well with the measured data at spatial points of wind fields, whereby the phase spectra at defined datum mark and objective point are feasibly obtained using the umerical scheme associated with the starting-time of phase evolution. In conjunction with the stochastic Fourier amplitude pectrum that we developed previously, the time history of luctuating wind speeds at any spatial points of wind fields can be readily simulated.

Key Words
phase delay spectrum; stochastic wind field; spatial coherence; coherence function; wind field simulation; spectral representation method

Address
Qi Yan and Jie Li : School of Civil Engineering, Tongji University, Shanghai 200092, China;
Yongbo Peng and Jie Li : State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, China;
Yongbo Peng : Shanghai Institute of Disaster Prevention and Relief, Tongji University, China

Abstract
This paper presents theoretical background for a semi-empirical, mathematical model of critical vortex excitation of slender structures of compact cross-sections. The model can be applied to slender tower-like structures (chimneys, towers), and to slender elements of structures (masts, pylons, cables). Many empirical formulas describing across-wind load at vortex excitation depending on several flow parameters, Reynolds number range, structure geometry and lock-in phenomenon can be found in literature. The aim of this paper is to demonstrate mathematical background of the vortex excitation model for a theoretical case of the structure section. Extrapolation of the mathematical model for the application to real structures is also presented. Considerations are devoted to various cases of wind flow (steady and unsteady), ranges of Reynolds number and lateral vibrations of structures or their absence. Numerical implementation of the model with application to real structures is also proposed.

Key Words
across-wind load; vortex excitation; lateral vibrations; sectional model; circular cross-section

Address
T. Lipecki : Faculty of Civil Engineering and Architecture, Lublin University of Technology, Lublin, Poland;
A. Flaga : Wind Engineering Laboratory, Cracow University of Technology, Cracow, Poland

Abstract
This paper presents results of calculations performed according to our own semi-empirical mathematical model of critical vortex excitation. All calculations are carried out using own computer program, which allows the simulation of both the across-wind action caused by vortices and the lateral response of analysed structures. Vortex excitation simulations were performed in real time taking into account wind-structure interaction. Several structures of circular cross-sections were modelled using a FEM program and calculated under the action of critical vortex excitation. Six steel chimneys, six concrete chimneys and two concrete towers were considered. The method of selection and estimation of the experimental parameters describing the model are also presented. Finally, the results concerning maximum lateral top displacements of the structures are compared with available full-scale data for steel and concrete chimneys.

Key Words
across-wind load; vortex excitation; steel chimneys; concrete chimneys; towers; lateral vibrations; circular cross-section

Address
T. Lipecki : Faculty of Civil Engineering and Architecture, Lublin University of Technology, Lublin, Poland;
A. Flaga: Wind Engineering Laboratory, Cracow University of Technology, Cracow, Poland

Abstract
Wind pressure characteristics on a double tower high-rise structure, which is disturbed by surrounding buildings, were investigated using large eddy simulation (LES) and 1:300 scale wind tunnel experiments. The computational simulation technique and wind tunnel experimental technique were described in detail initially. Comparisons of computational results with the experimental data have subsequently been carried out to validate the reliability of LES. Comparisons have been performed in detail for the mean and fluctuating pressure coefficients. Detailed explanations of each comparison were given in the paper. To study further on the pressure coefficients on the building surfaces, parametric studies on shape coefficient and spatial correlation were performed and investigated. The numerical and experimental results presented in this paper advance understanding on wind field around buildings and the application of LES and wind tunnel tests.

Key Words
computation; building; disturbance; pressure coefficient; spatial correlation; wind tunnel

Address
K.T. Tse : Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong;
D.Y. Wang and Y. Zhou : School of Civil Engineering, Guangzhou University, Guangdong, P.R. China

Abstract
This paper presents applications of proper orthogonal composition in both the time and frequency domains based on both cross spectral matrix and covariance matrix branches to analyze multi-variate unsteady pressure fields on prisms and to study spanwise and chordwise pressure distribution. Furthermore, modification of proper orthogonal decomposition is applied to a rectangular spanwise coherence matrix in order to investigate the spanwise correlation and coherence of the unsteady pressure fields. The unsteady pressure fields have been directly measured in wind tunnel tests on some typical prisms with slenderness ratios B/D=1, B/D=1 with a splitter plate in the wake, and B/D=5. Significance and contribution of the first covariance mode associated with the first principal coordinates as well as those of the first spectral eigenvalue and associated spectral mode are clarified by synthesis of the unsteady pressure fields and identification of intrinsic events inside the unsteady pressure fields. Spanwise coherence of the unsteady pressure fields has been mapped the first time ever for better understanding of their intrinsic characteristics.

Key Words
unsteady pressure; pressure distribution; spanwise correlation; spanwise coherence; coherence mapping; covariance proper orthogonal decomposition; spectral proper orthogonal decomposition

Address
Le Thai Hoa: Wind Engineering Research Center, Tokyo Polytechnic University,1583 Iiyama, Atsugi, Kanagawa 243-0297, Japan; Hanoi University of Engineering and Technology, Vietnam National University, Hanoi,144 Xuan Thuy,Cau Giay, Hanoi, Vietnam;
Yukio Tamura: Department of Architectural Engineering, Tokyo Polytechnic University,1583 Iiyama, Atsugi, Kanagawa 243-0297, Japan;
Masaru Matsumoto and Hiromichi Shirato: Department of Civil Engineering, Kyoto University, Kyoto-Daigaku-Katsura, Nishikyo, Kyoto 615-8530, Japan


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