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CONTENTS
Volume 17, Number 1, July 2013
 

Abstract
A multiscale finite element method is applied to the Spalart-Allmaras turbulence model based detached-eddy simulation (DES). The multiscale arises from a decomposition of the scalar field into coarse (resolved) and fine (unresolved) scales. It corrects the lack of stability of the standard Galerkin formulation by modeling the scales that cannot be resolved by a given spatial discretization. The stabilization terms appear naturally and the resulting formulation provides effective stabilization in turbulent computations, where reaction-dominated effects strongly influence near-wall predictions. The multiscale DES is applied in the context of high-Reynolds flow over the Commonwealth Advisory Aeronautical Council (CAARC) standard tall building model, for both uniform and turbulent inflows. Time-averaged pressure coefficients on the exterior walls are compared with experiments and it is demonstrated that DES is able to resolve the turbulent features of the flow and accurately predict the surface pressure distributions under atmospheric boundary layer flows.

Key Words
multiscale method; detached-eddy simulation; computational wind engineering; inflow turbulence generation; CAARC model

Address
Yue Zhang, Rooh A. Khurram and Wagdi G. Habashi : NSERC-J.-Armand Bombardier Industrial Research Chair for Multi-disciplinary Analysis and Design of Aerospace Systems CFD Lab, Department of Mechanical Engineering, McGill University Montreal, QC, Canada H3A 2S6

Abstract
Wind-induced vibrations of mast arms of cantilever traffic signal structures can lead to fatigue failure. Two such structures were instrumented each with a sonic anemometer and a camera that records the motions of the tip of the arm. It was observed throughout this experiment that large amplitude vertical vibrations of mast arms with signals with backplates occur for the most part at low wind speed ranges, between 2 to 7 m/s, and as the wind speed increases the amplitude of the vertical vibrations decreases. The results of these experiments contradict the generally accepted belief that vortex shedding does not cause significant vibrations of mast arms that could lead to fatigue failure, which have been attributed to galloping in the past . Two damping devices were tested with mixed results.

Key Words
cantilevered traffic signal structures; fatigue; wind-induced vibrations; galloping; vortex shedding; vented backplates; damping plate

Address
Hector J. Cruzado : Department of Civil and Environmental Engineering, Polytechnic University of Puerto Rico, San Juan, PR, USA
Chris Letchford : Department of Civil and Environmental Engineering, Rensselaer Polytechnic Institute, Troy, NY, USA

Abstract
The fluctuating wind induced vibration is one of the most important factors which has been taken into account in the design of long-span bridge due to the low stiffness and low natural frequency. Field measurement characteristics of sustained wind on structure site can provide accurate wind load parameters for wind field simulation and structural wind resistance design. As a suspension bridge with 1490 m main span, the Runyang Suspension Bridge (RSB) has high sensitivity to fluctuating wind. The simultaneous and continuously wind environment field measurement both in mid-span and on tower top is executed from 2005 up to now by the structural health monitoring system installed on this bridge. Based on the recorded data, the wind characteristic parameters, including mean wind speed, wind direction, the turbulence intensity, the gust factors, the turbulence integral length, power spectrum and spatial correlation, are analyzed in detail and the coherence functions of those parameters are evaluated using statistical method in this paper. The results indicate that, the turbulence component of sustain wind is larger than extremely strong winds although its mean wind speed is smaller; the correlation between turbulence parameters is obvious; the power spectrum is special and not accord with the Simiu spectrum and von Karman spectrum. Results obtained in this study can be used to evaluate the long term reliability of the Runyang Suspension Bridge and provide reference values for wind resistant design of other structures in this region.

Key Words
long-span bridge; structural health monitoring; wind environment; field measurement; turbulence characteristics; power spectrum

Address
Youliang Ding, Guangdong Zhou, Aiqun Li and Yang Deng : Key Laboratory of Concrete and Prestressed Concrete Structures of Ministry of Education, Southeast University, Nanjing, 210096, China

Abstract
With the sustainability movement, vegetated building envelopes are gaining more popularity. This requires special wind effect investigations, both from sustainability and resiliency perspectives. The current paper focuses on wind load estimation on small- and full-scale trees used as part of green roofs and balconies. Small-scale wind load assessment was carried out using a wind tunnel testing in a global-effect study to understand the interference effects from surrounding structures. Full-scale trees were investigated at a large open-jet facility in a local-effect study to account for the wind-tree interaction. The effect of Reynolds number combined with shape change on the overall loads measured at the base of the trees (near the roots) has been investigated by testing at different model-scales and wind speeds. In addition, high-speed tests were conducted to examine the security of the trees in soil and to assess the effectiveness of a proposed structural mitigation system. Results of the current research show that at relatively high wind speeds the load coefficients tend to be reduced, limiting the wind loads on trees. No resonance or vortex shedding was visually observed.

Key Words
green building envelope; full-scale testing; tall buildings; tree; wind loading; wind tunnel

Address
Aly Mousaad ALY : Louisiana State University, Louisiana, USA; Politecnico di Milano, Milano, Italy; Western University, Ontario, Canada
Fabio Fossati, Sara Muggiasca and Tommaso Argentini: Politecnico di Milano, Milano, Italy
Girma Bitsuamlak : Western University, Ontario, Canada
;Florida International University, Florida, USA
Alberto Franchi, Nicola Longarini and Pietro Crespi: Politecnico di Milano, Milano, Italy
Arindam Gan Chowdhury : Florida International University, Florida, USA

Abstract
Modeling an equilibrium atmospheric boundary layer (ABL) in an empty computational domain has routinely been performed with the k-e turbulence model. However, the research objects of structural wind engineering are bluff bodies, and the SST k-w turbulence model is more widely used in the numerical simulation of flow around bluff bodies than the k-e turbulence model. Therefore, to simulate an equilibrium ABL based on the SST k-w turbulence model, the inlet profiles of the mean wind speed U, turbulence kinetic energy k, and specific dissipation rate w are proposed, and the source terms for the U, k and w are derived by satisfying their corresponding transport equations. Based on the proposed inlet profiles, numerical comparative studies with and without considering the source terms are carried out in an empty computational domain, and an actual numerical simulation with a trapezoidal hill is further conducted. It shows that when the source terms are considered, the profiles of U, k and w are all maintained well along the empty computational domain and the accuracy of the actual numerical simulation is greatly improved. The present study could provide a new methodology for modeling the equilibrium ABL problem and for further CFD simulations with practical value.

Key Words
equilibrium atmospheric boundary layer; SST k-

Address
Peng Hu, Yongle Li andH Haili Liao : Department of Bridge Engineering, Southwest Jiaotong University, Chengdu, Sichuan 610031, China
C.S. Cai and G.J. Xu : Department of Civil and Environmental Engineering, Louisiana State University,
Baton Rouge, Louisiana 70803, USA


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