2.4.2. Details of Wind Environment Experiment and Simulations
- (1)
Measured Points Selection
CFD was used to simulate the wind environment in the outdoor enclosure space for wind measurement Case No. 3 (the outdoor enclosure space of Kaiyuan Temple). The representative wind speed points ABCDEFGHI are selected according to the different areas formed when the natural wind meets the outdoor enclosed space of Kaiyuan Temple No. 3 in Quanzhou during winter (
Figure 6a).
- (2)
Field Experiment Method
To confirm the correlation between the recorded wind speeds and the wind speeds generated by simulations, the wind conditions in the outdoor enclosed space of Kaiyuan Temple in Quanzhou were measured on a winter day with clear and cloudy skies. On 10 January 2024, the NK5500LINK anemometer was used for fixed-point measurements at nine measuring points (ABCDEFGHI) in the outdoor enclosed space of the Shakya Mani Hall of Kaiyuan Temple in Quanzhou from 9:00 a.m. to 5:00 p.m. The method employed was simultaneous fixed-point observation by multiple individuals, setting the standard for all wind environment simulation scenarios since then. The anemometer was fixed at nine measuring points for testing using a tripod, with the inlet of the anemometer positioned 1.5 m above the ground. Wind speed data are gathered hourly, and the average wind speed within one minute of each measurement point is computed for every data point. A set of minimum, average, and maximum wind speed values is recorded every 10 s within 1 min. Between 9:00 and 17:00, 9 sets and 486 valid wind speed data points were collected, with average wind speed data calculated for each time period based on the measured data (The final recorded measured data are the average wind speed of each group). A comparison and analysis were performed on the wind environment in enclosed outdoor spaces at various measurement points, utilizing the NK500LNK anemometer, which provides wind speed measurements with an accuracy of ±3%.
On 10 December 2024, measurements of Leaf Area Index (LAI) were taken for 23 plant species at Kaiyuan Temple in Quanzhou (
Figure 7). Measurements were taken using the LAI-2200C Plant Canopy Analyzer from LI-COR Biosciences in Lincoln, NE, USA. The analyzer was placed at four different points (north, south, east, and west) beneath the canopy of each plant to determine the leaf area index. After completing the actual measurement, connect the plant canopy analyzer to the computer to read the measured data of 23 plants. Then, install FV2200 V 2.1.1 software on the computer to analyze the measured data of the plants and obtain the LAI values of the 23 plants.
- (3)
Three-Dimensional Model Construction
Firstly, a plant configuration model was established for Kaiyuan Temple in Quanzhou. Upon conducting an on-site investigation, it was observed that the plants can be categorized into herbs, shrubs, and trees. Among them, herbs include carpet grass, shrubs include areca palm, and trees include the Banyan, Sabina chinensis, Podocarpus macrophyllus, Bodhi tree, Longan tree, Byakuran, Bambusa ventricosa, Camphor tree, Mango tree, Cherry blossom, Mulberry, Magnolia, Banana shrub, Cycad, Osmanthus flower, Averrhoa carambola L, Camellia flower, Poinciana, Syzygium jambos, Jackfruit, Hoop pine, and Southern Magnolia. Additionally, field studies were conducted to measure the canopy height, width, and trunk height of various plants. The leaf area index of different plants was also measured using the LAI-2200C plant canopy analyzer (
Table 2). Liang Li’s study indicates that rectangular models have advantages, including uncomplicated modeling, rapid computation, and excellent convergence [
35]. Hence, the plant model adopts the technique of rectangular simplification and utilizes SketchUp v.2020 software to establish a three-dimensional model.
Secondly, 50 real models were constructed for the outdoor enclosed space of Kaiyuan Temple in Quanzhou. Based on the CAD topographic maps of 50 outdoor enclosed spaces in Kaiyuan Temple, Quanzhou, SKETCHUP software was used to establish 3D models for outdoor wind field calculation of 50 outdoor enclosed space scenes (
Table 3,
Figure 6b).
Thirdly, 15 index models will be constructed for the outdoor enclosed space of Kaiyuan Temple in Quanzhou. Based on
Table 1, the highest frequency index values of the three key factors for the outdoor enclosed space layout of Kaiyuan Temple in Quanzhou were statistically obtained. From this, a typical original model for simulating the wind comfort level of the key factors for the outdoor enclosed space layout of Kaiyuan Temple in Quanzhou was summarized. This model has a certain universal applicability. The outdoor enclosed space of Kaiyuan Temple in Quanzhou has the highest proportion of 42.00% in terms of height-to-cross-section ratio, falling within the range of 0.12 to 0.22. In terms of enclosure rate, the outdoor enclosure space of Kaiyuan Temple in Quanzhou has the highest proportion of 28.00% in the range of 0.12 to 0.24. In terms of permeability, the outdoor enclosed space of Kaiyuan Temple in Quanzhou has the highest proportion of 34.00% within the range of less than 0.10. A typical original model of the outdoor enclosed space of Kaiyuan Temple in Quanzhou was established based on a height-to-cross-section ratio of 0.17, an enclosure rate of 0.18, and a permeability value of 0.03 (
Figure 8). Further classify and analyze the three indices of 50 outdoor enclosed spaces in Kaiyuan Temple, Quanzhou. There are five types of height-to-cross-section ratio indices: 0.07, 0.17, 0.27, 0.37, and 0.47. There are five types of enclosure rate indexes, with values of 0.06, 0.18, 0.30, 0.42, and 0.54, respectively. There are five types in the permeability index classification, with permeability indices of 0.03, 0.17, 0.31, 0.45, and 0.59. Fifteen index models were established by referring to typical original models, as indicated in
Table 4 and
Figure 8, corresponding to the height-to-cross-section ratio indices of 0.07, 0.17, 0.27, 0.37, and 0.47 (
Figure 9a–e), enclosure rate indices of 0.06, 0.18, 0.30, 0.42, and 0.54 (
Figure 9f–j), and permeability indices of 0.03, 0.17, 0.31, 0.45, and 0.59 (
Figure 9k–o). Thirdly, convert 50 real models and 15 exponential models into STL format and import them into PHOENICS software. Adjust the length, width, and height of the calculation domain, set the center and edge area grids, determine the input wind speed, wind direction, and profile type (power law), and include open sky (its setting parameters do not change with height). Then, simulate the wind environment and obtain the wind environment simulation results at a height of 1.5 m (Z = 1.5 m) from the ground for pedestrians.
- (4)
CFD Simulation Settings
PHOENICS was used for CFD simulation, revealing that varying height distribution patterns affect wind speed and pressure alterations [
36]. Altering the design of structures can effectively reduce overall energy usage and carbon emissions within a community [
37]. The placement of plants in the courtyard can significantly impact the outdoor microclimate and the thermal comfort of residents [
38]. The layout of architectural spaces and changes in aspect ratios can create effective ecological buffer spaces [
39]. Based on the above and existing research [
15]. The PHOENICS software is a perfect choice for simulating the wind environment in enclosed outdoor spaces for this research. Below are the details for the CFD simulation setup and model creation:
Model Selection: Perform a simulation using the RNG
k-ε turbulence model in PHOENICS software (the RNG
k-ε model has been widely studied and has accuracy in fields such as architecture and landscaping. The y+ value is approximately between 30 and 300, and the Reynolds number is greater than 10
6) [
15,
26]. The RNG
k-ε model has good predictive ability, and its equation form is relatively simple compared to more complex models such as the SST
k-ω model. It also has lower computational resource consumption, making it suitable for large-scale grids or steady-state simulations. Following Equations (4) and (5). The simulation for velocity–pressure coupling was executed by utilizing the PRESTO discrete equation and configuring the PARSOL function settings. The automatic convergence detection feature in PHOENICS guarantees effective convergence of simulation results with an accuracy level of 10
−5 [
40].
The software PHOENICS v.2016 is used to solve equations that use the symbols k for turbulent kinetic energy and ε for the rate of turbulent dissipation.
Grid Setting: The computing domain of the scene has dimensions that are five times larger in length and width compared to those of the corresponding scene model, and three times greater in height. Previous research has shown that simulation results are not affected by the height of the computational domain [
15]. The calculation domain size is 5W × 5L × 3H (
Figure 6b). To determine the mesh of the simulation area, divide the computational domain into two sections: the central area and the edge area. The grid densities for coarse meshes, fine meshes, and finest meshes are as follows: X
min = Y
min = Z
min = 0.27 H, X
min = Y
min = Z
min = 0.13 H, and X
min = Y
min = Z
min = 0.06 H, respectively (the comparison in
Table 5 reveals that simulated data in diverse computational domains exhibit improved accuracy with finer grid densities). With a ground roughness parameter α of 0.2, the inflow boundary condition was set to a fixed pressure and zero gradient (the previous setting of boundary conditions has proven the reliability of CFD simulation) (
Table 6) [
15].
The inflow profile had a constant horizontal velocity and turbulent kinetic energy at the top boundary, while the slip walls on both the left and right were symmetrical and without gradients.
Using Equation (6), we can determine the gradient of the oncoming wind at the inlet:
where
u(
z) is the horizontal velocity at height
z, and
u0 is the horizontal velocity at height
z0. In this model,
u0 = 2.0 m/s (winter),
z0 = 65.1 m, and
α = 0.25 [
41].
The turbulent kinetic energy,
k (m
2/s
2), and its dissipation rate,
ε (m
2/s
3), are set as follows:
where
u* is the friction velocity,
δ is the depth of the boundary layer, and
K is the von Karman’s constant. In this model,
u* = 0.1 m/s (winter),
K = 0.4, and
Cµ = 0.09 [
15].
Trees in the model were parameterized as a one-dimensional column, where the tree height was used to scale the normalized LAI. The vertical distribution of LAI is not consistent, creating difficulties in distinguishing between different vegetation types. The crown shape, height, and canopy edges influence the fluctuation of LAI with height. Each tree branch is likened to the crown in turbulence models, where tree crowns are seen as porous media [
15]. The tree canopy causes a reduction in the kinematic energy of airflow due to drag and pressure, leading to the need for resistance to be included in the momentum equation to address the impact of vegetation on turbulent flow patterns. A sink term is added to the momentum equation to accommodate turbulence resistance from the canopy layer:
where
Cd is the drag coefficient,
is the vector speed on foliage surface (m/s), and
is the Cartesian velocity in
direction (m/s).
Additional source terms in the momentum equation can show the correlation between airflow and tree canopy turbulence in the following way:
In this investigation,
βp,
βd,
C4ε, and
C5ε are empirical constants with values of 1.0, 3.0, 1.5, and 1.5, respectively.
βp denotes the average fluid kinetic energy of the wake flow,
k, produced by the drag force of the canopy, whereas
βd indicates the kinetic energy,
k, dissipated by the short circuits of Kolmogorov energy gradients [
15].
Sensitivity analysis: It can be calculated using Equation (12):
where
Cpi is the mean wind pressure coefficient of a specific point
i,
Pi represents the wind pressure at the point
i,
is the static pressure at reference height,
is air density, set as 1.225 kg/m
3,
is the wind speed at the reference height, and the wind speed is 6.13 m/s at the top of the outdoor enclosed space (No. 3) at Kaiyuan Temple in Quanzhou, 21.7 m [
42].
For the sensitivity analysis, experimental results of the TJ case were used for comparative analysis, utilizing Equations (13) and (14), which were the earliest wind tunnel test results on CAARC in China [
42].
where
n is the number of measuring points included in the comparison, set as 5 on each surface,
CpiCFD is the mean wind pressure coefficient of point
i in CFD simulation and
Cpitunnel represents the mean wind pressure coefficient of point
i in wind tunnel test [
42]. Generate a surrogate model using Kriging substitution model and Monte Carlo sampling (MCS) to calculate the uncertainty level of material properties (UTS COV of approximately 25%) [
43,
44].
According to Lader and Fredheim (2006) [
45], the material of nets follows the non-linear constitutive law:
With
the unstretched bar length and (
C1,
C2) = (1160
N, 37,300
N) for squared meshes [
43].
Iterative solution of Equation (16) through nonlinear equations: addressing numerical instability and time step limitations caused by nonlinear deformation [
46].
Failure mechanisms: By combining finite element analysis (FEA) and computational fluid dynamics (CFD), parameters such as the local pressure peak, temperature gradient, cavitation number (σ), and vorticity intensity are determined to identify critical points beyond material limits. Sensitivity analysis can be used to quantify input parameters such as inlet turbulence intensity and material property deviations to predict failure modes [
47].
The standard wall functions are employed together with roughness modification on the ground surface. The values of the roughness parameters, i.e., the sand-grain roughness height
ks (m) and the roughness constant
Cs, are determined using their consistency relationship with the aerodynamic roughness length
z0, Equation (17):
In this study,
ks = 0.0007 m and
Cs = 0.13 for the ground surface. The building walls have roughness
ks = 0 and
Cs = 0.5. Zero static gauge pressure is applied at the outlet plane. Symmetry conditions, i.e., zero normal velocity and zero normal gradients of all variables, are imposed on the top and lateral sides of the domain [
48].
Wind Condition Setting: As per the “Design Code for Heating, Ventilation and Air Conditioning of Civil Buildings” (GB5073602012, China) [
49], “China Weather Network”, “China Meteorological Network”, and “China Meteorological Data Network”, in the Quanzhou area during winter, the average wind speed is 6.13 m/s. The dominant wind direction for the season is the northeast direction (selecting the northeast direction with the highest frequency of occurrence in winter as the simulated wind direction). Kaiyuan Temple in Quanzhou has a relatively low latitude, flat terrain, and a coastal area. Spring is usually warm and humid, summer is high in humidity and heat, autumn is sunny and cloudy with little rain, and winter is mild but often has strong winds.