Techno Press
Tp_Editing System.E (TES.E)
Login Search


was
 
CONTENTS
Volume 17, Number 4, October 2013
 

Abstract
Wall functions have been widely used in computational fluid dynamics (CFD) simulations and can save significant computational costs compared to other near-wall flow treatment strategies. However, most of the existing wall functions were based on the asymptotic characteristics of near-wall flow quantities, which are inapplicable in complex and non-equilibrium flows. A modified wall function is thus derived in this study based on flow over a plate at zero-pressure gradient, instead of on the basis of asymptotic formulations. Turbulent kinetic energy generation (GP), dissipation rate (e) and shear stress (tw) are composed together as the near-wall expressions. Performances of the modified wall function combined with the nonlinear realizable k -e turbulence model are investigated in homogeneous equilibrium atmosphere boundary layer (ABL) and flow around a 6 m cube. The computational results and associated comparisons to available full-scale measurements show a clear improvement over the standard wall function, especially in reproducing the boundary layer flow. It is demonstrated through the two case studies that the modified wall function is indeed adaptive and can yield accurate prediction results, in spite of its simplicity.

Key Words
wall function; k -e model; computational fluid dynamics (CFD); atmosphere boundary layer; bluff body

Address
Jian Zhang : School of Civil Engineering, Beijing Jiaotong University, Beijing, China;
Department of Civil and Architectural Engineering, City University of Hong Kong, Hong Kong
Qingshan Yang: School of Civil Engineering, Beijing Jiaotong University, Beijing, China
Q.S. Li: Department of Civil and Architectural Engineering, City University of Hong Kong, Hong Kong

Abstract
Large Eddy Simulations (LES) were carried out to investigate the aerodynamic characteristics of a rectangular cylinder with side ratio B/D=5 at Reynolds number Re=22,000 (based on cylinder thickness). Particular attention was devoted to the effects of velocity shear in the oncoming flow. Time-averaged and unsteady flow patterns around the cylinder were studied to enhance understanding of the effects of velocity shear. The simulation results showed that the Strouhal number has no significant variation with oncoming velocity shear, while the peak fluctuation frequency of the drag coefficient becomes identical to that of the lift coefficient with increase in velocity shear. The intermittently-reattached flow that features the aerodynamics of the 5:1 rectangular cylinder in non-shear flow becomes more stably reattached on the high-velocity side, and more stably separated on the low-velocity side. Both the mean and fluctuating drag coefficients increase slightly with increase in velocity shear. The mean and fluctuating lift and moment coefficients increase almost linearly with velocity shear. Lift force acts from the high-velocity side to the low-velocity side, which is similar to that of a circular cylinder but opposite to that of a square cylinder under the same oncoming shear flow.

Key Words
rectangular cylinder; shear parameter; large eddy simulation; aerodynamic forces; vortex shedding; flow reattachment

Address
Qiang Zhou, Shuyang Cao and Zhiyong Zhou : State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University,Shanghai 200092, P. R. China

Abstract
The structural vibration suppression with active constrained layer damping (ACLD) was widely studied recently. However, the literature seldom concerned with the vibration control on flow-induced vibration using active constrained layer. In this paper the wind induced vibration of cantilevered beam is analyzed and suppressed by using random theory together with a velocity feedback control strategy. The piezoelectric material and frequency dependent viscoelastic layer are used to achieve effective active damping in the vibration control. The transverse displacement and velocity in time and frequency domains, as well as the power spectral density and the mean-square value of the transverse displacement and velocity, are formulated under wind pressure at variable control gain. It is observed from the numerical results that the wind induced vibration can be significantly suppressed by using a small outside active voltage on the constrained layer.

Key Words
wind induced vibration; active control; fluctuating wind; cantilevered beam; velocity feedback

Address
Jinqiang Li : College of Mechanics, Taiyuan University of Technology, China;
Faculty of Engineering, Hokkaido University, Japan
Yoshihiro Narita : Faculty of Engineering, Hokkaido University, Japan

Abstract
This paper investigates the vertical profiles of horizontal mean wind speed and direction based on the synchronized measurements from a Doppler radar profiler and an anemometer during 16 tropical cyclones at a coastal site in Hong Kong. The speed profiles with both open sea and hilly exposures were found to follow the log-law below a height of 500 m. Above this height, there was an additional wind speed shear in the profile for hilly upwind terrain. The fitting parameters with both the power-law and the log-law varied with wind strength. The direction profiles were also sensitive to local terrain setups and surrounding topographic features. For a uniform open sea terrain, wind direction veered logarithmically with height from the surface level up to the free atmospheric altitude of about 1200 m. The accumulated veering angle within the whole boundary layer was observed to be 30o. Mean wind direction under other terrain conditions also increased logarithmically with height above 500 m with a trend of rougher exposures corresponding to lager veering angles. A number of empirical parameters for engineering applications were presented, including the speed adjustment factors, power exponents of speed profiles, and veering angle, etc. The objective of this study aims to provide useful information on boundary layer wind characteristics for wind-resistant design of high-rise structures in coastal areas.

Key Words
tropical cyclone; wind profile; boundary layer; hilly terrain; Doppler radar

Address
Y.C. He and Q.S. Li : Department of Civil and Architectural Engineering, City University of Hong Kong, Hong Kong
P.W. Chan : Hong Kong Observatory, Kowloon, Hong Kong

Abstract
The vehicle-induced aerodynamic loads bring vibrations to some of the highway sound barriers, for they are designed in consideration of natural wind loads only. A field experiment is carried out with respect to three important factors: vehicle type, vehicle speed and the vehicle-barrier separation distance. Based on the results, the time-history of pressures is given, showing identical characteristics in all cases. Therefore, the vehicle-induced aerodynamic loads acting on the highway sound barrier are summarized as the combination of \"head impact\" and \"wake impact\". The head impact appears to have potential features, while the wake impact is influenced by the rotational flow. Then parameters in the experiment are analyzed, showing that the head impact varies with vehicle speed, vehicle-barrier separation distance, vehicle shape and cross-sectional area, while the wake impact is mainly about vehicle-barrier separation distance and vehicle length.

Key Words
vehicle-induced aerodynamic loads; highway sound barriers; field experiment

Address
Dalei Wang, Benjin Wang and Airong Chen : Department of Bridge Engineering, Tongji University, Shanghai, 200092, China

Abstract
This study develops an extended inverse input estimation algorithm with intelligent adaptive fuzzy weighting to effectively estimate the unknown input wind load of nonlinear structural systems. This algorithm combines the extended Kalman filter and recursive least squares estimator with intelligent adaptive fuzzy weighting. This study investigated the unknown input wind load applied on a tower structural system. Nonlinear characteristics will exist in various structural systems. The nonlinear characteristics are particularly more obvious when applying larger input wind load. Numerical simulation cases involving different input wind load types are studied in this paper. The simulation results verify the nonlinear characteristics of the structural system. This algorithm is effective in estimating unknown input wind loads.

Key Words
fuzzy estimator; fuzzy Kalman filter; least square method; fuzzy logic

Address
Ming-Hui Lee : Department of Civil Engineering, Chinese Military Academy, Fengshan, Kaohsiung, Taiwan, R.O.C.


Techno-Press: Publishers of international journals and conference proceedings.       Copyright © 2017 Techno-Press
P.O. Box 33, Yuseong, Daejeon 34186 Korea, Tel: +82-42-828-7996, Fax : +82-42-828-7997, Email: info@techno-press.com