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4th International Conference On Building Energy, Environment

CFD Study on the Characteristics of Temperature and Velocity in a Environmental Wind Tunnel

L.L. Li, L. Zhang, Q. Li, R. Peng, L. Zhao, Y.F. Zhang and Q.L. MengState Key Laboratory of Subtropical Building Science

South China University of Technology, Guangzhou, Guangdong, 510640, China

SUMMARY The majority of existing environmental wind tunnels do not consider temperature. However, atmospheric temperature is very important for urban physics research. This paper introduces a hot-humid-climate environmental wind tunnel, which is a non-neutral atmospheric boundary layer (ABL) wind tunnel. For this wind tunnel, temperature and velocity fields are extremely significant. To understand the characteristics of them, this study couples the modelling of the two fields by Computational fluid dynamics (CFD) software with the aim of developing control methods for the construction of wind tunnel lab, which can also reduce manpower and material consumption in wind tunnel debugging.

INTRODUCTION Wind tunnels are important test equipment that are widely used in urban physics investigation to analyse problems in the urban wind environment (Tsang C W. et al. 2012, Leitl B. et al. 2007), wind effect on buildings (Cermak. et al.2011), air pollution diffusion (Civiš S. et al. 2002, Melo A.M.V., et al. 2012.), and others. The accuracy of the wind tunnel experiment results is related to the flow field quality of the wind tunnel. To obtain a better flow field in the wind tunnel, it is necessary to improve the flow field in the early design stage and the later commissioning stage. CFD numerical simulation is a convenient and quick method to model the flow field in a wind tunnel. Furthermore, most researchers have adopted the CFD numerical simulation method to optimize the design of wind tunnel equipment (Calautit J K. et al. 2014, S.A.A.A. Ghani. et al. 2001), and to perfect the design of the wind tunnel test (S.S. Desai 2003). These activities immensely reduce manpower and material consumption of wind tunnel test debugging. Existing CFD simulation literatures of wind tunnels focus predominantly on the velocity field, turbulence profile, and other flow field characteristics in neutral ABL wind tunnels. There are limited simulations of temperature fields in wind tunnels (Chaudhry H N.et al. 2015).

However, the neutral atmospheric conditions only accounted for 20%–40% (Walczewski.et al.1998). Some experts have studied the influence of temperature field in ABL. Snyder (Snyder W H. 1994) verified the effect of thermal stratification on pollutants in the wakes area of buildings using water tunnel experiment. Uehara (Uehara Kiyoshi. et al. 2000) analysed the influence of atmospheric thermal stratification on the flow pattern in street valleys and found that the vortex in the street valley became weaker when the atmosphere was stable, and increased when the atmosphere was unstable. Kovar (Kovar-Panskus A. et al. 2001) studied the effect of solar radiation heat on the flow field of the street valley by a wind tunnel test, and obtained the Froude number threshold when the thermal effect could not be neglected. Hancock (Hancock P E and Pascheke F. 2014, Hancock P E and Zhang S. 2015) studied

the influence of atmospheric stability on the wake of a wind turbine.

Therefore, the temperature field in ABL remains an important factor in urban physics investigation. The CFD simulation of environmental wind tunnels should include not only the velocity profile, but also the temperature profile of non-neutral ABL under different stabilities. This paper introduces a hot-humid-climate environmental wind tunnel, which can simulate the non-neutral ABL. In this study, the temperature and velocity fields in the wind tunnel are a coupled analysis of theoretical calculation and CFD numerical simulation. Then, this paper provides the basis for the wind tunnel debugging to reduce manpower and material consumption in the wind tunnel test.

DESCRIPTION OF THE WIND TUNNNEL As shown in Figure 1, the hot-humid-climate environmental wind tunnel is located at the State Key Laboratory of Subtropical Building Science, South China University of Technology. This wind tunnel is a vertical closed-loop low-speed wind tunnel. The total length of the wind tunnel is 40.5 m with a widest and highest parts of 4.9 and 10.4 m, respectively. The wind tunnel consists of eight parts: first stable section, first test section, diffusion section, fan section, filter section, contraction section, second stable section, and second test section.

Figure 1. Sketch of hot-humid-climate environmental wind tunnel

Figure 2. Top view of the second test section

This study mainly analyses the second test section, which can simulate the non-neutral ABL. The second test section is on the second floor of the wind tunnel building, which has a boundary dimension of 20×3×(2–3) m (length-width-height). In the test section, the temperature profile cart, spires, roughness elements, heating/cooling floor, and model turntable (diameter 2.5 m) are arranged along the direction of flow (Figure 2). The temperature profile cart and

ISBN: 978-0-646-98213-7 COBEE2018-Paper078 page 214


heating/cooling floor make up a temperature stratification system, which can build up the temperature of non-neutral ABL in the wind tunnel. The specific control parameters of the second test section are shown in Table 1.

Table 1. Control parameters of the second test section

Parameter Range

Velocity 0.5~10m/s

Air temperature 10~40ºC

Floor temperature 10-90ºC

TPC temperature gradient 80 ºC/m

CFD NUMERICAL SIMULATION This simulation corresponds to actual atmospheric boundary conditions, which correspond to a stable ABL with a temperature gradient of 0.06/100 ºC/m and B terrain (Standard for wind tunnel test of buildings and structures in China). In the following sections, the basic parameters of the simulation are determined by theoretical calculation. Then, simulation cases and boundary conditions are set. Finally, the simulation results are analysed to provide a reference for the debugging of the wind tunnel.

Sub-heading: parameters calculation

The fitted mean velocity and turbulence intensity curve of B terrain can be expressed by the power law as Eq.(1-2), which is referred to the literature.

𝑈

𝑈10= (

z

10)

0.15 (1)

𝐼

𝐼10= (

z

10)

−0.15 (2)

where U10 and I10 are the mean velocity and turbulence intensity at a height of 10 m.

ℎ = 1.39𝛿/(1 + 𝛼/2) (3)

𝑏/ℎ = 0.5𝜓(𝐻/𝛿)(1 + 𝛼/2)/(1 + 𝜓) (4)

𝜓 = β [2

1+2𝛼+ 𝛽 −

1.13𝛼

(1+𝛼)(1+𝛼/2)] /(1 − 𝛽)2 (5)

𝛽 = 𝛼(𝛿/𝐻)/(1 + 𝛼) (6)

k/𝛿 = exp {2

3ln (

𝐷

𝛿) −0.1161 [

2

𝐶𝑓+ 2.05]

1/2

} (7)

𝐶𝑓 = 0.136 [𝛼

(1+𝛼)]

2 (8)

where h and b are the height and width of the spires respectively, α the wind profile index (0.15), H is the height of the wind tunnel, δ is the height of the ABL in the wind tunnel,

k is the height of the roughness element, D is the distance between the roughness elements, and Cf is the ground roughness coefficient.

To acquire the fitted curve of the wind tunnel, the spires and roughness element need to be arranged. Their sizes can be calculated using Eq.(3-8), which refers to the design method of Irwin (Irwin.1981); in this case, the scale of model is 1:350 and δ is 1 m. The results are shown in the Table 2. There are four spires along the width direction of second test section, and the centre distance is 0.6 m. The rough element is arranged according to the plum blossom type. The transverse distance between the rough elements is D1 and the longitudinal spacing is D2.

Table 2. Parameters of the spires and roughness element

ℎ 𝑏 k D1 D2

1.4m 0.14m 40mm 0.5m 1m

The fitted temperature curve is expressed as Eq.(8), which is calculated according to the thermal similarity Richardson number of Eq.(9). The result of temperature gradient in the test section is 69.4 °C/m, which corresponds to 0.06/100 °C/m of actual ABL. Here, we choose an initial temperature of 15 °C.

𝑇𝑧 = 15 + 69.4𝑧 (8)

𝑅𝑖 =∆𝑇

𝑇0⋅

𝐿𝑔

𝑈2 (9)

where To is the average absolute temperature over the entire boundary layer depth, ∆T is the temperature difference of the boundary layer, L is the boundary layer thickness, U is the velocity magnitude, and g is the acceleration due to gravity.

Sub-heading: numerical model setting

The computational settings SST k-ω model is used in the CFD simulation; it is regarded as a better RANs model for simulating blunt body flow than the standard k-ω model (Yang .et al.2009, Yang .et al.2017). We choose the SIMPLE algorithm to solve and initialize the flow field by using the values set for the inlet boundary conditions. Moreover, we set the energy equation for the temperature field, in addition to the Navier-Stokes and turbulence equations.

Table 3. Parameters of the spires and roughness element

case spires Roughness

element

k(mm)

Inlet temperature

(ºC) h(m) b(m)

0 - - 40 𝑇𝑧 = 15 + 69.4𝑧

1 1.4 0.14 - 𝑇𝑧 = 15 + 69.4𝑧

2 1.4 0.14 40 𝑇𝑧 = 15 + 69.4𝑧

3 1.4 0.14 60 𝑇𝑧 = 15 + 69.4𝑧

4 1.4 0.14 40

𝑇𝑧1 = 6 + 119.7𝑧

（0≤z≤0.64m）；

𝑇𝑧2 = 32.4 + 60.4𝑧

（0.64＜z≤1.02m）



The TPC has 50 floors each with a width of 3 m and height of 4 cm, which are simplified as the velocity inlet of the test section. as indicated in Table 4 According to previous calculation, we set the simulation as Case 2. Then, based on this, we changed the size of the spires and roughness elements. The corresponding cases are 0, 1, and 3, respectively (Table 3). Case 4 is set based on the changing inlet temperature curve (Table 3).

Table 4. Setting of boundary condition in numerical model

turbulence model (SST) k- ω

Inflow boundary u=1m/s v=0,w=0, Iu=5%,T=Tz

Downstream boundary out flow

Wall condition non slip solid wall

Sub-heading: grid generation

The calculation domain is a rectangular parallelepiped: 20 m long (x), 3 m wide (y), 2 m high (z).The sections of the geometry apply structured grid (Hooff and Blocken.2010). To speed up the calculation and to ensure the accuracy result, a dense mesh resolution is applied at the spires, roughness element, and bottom wall of the test section. The complete meshed model has approximately 4,500,000 cells.

Figure 3. Griding of numerical domain

Figure 4. Local grid refinement

RESULTS This To analyse the influencing factors of velocity and temperature field in the test section, we choose the front edge of the model turntable (x=9.5 m, y=1.5 m) as the measurement line in Figure 2, and extract the average velocity (U), turbulent kinetic energy value (k), and temperature (T) parameters. The reference height is 0.7 m (Zr), and the velocity at this height is the reference velocity (Ur). Referring to Load Code for the Design of Building Structures of China, the turbulence intensity (Iu) is calculated in terms of its relation to turbulent kinetic energy (Iu=(1.01k)1/2/Uz).

Sub-heading: velocity field

In Figures 5 and 6, the comparison of Case 0 with the others shows that the spires have a greater effect on the velocity field and turbulence field in this test section. This is because spires have a larger disturbance on the velocity field, as shown in Figures 7-1 and 7-2. Comparison of Cases 2 and 3 shows that increasing the size of the roughness element changes the velocity field changed very little, as shown in Figures 7-3 and 7-4.

The roughness element can increase turbulence intensity at relative altitudes below 0.4; however, a value of Iu remains below the fitted curve, as shown in Figure 6. Thus, to form a fitted velocity curve in a wind tunnel, we can appropriately relax the requirements of turbulence. Figures 7-3 to 7-5 show that inlet temperature has little effect on the velocity field.

Figure 5. X=9.5m, Y=1.5m Vertical Velocity curve

Figure6. X=9.5m, Y=1.5m Vertical Turbulence curve

Figure 7-1. case0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.0 0.2 0.4 0.6 0.8 1.0

Z/Z r

U/Ur

case0case1case2case3case4Fitted curve

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.00 0.05 0.10 0.15 0.20

Z/Z r

Iu

case0

case1

case2

case3

case4

Fitted curve



Figure7-2. case1

Figure7-3. case2

Figure7-4. case3

Figure7-5. case4

Figure7. Y=1.5m Velocity Plane

Sub-heading: temperature field

Figure 8-1 shows that if the inlet temperature is set to the fitted curve, the test section cannot form the fitted temperature curve at the front edge of the model turntable. However, if the slope of the temperature curve is increased as in Case 4, the test section can form the close fitted temperature curve at the front edge of model turntable. However, the slope of the inlet temperature curve is not maximised; it requires segmentation to set the slope according to the simulation results of the temperature difference. For example, the inlet temperature curve set in accordance with Tz1=6+119.7z at a height of 0–0.64 m and Tz2=32.4+60.4z at a height of 0.64–1.02 m in Case4. Then, it can achieve a similar fitted temperature curve in the test section, as shown in Figure 8-2.

The slope of the inlet temperature curve is increased owing to the heat loss and the effect of the spires and roughness element. As shown in Figures 9-1–9-4, the test section has the same inlet temperature as the fitted temperature curve because different turbulence devices produce different wind fields in the wind tunnel, resulting in different heat losses and temperature fields. Figures 9-2–9-4 show that the temperature near the ground surface in Case 2 is slightly lower than that of Cases 1 and 3. One possible reason is that there was no roughness element in Case1, and the overall wind speed was slightly lower. Thus, the low- temperature air near the ground only reaches downstream at approximately x=5 m. For Case 2, the roughness was increased, increasing the wind speed, and the low-temperature air reached a distance of

approximately x=9.5 m. For Case 3, the roughness and the field disturbance were increased. Thus, the flow increases the air temperature and rapidly mixes the different temperature air. Then, the near-ground low-temperature air only reaches downstream at approximately x=5.5 m.

Figure8-1. Temperature curve

Figure 8-2. Temperature curve

Figure8.X=9.5m, Y=1.5m Vertical Temperature curve

Figure 9-1. case0

Figure 9-2. case1

0.0

0.2

0.4

0.6

0.8

1.0

1.2

10 20 30 40 50 60 70 80 90

Z(m

)

T(ºC)

case1case2case3Fitted curve

0.0

0.2

0.4

0.6

0.8

1.0

1.2

10 20 30 40 50 60 70 80 90

Z(m

)

T(ºC)

Fitted curvecase2case4 inletcase4



Figure 9-3. case2

Figure 9-4. case3

Figure 9-5. case4

Figure9. y=1.5mTemperature plane

The turbulence generation service has an effect on temperature uniformity in the cross-section. Comparison of Figures 10-1 and 10-2 shows that Case 0 only has roughness elements, and its temperature stratification is obvious and uniform. In contrast, the uniformity of temperature stratification in Case 1 is poor. Case 2 has both spires and roughness elements. However, compared with Case 1, its uniformity changed only slightly, as Figure 10-3 .Therefore, the spires have a greater effect on the uniformity of the temperature distribution in the cross-section. Moreover, we need to find alternative spire forms that are much better for the uniformity of the cross-section temperature.

Figure 10-1. case0

Figure 10-2. case1

Figure 10-3. case2

Figure10. x=9.5m Temperature plane

DISCUSSION The existing ABL wind tunnel is a predominantly neutral ABL wind tunnel, which does not consider atmospheric temperature. However, the atmospheric temperature boundary layer is very important for urban physics research. This study carried out the coupled temperature and velocity field analysis in a non-neutral ABL wind tunnel by CFD numerical simulation, and analysed influencing factors and setting methods in the wind tunnel. The results of the CFD simulation can help solve problems which require use of the non-neutral ABL wind tunnel, and reduce the manpower and material consumption of the wind tunnel debugging.

At present, the simulation cases listed in this paper are only typical cases, and the follow up research will conduct more simulation cases and analysis. In addition, since the wind tunnel is still under construction, the present simulation results cannot be verified by wind tunnel tests. In subsequent studies, we will use wind tunnel tests to verify these results.

CONCLUSIONS There is a contradiction between the coupled formation of velocity and temperature fields in non-neutral ABL wind tunnels. It is difficult to control the velocity, turbulence intensity, and temperature curves simultaneously. According to the analysis of the simulation results, we can use a method that we first arrange the turbulence generating device to form a target velocity curve, then relax the requirements of turbulence, and finally debug the temperature curve, to achieve a better temperature and velocity field in the wind tunnel. Owing to the heat loss and the effect of the turbulence generating device, the inlet temperature curve needs to improve the slope of the Z-T temperature curve, which is based on the fitted curve from the wind tunnel.

ACKNOWLEDGEMENT This research work was funded by the Major Program of the National Natural Science Foundation of China (No. 51590912), National Natural Science Foundation of China (No. 51308223, 51678243 and 51308222), Guangdong Natural Science Foundation (No. 2016A030313506) and State Key Lab of Subtropical Building Science, South China University of Technology (No. 2015ZC14), Fundamental Research Funds for the Central Universities (No. 2017ZD017 and 2017ZD039)

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