2017 2nd International Conference on Industrial Aerodynamics (ICIA 2017) ISBN: 978-1-60595-481-3 CFD/FEM Based Analysis Framework for Wind Effects on Tall Buildings in Urban Areas Qiao Yan, Dalong Li, Kangkang Liu and Yan Bowen ABSTRACT In this paper, the one-way coupled numerical simulation with computational fluid dynamics (CFD) and finite element method (FEM) is used to evaluate the wind load and wind-induced vibration response of high-rise buildings in urban area. Firstly, this paper presents a complete framework for the numerical simulation of the wind effect of high-rise buildings, which mainly includes the module for the shape coefficient of structural members, the module for fluctuating wind load analysis of structural members and the module for the wind-induced vibration response analysis. Then, based on the proposed numerical simulation analysis framework, this paper selects one typical high-rise building located in the center of the high-density city to carry out the systematic analysis in terms of the shape coefficient, the wind pressure coefficient, the interference factor, the base moment spectrum and structural wind-induced vibration response, and combined with high-rise building structure specifications for its wind-induced vibration of a reasonable assessment. The results show that the proposed framework can effectively assess the wind effect of the high-rise building in the urban area and the accurate evaluation of the wind-induced vibration comfort, and provide a highly efficient and reliable tool for the wind resistance design of the high-rise building. 452
INTRODUCTION High-rise buildings are usually the landmarks of the modern urbanization of metropolitan cities, which are often located in the city center surrounded by many existing tall buildings. The surrounding buildings and complex terrain conditions make the effective and efficient evaluation of wind effects on the tall buildings quite cumbersome. In addition, the current codes or standards of the wind loads on structures and buildings [1, 2] have highlighted the spotlight of the standalone tall buildings featured by cuboidal shapes and lack of thorough and detailed specifications regarding to the topographical effects on the wind fields and interference effects of neighboring buildings, although some significant progresses have been made towards such issues [1]. As a consequence, it is of great necessity to develop the comprehensive methodology for the reliable evaluations of wind effects on tall buildings. In the past five decades, meeting the convenient computing power while enhancing the capabilities of numerical computational methods has led to surge of computational fluid dynamics (CFD) as a complementary tool to the traditional approaches (wind tunnel experiments and on-site measurements) for wind engineering applications, such as air pollutant dispersion and flow over complex topography [3]. With increasing computing power and advanced numerical techniques, some encouraging studies for LES of wind effects on buildings were reported [4-6]. Recently, Blocken et al. conducted a verification and validation study to investigate the wind flow around a bluff body by LES. Moreover, various uncertainty sources in LES including the grid resolution, inflow turbulence and sub-grid scale (SGS) models were quantitatively assessed [7]. By using a new inflow turbulence generation method, the fluctuating wind loadings predicted by LES on a standard tall building model were found to agree with the wind tunnel measurements [5]. From the perspective of engineering applications, the sustained and rapid development of computational methods enables the CFD to be a sustainable viable option for the wind-related issues. Therefore, the comprehensive analysis framework based on the CFD techniques has been proposed for the handy, efficient and reliable evaluations of wind loads and wind-induced vibrations on the tall buildings. In this study, the CFD modeling techniques are coupled in one-way with the FEM modeling techniques for the determination of wind-induced responses of tall buildings. In addition, the wind effects on twin tall buildings with surrounding buildings in urban areas are in detail analyzed using the analysis framework herein. Finally, he proposed framework could be integrated into the 453
platform of Building Information Management (BIM) system for engineers and researchers evolving in the wind-resistant design of tall buildings. Figure 1. Analysis framework for wind effects of tall buildings. ANALYSIS FRAMEWORK OF WIND EFFECTS ON TALL BUILDINGS The analysis framework proposed in this study is shown in Figure 1, which can provide the thorough evaluations of wind effects on tall buildings in terms of shape coefficients, wind pressure coefficients, wind-induced vibrations, and assessment of serviceability of tall buildings of concern. In addition, the main methodology includes three parts: RANS models for both shape coefficients and mean wind pressure coefficients; LES models for wind load fluctuations, and One-way coupled CFD/FEM for wind-induced vibrations. CFD simulations RANS MODELS Since most of the turbulent flows encountered in practical configurations, and particularly in the wind engineering applications are featured by high Reynolds number (typically 10 5 <Re L <10 8 ), it has clearly appeared necessary to develop some specific approaches to reducing the cost associated with the simulation of turbulent flows, leading to the emergence of RANS modelling techniques. Due to 454
their affordable computational cost, these approaches are still the most commonly used for industrial applications. RANS models rely on a statistical description of the flow. In practice, all turbulent motions of the flow are in fact unresolved and have to be described in a mathematical model. Therefore, the averaged flow field information can be obtained using RANS models including time averaged pressures and velocities. As shown in Figure 1, the shape coefficients and mean wind pressure coefficients could be assessed using RANS models at the significantly reduced cost of computational resources, thus leading to some RANS simulations that can be easily performed using usual computers at the preliminary stage. LES MODELS Large-eddy simulation (LES) models adopts a scale separation between the largest energy-containing eddies of the flow and the smallest energy dissipation ones of the flows. The scale separation operator, in fact, is defines as a low-pass filter in the wave number space. Since the cutoff wave number can be arbitrarily fixed, this approach can provide a full description of unsteady flow characteristics, up to the cutoff frequency related to the cutoff wave number. Compared with RANS models, the LES is a fascinating compromise between the prohibitive cost of DNS and the averaged description of the flow provides by RANS. And thus, the LES as shown in Figure 1 enables the users to obtain the fluctuating wind loads in terms of base moment spectrum for structural resistance design and peak wind pressure coefficients for cladding design. One-way coupled CFD/FEM simulations To evaluate the wind-induced response of a tall building, the fluid-solid transfer scheme is required to transfer the time history of external loads determined from CFD solver to the structural nodes. As shown in Figure 1, the external forces come from wind loads including flow pressures and shear stresses on the surface of the tall building. Apparently, the accurate transfer of wind loads from fluid to structure is of great importance in evaluating the wind-induced responses of a tall building. In this study, the quadrature-project scheme [8] is adopted with the advantages of easy and efficient implementation for three-dimensional simulations and relatively low numerical errors. 455
CASE STUDY OF WIND EFFECTS ON TWIN TALL BUILDINGS IN URBAN AREAS In this section, the wind effects on the twin tall buildings in urban areas are investigated using the proposed framework herein. The height of the twin tall buildings is 234.6 m high of No. 1 building and 234 m high of No. 2 building as shown in Figure 2. Numerical methodology CFD MODELS OF TWIN TALL BUILDINGS Figure 2. Approaching wind flow directions of numerical simulations. The geometric models of twin tall buildings and their surrounding podium buildings are firstly established as shown in Figure 3(a); meanwhile, the dimensions of the computational domain are 2,520 m (Length) 1,680 m (Width) 400 m (Height) as shown in Figure 3(c). The inner region surrounding the building models is cylindrical with the advantages of variations of approaching wind directions. The hybrid tetrahedral and prismatic grids are filled in the inner region and the surface meshes of twin tall buildings are demonstrated in Figure 3(b), while the outer regions away from the central area are discretized by the structured elements to reduce the total grid number. And the total number of grids is 3.9 10 6. The numerical simulation grid is shown in Figure 3. (a) Geometric models (b) Surface meshes of tall buildings 456
(c) Computational domain Figure 3. Schematic diagram of geometric models, mesh arrangement and computational domain. The RNG k-ε turbulence model is used to calculate the averaged wind pressure coefficients, shape coefficients and interference factors of the twin tall buildings with increment of 15 o and a total of 24 numerical simulations are performed as shown in Figure 2. The mean velocity profile follows the exponent power law as prescribed in Chinese wind code for Terrain B. And the other boundary conditions are specified according to our previous work [9, 10]. FEM MODELS OF TWIN TALL BUILDINGS In the study of target building response to the wind-induced vibration, two kinds of element types: Beam-188 and Shell-181 were used to model in ANSYS software. At the same time, two simplified treatments were carried out: 1. Ignore the weight of filler wall and glass curtain wall; 2. Ignore the constraint of the podium buildings. The model is shown in Figure 4. For designing the building structure, the ANSYS software can be used to calculate the natural frequency and mode of the column-core tube structure. And the reciprocal of the natural frequency is the self-oscillation period. Combining with the results of LES transient analysis, the wind-induced vibration response time-history analysis can be carried out under three conditions: (1) 0, No. 1 building; (2) 270, No. 1building; (3) 270, No. 2 building. (a) Vertical view (b) Frontal view (c) Side view (d) Global model Figure 4. The finite element model of 1# building. 457
RESULTS AND DISCUSSIONS RANS analysis of averaged wind loads of twin tall buildings The calculation convergence criterion is that the dimensionless residual of all variables reduce to 10-4. All the conditions become steady after about 6000 steps and ultimately stabilize. Figure 5 shows the contours of the averaged wind pressure coefficients of No. 1 and No. 2 buildings at the wind direction of 0 o based on the RNG k-ε turbulence model. It can be seen that the maximum positive pressure is recorded at 0.8H (H is the height of the building) on the windward surface of the building, and the maximum negative pressure occurring position is not uniform and needs to be calculated. The absolute value of the negative pressure is usually higher than the positive pressure, and sometimes can reach the twice or three times the positive pressure coefficients. (a) Windward (b) Leeward Figure 5. Contours of Averaged wind pressure coefficients in the wind direction of 0. LES analysis of fluctuating wind loads of twin tall buildings The surface wind pressure distribution coefficients of No. 1 and No. 2 buildings and the characteristics of the flow field in the adjacent area are obtained by 670-hour large-scale distributed numerical simulation. The results show that the maximum averaged positive pressure coefficient on the whole structure is 1.1, which appears in the upper part of the building when the wind direction is 0. The minimum negative average wind pressure coefficient is -2.2, appearing in the building at the upper corner when the wind direction is 150. The average wind pressure coefficient of the side wind surface and the leeward surface is mostly a minus, especially near the corner of the building surface. 458
By using a writing program in User defined function (UDF), the target building is divided into 28 layers according to the building design drawings. The wind-induced time history loads including along-wind, across-wind and twisting are respectively recorded to conduct subsequent analysis of wind-induced response. The results are shown in Figure 6. (a) Along-wind (b) Across-wind (c) Torsion Figure 6. Base moment spectrum of No. 1 building in the wind direction of 0. TABLE I. FUNDAMENTAL PERIODS AND FREQUENCIES OF THREE FORMATIONS. Mode Cycle Frequency Formation participation coefficient 1 0.17054 5.863727 1 2 0.17326 5.771673 0.7333 3 0.4359 2.294104 0.7536 TABLE II. TIME-HISTORY ANALYSIS RESULTS UNDER THREE CONDITIONS. Condition Maximum acceleration /m s -2 Minimum acceleration / m s -2 Maximum displacement /m Minimum displacement /m Condition 1 Along-wind 0.1092-0.1051 0.1126 0.000002 Across-wind 0.0594-0.0648 0.0039-0.021 Condition 2 Along-wind 0.0321-0.0287 0.0078-0.0032 Across-wind 0.054-0.0742 0.00017-0.0148 Condition 3 Along-wind 0.0664-0.0597 0.0772-0.00095 Across-wind 0.0641-0.0498-1.63E-06-0.0464 One coupled CFD/FEM analysis of wind-induced response analysis In this study, the Lanczos method is used to solve the general physical dynamics equation. The results are shown in TABLE I. 459
The elastic response of structures is calculated under 30 seconds of wind loading. The Along-wind wind load, Across-wind wind load and torsional wind load are applied to the representative beam-column joints of every two layer. The results, as shown in TABLE II, indicate that the maximum acceleration appearing at 0 condition, which is 0.1092m/s 2, meet the comfort requirements. The maximum vertex displacement is 0.1126m, which also meets the requirement of the vertex displacement of the elastic stage of concrete structure [2]. Figure 7. Comparison of wind vibration coefficients in different conditions with normative value. To calculate the displacement wind vibration coefficient, since only the first order formation is considered in the norm, it is essential to take higher order formation into account. Therefore, the displacement wind vibration coefficient is adopted as an important indicator of wind-induced structural response. Suppose the vertical displacement caused by average wind is U si, while the vertical displacement caused by the equivalent force of fluctuating wind is U Di. Therefore, the total wind-induced displacement is. According to its definition, the displacement wind vibration coefficient is: (1) At the same time, the defined acceleration wind vibration coefficient is compared with the normative value. (2) The comparison is shown in Figure 7. Compared with the wind-induced vibration coefficient of the current codes, the 460
results of the time-history analysis are smaller than the normative value. Therefore, the wind resistance design of high-rise building structure is conservative, directly using the value provided by the current codes. CONCLUSIONS The proposed numerical simulation framework can be a feasible solution to precisely determine the wind effect of the high-rise buildings in urban area, meanwhile reasonably assessing the wind-induced vibration comfort of them. As a highly efficient and reliable tool for the wind resistance design of the high-rise building, this framework can be extensively used by structural engineers. REFERENCES [1] Ministry of Housing and Urban-Rural Development of the People s Republic of China. 2011. Technical Specification for Concrete Structures of Tall Building. China Architecture & Building Press. [2] Ministry of Housing and Urban-Rural Development of the People s Republic of China. 2012. Load Code for the Design of Building Structures. China Architecture & Building Press. [3] Blocken, B. 2014. 50 Years of Computational Wind Engineering: Past, Present and Future, Journal of Wind Engineering & Industrial Aerodynamics, 129(6), 69-102. [4] Huang, S., Li, Q. S., & Xu, S. 2007. Numerical Evaluation of Wind Effects on a Tall Steel Building by CFD, Journal of Constructional Steel Research, 63(5), 612-627. [5] Huang, S.H., & Li, Q.S. 2010. Large Eddy Simulation of Wind Effects on a Super-Tall Building, Wind and Structure, 13, 557-580. [6] Nozu, T., Tamura, T., Okuda, Y., & Sanada, S. 2008. LES of the Flow and Building Wall Pressures in the Center of Tokyo, Journal of Wind Engineering and Industrial Aerodynamics, 96, 1762-1773. [7] Gousseau, P., Blocken, B., & van Heijst, G.J.F. 2013. Quality Assessment of Large-Eddy Simulation of Wind Flow around a High-Rise Building: Validation and Solution Verification, Computers & Fluids, 79, 120-133. [8] Cebral, J., Lohner, R. 1997. Conservative Load Projection and Tracking for Fluid Structure Problems, Aiaa Journal, 35(4), 687-692. [9] B.W. Yan and Q.S. Li. 2015. Inflow Turbulence Generation Methods with Large Eddy Simulation for Wind Effects on Tall Buildings, Computers & Fluids, 116: 158-175. [10] B.W. Yan, Q.S. Li, Y.C. He and P.W. Chan. 2015. RANS Simulation of Neutral Atmospheric Boundary Layer Flows over Complex Terrain by Proper Imposition of Boundary Conditions and Modification on the k-ε Model, Environmental Fluid Mechanics, 1: 1-23. 461