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CONTENTS
Volume 15, Number 6, November 2012
 

Abstract
The effects of Reynolds number (Re), freestream turbulence intensity (Tu) and integral length scale (A) on the drag coefficient (Cd) of a circular cylinder in cross flow were experimentally studied for 6.45 x 103 < Re < 1.82 x 104. With the help of orificed plates, Tu was fixed at approximately 0.5%, 5%, 7% and 9% and the normalized integral length scale (L/D) was varied from 0.35 to 1.05. Our turbulent results confirmed the general trend of decreasing Cd with increasing Tu. The effectiveness of Tu in reducing Cd is found to lessen with increasing A/D. Most interestingly, freestream turbulence of low Tu (~5% ) and large A/D (~1.05 ) can increase the Cd above the corresponding smooth flow value.

Key Words
circular cylinder; turbulence; cross flow; turbulence intensity; integral length scale; drag

Address
Nibras Younis and David S.K. Ting : Mechanical, Automotive & Materials Engineering University of Windsor Windsor, Ontario, Canada N9B 3P4

Abstract
The greenhouse type metal structures are increasingly used in modern construction of livestock farms because they are less laborious to construct and they provide a more favorable microclimate for the growth of animals compared to conventional livestock structures. A key stress factor for metal structures is the wind. The external pressure coefficient (cpe) is used for the calculation of the wind effect on the structures. A high pressure coefficient value leads to an increase of the construction weight and subsequently to an increase in the construction cost. The EC1 in conjunction with EN 13031-1:2001, which is specialized for greenhouses, gives values for this coefficient. This value must satisfy two requirements: the safety of the structure and a reduced construction cost. In this paper, the Navier – Stokes and continuity equations are solved numerically with the finite element method (Galerkin Method) in order to simulate the two dimensional, incompressible, viscous air flow over the vaulted roofs of single span and twin-span with eaves livestock greenhouses\' structures, with a height of 4.5 meters and with length of span of 9.6 and 14 m. The simulation was carried out in a wind tunnel. The numerical results of pressure coefficients, as well as, the distribution of them are presented and compared with data from Eurocodes for wind actions (EC1, EN 13031-1:2001). The results of the numerical experiment were close to the values given by the Eurocodes mainly on the leeward area of the roof while on the windward area a further segmentation is suggested.

Key Words
Eurocode; external pressure coefficient; CFD; livestock building; wind flow

Address
D.L. Kateris, V.P. Fragos,T.A. Kotsopoulos, and D. Moshou : Department of Hydraulics, Soil Sciences and Agricultural Engineering, Aristotle University of Thessaloniki
Thessaloniki, Greece
V.P. Fragos and T.A. Kotsopoulos :Agricultural Structures Control Center (ASCC),Thermi-Thessaloniki, Greece
A.G. Martzopoulou : School of Architecture, Aristotle University of Thessaloniki, Thessaloniki, Greece

Abstract
The present paper is focused on the prediction of the acrosswind aeroelastic response of square tall buildings. In particular, a semi-analytical procedure is proposed based on the assumption that square tall buildings, for reduced velocities corresponding to operational conditions, do not experience vortex shedding resonance or galloping and fall in the range of positive aerodynamic damping. Under these conditions, aeroelastic wind tunnel tests can be unnecessary and the response can be correctly evaluated using wind tunnel tests on rigid models and analytical modeling of the aerodynamic damping. The proposed procedure consists of two phases. First, simultaneous measurements of the pressure time histories are carried out in the wind tunnel on rigid models, in order to obtain the aerodynamic forces. Then, aeroelastic forces are analytically evaluated and the structural response is computed through direct integration of the equations of motion considering the contribution of both the aerodynamic an aeroelastic forces. The procedure, which gives a conservative estimate of the aeroelastic response, has the advantage that aeroelastic tests are avoided, at least in the preliminary design phase.

Key Words
aeroelastic force; aerodynamic damping; tall buildings; wind tunnel tests; rigid models; square cross-section

Address
I. Venanzi and A.L. Materazzi : Department of Civil and Environmental Engineering, University of Perugia, Perugia, Italy

Abstract
Traditionally, a quasi steady response concerning the aerodynamic force and moment coefficients acting on a flat plate while \'flying\' through the air has been assumed. Such an assumption has enabled the flight paths of windborne debris to be predicted and an indication of its potential damage to be inferred. In order to investigate this assumption in detail, a series of physical and numerical simulations relating to flat plates subject to autorotation has been undertaken. The physical experiments have been carried out using a novel pressure acquisition technique which provides a description of the pressure distribution on a square plate which was allowed to auto-rotate at different speeds by modifying the velocity of the incoming flow. The current work has for the first time, enabled characteristic pressure signals on the surface of an auto-rotating flat plate to be attributed to vortex shedding.

Key Words
auto-rotation; surface pressures; coherent structures; vortex shedding

Address
P. Martinez-Vazquez, Sterling, C.J. Baker, A.D. Quinn and J.S. Owen : School of Civil Engineering, University of Birmingham, UK
B. Kakimpa : Department of Civil Engineering, University of Nottingham, UK
P.J. Richards : Department of Mechanical Engineering, University of Auckland, New Zealand

Abstract
Many tall buildings possess through-building gaps at middle levels of the building elevation. Some of these floors are used as sky gardens, or refuge floors, through which wind can flow with limited blockage. It has been reported in the literature that through-building gaps can be effective in reducing across-wind excitation of tall buildings. This paper systematically examines the effectiveness of two configurations of a through-building gap, at the mid-height of a tall building, in reducing the windinduced dynamic responses of the building. The two configurations differ in the pattern of throughbuilding opening on the gap floor, one with opening through the central portion of the floor and the other with opening on the perimeter of the floor around a central core. Wind forces and moments on the building models were measured with a high-frequency force balance from which dynamic building responses were computed. The results show that both configurations of a through-building gap are effective in reducing the across-wind excitation with the one with opening around the perimeter of the floor being significantly more effective. Wind pressures were measured on the building faces with electronic pressure scanners to help understand the generation of wind excitation loading. The data suggest that the through-building gap reduces the fluctuating across-wind forces through a disturbance of the coherence and phase-alignment of vortex excitation.

Key Words
through-building opening; wind loading; wind pressure; vortex shedding; across-wind excitation

Address
Alex P. To :Ove Arup & Partners Hong Kong Ltd, 5/F, Festival Walk, Tat Chee Avenue, Kowloon Tong, Hong Kong
K.M. Lam and S.Y. Wong : Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong
Z.N. Xie: State Key Laboratory of Subtropical Architecture Science, South China University of Technology, Guangzhou
510641, China


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