Keywords

1 Introduction

The surface aerodynamic characteristics of high-rise circular buildings under wind load are very complex [1, 2], and wind load has become one of the main loads in structural design and analysis. According to the “Load Code for the Design of Building Structures” (GB50009-2012) [3], there is no similar wind load shape coefficient value. This project is based on BIM technology and combines BIM technology with CFD technology [4, 5]. Through CFD numerical simulation method, SST k-w [6] turbulence model is used to simulate the surrounding wind field and 8 wind directions of the simplified single building model, and the overall and local wind loads of the building are studied and analyzed, To determine the wind load shape factor for design purposes.

2 Project Overview

The project is located in Bishan District, Chongqing, with a building area of 107900 m2 and a building height of 53 m. It is composed of underground garage, podium and tower. Its main functions are exhibition hall, conference and exhibition, theater, external exhibition service area, etc. The lower part of the tower in this project is a reinforced concrete cylinder, and the upper part is a spatial steel truss. The span of the outer ring main truss is 89 m, and the span of the inner ring main truss is 58 m. The upper plane of the tower is circular, with an inner ring radius of 51.6 m, an outer ring radius of 103.1 m, and a radial width of 51.5 m.

3 Model Establishment and Grid Partitioning

Using the Revit 2016 modeling software, create a building BIM model (see Fig. 1), and simplify the model through GUI operations (see Fig. 2), the model is directly exported to an STL format file through Revit with plugins, and then imported into ICEM CFD for calculation of watershed establishment and grid division [7].

Fig. 1.
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BIM Model and Renderings.

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Fig. 2.
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Target Building CFD Geometric Modeling.

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The numerical simulation of the flow around the roof includes two characteristics: on the one hand, the flow around the roof is turbulent, which itself contains vortices of different scales; On the other hand, the flow structure is also a problem of different scales in space. Both have corresponding requirements for mesh generation. Calculate the watershed as 1500 m × 1100 m × 203 m (flow direction x × Direction y × Vertical z) (see Fig. 3). Considering the complex shape of the target building, the grid generation scheme adopts regional blocking technology, dividing roofs or walls with similar sizes and shapes into groups to establish a part, with different parts defining different sizes of grids. Dense unstructured grids are used in the area near the buildings, and hybrid Hexahedron grids are used in other areas, with a total number of about 4 million grids.

Fig. 3.
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Calculate Watershed.

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4 CFD Numerical Simulation Analysis of Wind Load

4.1 Selection of Turbulence Models

The wind load numerical simulation of the target building adopts the large-scale general computational fluid dynamics (CFD) software Fluent. The coupling of pressure and velocity is solved using the SIMPLEC algorithm, and the control equation is solved using the Segregated method. The SST k-w model is selected as the turbulence model, which has a wide range of applications and high accuracy. It is a reliable turbulence model for simulating wind loads on building structures. The parameters of the turbulence model are taken from the corresponding values in the UDF file loaded in Fluent. The calculation result is taken as a steady-state result, and the number of iteration steps is taken as 3000. The calculation accuracy is first calculated using first-order accuracy of 1200 steps, and then using second-order accuracy of 1800 steps. The convergence criterion for calculation is taken as a residual value of 5 × 10–4 [8].

4.2 Selection and Handling of Boundary Conditions

Boundary conditions for incoming flow: Velocity inlet is used at the inlet of the flow field, and the average wind profile is represented by an exponential law:

$$ U\left( z \right) = U_{10} \left( \frac{z}{10} \right)^\alpha $$
(1)

U10 is the average wind speed of the incoming flow at a height of 10 m, calculated based on the local basic wind pressure; α is the ground roughness index [9].

The turbulent kinetic energy and dissipation rate of the inflow surface are expressed as follows：

$$ k\left( z \right) = \frac{3}{2} \cdot \left[ {U\left( z \right) \cdot I_u \left( z \right)} \right]^2 $$
(2)
$$ \varepsilon \left( z \right){ = }C_\mu^{3/4} \cdot \frac{{k^{2/3} \left( z \right)}}{K \cdot z} $$
(3)

In the formula, Cμ is a model constant with a value of 0.09; K is the Carmen constant, with a value of 0.42; Iμ(z) for the turbulence degree of incoming flow at height.

The outlet surface adopts pressure outlet boundary conditions; Symmetry boundary conditions are used for the upper and lateral surfaces; The use of non slip wall boundary conditions on the surface and ground of buildings [10].

4.3 Numerical Simulation of Working Conditions

This study conducted numerical simulation calculations for the target building in the range of 0°to 360°, with a total of 8 wind directions spaced at 45°intervals. The calculation of different wind directions was achieved through rotating the model.

4.4 Calculation of Average Wind Pressure Coefficient

Calculation of Wind Pressure Coefficient at Measuring Points.

The point average wind pressure coefficient Cpi on the surface of the structure is obtained from the following equation:

$$ C_{pi} = \frac{P_i - P_\infty }{{\frac{1}{2}\rho U_G^2 }} $$
(4)

In the formula, Pi is the pressure acting on the measuring point i, and P∞ is the static pressure at the reference height, ρ is the Density of air, UG is the average wind speed of incoming flow at the height of gradient wind.

Calculation of Weighted Average Wind Pressure Coefficient for Local Area.

The weighted average average wind pressure coefficient Cp of the local area on the structural surface is obtained from the following equation:

$$ C_p = \frac{{\sum {C_{pi} \cdot A_i } }}{{\sum {A_i } }} $$
(5)

In the formula, Cpi is the average wind pressure coefficient at measurement point i, and Ai is the surface area of the structure represented by measurement point i. Due to the fact that CFD calculation can obtain the wind pressure value at any point on the entire building surface, this project directly provides a contour cloud map of the wind pressure distribution on the building surface, without the need to calculate the weighted average wind pressure coefficient of the area.

4.5 Numerical Simulation Results

Wind Load Indicated by Flow Field and Building.

This article uses CFD numerical simulation to calculate the wind speed field around the target building, the average wind pressure coefficient on the building surface, the local shape coefficient, and the distribution of equivalent static wind load under 8 wind direction conditions within the range of 0°to 360°. The wind load is the most unfavorable under the 45° wind direction condition (see Fig. 4).

Fig. 4.
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45°Wind Direction Working Condition.

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Overall Shape Coefficient of the Building.

The overall shape coefficients of the circular building at various wind directions were obtained through CFD numerical simulation calculations (see Fig. 5). It can be seen from the figure that the maximum Drag coefficient of the annular building is 1.12, which occurs under the condition of 45° wind angle. The average value of the overall shape coefficient of circular buildings at various wind directions is about 0.92.

Fig. 5.
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Numerical Simulation Results of Overall Shape Coefficient of Buildings.

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5 Conclusion

In view of the fact that there is no wind load shape coefficient of similar shape in the high-rise ring Building code, based on the BIM technology, this project combines the BIM technology with the CFD technology, analyzes the actual wind load CFD numerical simulation, and determines the design wind load shape coefficient: the overall shape coefficient of the ring building can be 1.2 for partial safety consideration; The windward walls and lower surfaces of circular buildings are mainly under positive pressure, while the leeward walls, sides, lower surfaces, and roofs are generally under negative pressure. The maximum positive pressure appears on the windward wall and lower surface of the outer ring, while the absolute value of the maximum negative pressure of the circular building appears at the edge of the inner and outer rings of the roof; For the wind resistance design of the enclosure structure, the wind load is considered based on a 50 year return period. The design wind pressure on the wall and lower surface of the circular building can be taken as 0.6 kPa, the negative pressure at the edge of the roof can be taken as 1.5 kPa, and the middle of the roof can be taken as 1.0 kPa. These conclusions provide theoretical and practical significance for similar projects in the future.