A COMPARATIVE STUDY ON INTERFERENCE FACTORS OF BUILDINGS

Transcription

1 The Eighth Asia-Pacific Conference on Wind Engineering, December 10 14, 013, Chennai, India A COMPARATIVE STUDY ON INTERFERENCE FACTORS OF BUILDINGS Ankit Singh 1 & S. Mandal 1 IDD (B. Tech. - M.Tech.) Student, Department of Civil Engineering, IIT(BHU), Varanasi, UP, India, ankit.singh.civ09@iitbhu.ac.in Associate Professor, Department of Civil Engineering,IIT(BHU),Varanasi, UP, India, smandal.civ@iitbhu.ac.in ABSTRACT Wind loads on buildings in real environments can be quite different from those measured on isolated buildings in wind tunnels. Surroundings can significantly increase or decrease the wind forces on the interfered buildings. The parameters affecting the wind forces may include the geometries and orientations of these buildings, the reduced wind velocity number of the adjacent buildings and the upstream terrain condition [Xie&Gu (007)]. Due to complex arrangement of buildings in urban scenario, different interference factors (IF) have been proposed by researchers to account for interference effects among various building configurations such as mean interference factor (MIF), envelope interference factor (EIF), dynamic interference factor (DIF), regression interference factors (RIF) for different cases of mean and dynamic interference effects with varying heights of buildings. Also included are certain other parameters like interference index which also give a qualitative estimate of the interference effects. Keywords: Meaninterference factor, Dynamic interference factor, Envelope interference factor, Regression interference factor. INTRODUCTION The present paper looks into different magnitudes of interference factors obtained by various wind tunnel experiments as well as computational modeling studies. These studies are reviewed briefly and the recommended values of interference factors are compared. Interference factors given by the draft IS:875(Part 3): 011 for tall as well as short buildings are compared with the other various interference factors to determine the relevance of these IFs in Indian context. IS:875 (Part 3) : 011provides the following interference factors for low roof buildings. Definition of different types of IF Interaction between two building models has typically been reported in the form of a non-dimensionalized shielding, buffeting or interference factor. When the response of the downstream model reduces, it is commonly called as shielding (or sometimes sheltering), buffeting when interference causes a cross wind or resonant response and interference when wind effects on either the upstream or downstream model become more severe [English & Fricke (1999)]. Also, this effect has been differently quantified as mean interference factor (MIF), envelope interference factor (EIF), regression interference factor (RIF), envelope interference factor (EIF)[Xie&Gu (007)]. They have introduced another term interference index as Interference Index = Shielding (or Buffeting, etc.) Factor 1.0 Proc. of the 8th Asia-Pacific Conference on Wind Engineering Nagesh R. Iyer, Prem Krishna, S. Selvi Rajan and P. Harikrishna (eds) Copyright c 013 APCWE-VIII. All rights reserved. Published by Research Publishing, Singapore. ISBN: doi: /

2 The interference effects among three buildings are very complex. An Envelope interference factor (EIF) is proposed and is obtained by maximizing the IFs in the reduced velocity ranges of V r = 9.Reduced velocities >9 rarely happen for the practical structures. RIF is the linear regression interference factor that is used for correlating IFs such as MIF and EIF for variables like different height ratio [Xie and Gu, (007)]. Comparative values of interference factors Table 1: Comparison of IF given by IS:875( Part 3), Thoroddsen et al.(1985), Khanduri et al.(1998) Spacing IS Code I.F. r.m.s. IF by IF for mean drag on IF for mean drag on Thoroddsen et al. upstream building downstream building (Khanduri (1985) (Khanduri et al., 1998) et al., 1998) x= 0.5b x= b x= 3b x= 4b x= 5b x= 10b Fig. 1 : Comparison of mean drag and due to interference: (a) mean-drag interference factors for upstream model; (b) mean drag interference factors for downstream model (Khanduri et al. 1998) Xie and Gu (007) provided contours of interference factors for two square shape buildings of equal height undergoing both along and across wind interference. The principal building is fixed at (0,0) while the position of the interfering building is changed. IS code specifies IFs for only along wind loading. Hence, comparison for only along wind IFs are done here. Table 3: Comparison between IF given by IS: 875 (Part 3) : 011 (draft code) and Xie and Gu(006) for low roof buildings Spacing IS:875 (Part 3) : 011 Xie and Gu (007) Remarks (draft) X <=0.5b Max. value by Xie and X= b Gu(006) =.4 at x= 4.1b, y= -0.8b X= 3b X= 4b X= 5b X= 10b

3 CFD simulation of interference effects An attempt has been made to model the interference effect using ANSYS 1. Correct specification of inlet boundary conditions of the turbulent flow field is most important issue [Ojha et al. (001)]. Interference effects between two and three buildings in the computational domain are being investigated.the main parameters and dynamic characteristics of the prototype are 40m in height, 40m in breath, % for structural damping ratio and 0. Hz for natural frequencies for both the sway fundamental modes.dimensions of the buildings are adopted similar to Xie and Gu (007).A wind tunnel building model of height 600 mm and width 100 mm is modeled in ANSYS FLUENT using RNG k-epsilon model. Fig. Wind profiles and turbulence intensity distributions in terrain B and D. Fig. 3 x-y coordinate grid for locating interfering buildings The turbulence model used in the present study is RNG k-epsilon model. U t U t 0 z z z 951

4 k 3 z 0 t k 1 t t k k k U 0 z z z z 1 t U c 1 t c z z z k z k t 0 Contours for different input boundary conditions 1. Aroussi and Ferris(1987) Fig. 4 (a) Contours of turbulence kinetic energy for three building configuration. (b) Contours of Streamline for three building configuration. Fig. 5 (a) Contours of turbulence kinetic energy for two building configuration. (b) Contours of Streamline for two building configuration. 95

5 Fig. 6 (a) Contours of turbulence kinetic energy for single building configuration. (b) Contours of Streamline for single building configuration.. MARUYAMA(199) Fig. 7 (a) Contours of turbulence kinetic energy for three building configuration. (b) Contours of Streamline for three building configuration 953

6 Fig. 8 (a) Contours of streamline for two building configuration. (b) Contours of turbulence kinetic energy for two building configuration Fig. 9 (a) Contours of streamline for single building configuration. (b) Contours of turbulence kinetic energy for single building configuration 3. Huang et al (007) Fig. 10 (a) Contours of streamline for three building configuration. (b) Contours of turbulence kinetic energy for three building configuration 954

7 Fig. 11 (a) Contours of streamline for two building configuration. (b) Contours of turbulence kinetic energy for two building configuration 4. LAMINAR BUILDING Fig. 1 (a) Contours of streamline for three building configuration. (b) Contours of velocity for three building configuration. Fig. 13 (a) Contours of streamline for two building configuration. (b) Contours of velocity for three building configuration. 955

8 Fig. 14 (a) Contours of streamline for single building configuration. (b) Contours of velocity for single building configuration. Results Table. Different IF for various input values computed through CFD study Author building IF 3 building IF Laminar flow Aroussi and Ferris(1987) Maruyama ( 199) Huang et al (007) Conclusions The above magnitudes of IF taken from different studies shows wide variability. Several boundary layer wind tunnel studies have duly considered the interference effects. Computational wind engineering, involving several types of turbulence models, also provides an alternate approach to evaluate IF. The present study briefly reviewed the different magnitudes of interference factors. It also envisages using CFD applications to estimate IF. The CFD study is done for critical locations of two and three building configuration reported by Xie and Gu (007). References Aroussi, A. and Ferris, S.A.(1987). Air flow over buildings: A computer simulation of LDA measurements. Proc. Second Int. Conf. on Laser Anemometry- Advances and Applications, Strathclyde, U.K., Bailey, P. A., and Kwok, K. C. S. (1985) Interference excitation of twin tall buildings. J. Wind Engg. & Indus.Aerodynamics English, E.C. and Fricke, F.R. The interference index and its prediction using a neural network anlaysis of wind -tunnel data.j. WindEngg. & Indus. Aerodynamics, 83, Haggkvist, K., Svensson, U. and Taeslar, R. (1989). Numerical simulations of pressure fields around buildings. Building and Environment, 4(1),

9 IS : 875 (Part 3) : 011 : DraftCode of practice for design loads (other than earthquake) for buildings and structures : Wind loads, Bureau of Indian Standards, New Delhi. Khanduri, A. C., Stathopoulos, T. and Bedard, C. (1998). Wind induced interference effects on buildings : A review of the state of the art. Engg. Structures, 0 (7), Lee, B. F., and Fowler, G. R. (1975). The mean wind forces acting on a pair of square prisms. Building Science, 10, Maruyama,T. (199). Numerical simulation of boundary ayer wind tunnel. J. Wind Engg. And Industrial Aerodynamics,41-44, Ojha, C. S. P., Ojha, C. S.P., Mandal, S., and Bhargava, P. (001). Aspects of Inlet boundary prescription for a turbulent flow field. J. Hydraulic Engineering, ASCE, 17(8), Sakamotu, H. and Haniu, H (1988). Aerodynamic forces acting on two square prisms placed vertically in a turbulent boundary layer. J. Wind Engg. & Indus. Aerodynamics Saunders, J. W., and Melbourne, W. H. (1979). Buffeting effects of upwind buildings. Proc. 5 th Int. Conf. Wind Engg., Fort Collins, CO, Singh,A.,andMandal,S.(01)."Astateoftheartreviewoninterferenceeffectofstructuressubjectedto windloads"proceed.of6thnationalconfonwindengineering,crri,newdelhi,1415dec,01,pp Taniike, Y. (199) Interference mechanism for enhanced wind forces on neighboring tall buildings. J. Wind Engg. & Indus. Aerodynamics, 4, Taniike, Y. and Inaoka, H. (1988). Aeroelasticbehaviour of tall buildings in wakes. J. Wind Engg. & Indus. Aerodynamics. 8, Thoroddsen, S. T., Cermak, J. E., and Peterka, J. A. (1985). Mean and dynamic wind loading caused by an upwind structure. Proc. Of 5 th U.S. National Conf. on Wind Engg., Texas Tech. Univ., 4a 73 to 4a 80. Xie, Z. N. and Gu, M. (007). Simplified formulas for evaluation of wind induced interference effects among three tall buildings. J. Wind Engg. & Indus. Aerodynamics, 95,